HK1070191B - Method and apparatus for determining time difference between multiple base stations in a cdma communication system - Google Patents

Description

Method and apparatus for determining time difference between multiple base stations in CDMA communication system

Technical Field

The present invention relates generally to data communications, and more particularly to techniques for time-aligned transmission from multiple base stations in a CDMA communication system.

Background

Widespread deployment of wireless communication systems provides various types of communication, including voice and packet data services. These systems may be based on Code Division Multiple Access (CDMA), Time Division Multiple Access (TDMA), or some other multiple access technique. CDMA systems may provide certain advantages over other types of systems, including increased system capacity. CDMA systems are typically designed to conform to one or more standards, such as IS-95, CDMA2000, and W-CDMA standards, which are well known in the art and are incorporated by reference herein.

A CDMA system may be operated to support both voice and data communications. During a communication session (e.g., a voice call), a terminal may be in active communication with one or more base stations placed in an "active" group of the terminal. In soft handoff (or soft handoff), the terminal communicates with multiple base stations simultaneously, which may provide diversity against deleterious path effects. The terminal may also receive other types of transmitted signals, e.g., pilot reference, paging, broadcast messages, etc., transmitted from one or more other base stations.

According to the W-CDMA standard, the base stations do not need to operate synchronously. When operating asynchronously, the timing of the base stations (or radio frames) may not be aligned from the point of view of the terminal, and the reference time for each base station may be different from the reference times for the other base stations.

In soft handoff, the terminal receives data transmissions (i.e., radio frames) from multiple base stations simultaneously. In order to ensure that radio frames arrive at the terminal within a certain time window so that they can be processed and reproduced correctly, the W-CDMA standard provides a mechanism to adjust the start time of the terminal-specific radio frame from each base station to the terminal. Generally, before the new base station is added to the active set of the terminal, the terminal determines the timing of the base station relative to the reference base station and reports to the system. The system then commands the new base station to adjust its transmission timing to the terminal so that the radio frames transmitted from this new base station are approximately aligned in time with the radio frames from the other active base stations.

For the W-CDMA standard, the time difference between the new candidate base station and the reference base station can be reported by "SFN-SFN observed time difference type 1 measurements" (where SFN represents the system frame number). This measurement consists of two parts. The first part provides chip-level timing between two base stations, which can be obtained by detecting the timing of pseudo-noise (PN) sequences used to descramble downlink signals from these base stations. The second part provides frame-level timing between the two base stations, which may be obtained by processing (i.e., demodulating and decoding) the broadcast channel transmitted by the base stations. These two parts are encapsulated in a report message sent from the terminal to the system.

In some W-CDMA system configurations, only chip-level timing is required to properly time-align the newly added base station radio frame. This is true, for example, as long as the base stations can be operated synchronously and the frame level timing is known to the system. In such cases, requiring the terminal to measure and report both frame-level timing and chip-level timing (as required by current W-CDMA standards) may degrade performance. First, if a terminal is forced to process the broadcast channel of a base station before a candidate base station can be selected for communication, the soft handoff area may be limited to only a small portion of the base station's coverage area and may also be constrained where the broadcast channel may be received. Second, the processing of the broadcast channel may generate other delays that degrade performance.

There is therefore a need in the art for techniques to achieve time-alignment of transmissions from multiple base stations to a terminal. Such a technique provides the required time difference (i.e., only chip-level timing or both chip-level timing and frame-level timing) from the terminal in a W-CDMA system, which may be suitable for handoff or other applications.

Disclosure of Invention

Aspects of the present invention provide various schemes for time-aligned data transmission from multiple base stations to a terminal. To achieve time-alignment, the time difference observed at the terminal between the arrival times of the transmissions of the base stations must be determined and provided to the system. The system then uses this timing information to adjust the timing at the base station so that the terminal-specific radio frame transmitted by the base station can arrive at the terminal within a specific time window.

In the first scheme, the time difference between two base stations is divided into a "high resolution" part and a "low resolution" part, and only the required part is reported when requested. For a W-CDMA system, SFN-SFN type 1 measurements can be framed-level time differences and chip-level time differences. Whenever a request is made to perform and report a time difference measurement for a list of one or more base stations, the terminal measures chip-level timing relative to a reference base station for each base station in the list. The terminal also measures frame-level timing and includes this information in SFN-SFN type 1 measurements only when needed (e.g., by system direction). Otherwise, if frame level timing is not needed, the terminal may set the frame level portion to a predetermined value, which may be a fixed value known (e.g., zero), an arbitrary value selected by the terminal and ignored by the system, a value of frame level timing known in advance by various means (e.g., past measurements of the same cell, sent by the system, etc.), or some other value.

In a second scheme, the terminal determines the time difference between two base stations based on the partial decoding of some base stations received by the terminal. For a W-CDMA system, the terminal may decode the primary common control channel (P-CCPCH) of many base stations selected according to a particular standard (e.g., received signal strength). If a particular number (e.g., two or more) of decoded base stations have the same System Frame Number (SFN) value in a particular time instance, the terminal can infer a synchronous system configuration and determine chip-level timing for the remaining base stations without determining frame-level timing.

In a third aspect, a base station determines the timing of a terminal based on uplink transmissions from the terminal. The timing information recovered by the system can then be used to adjust the timing of the downlink transmission to the terminal.

In a fourth aspect, the system determines the time difference between two base stations based on prior knowledge of the layout and size of the cells in the system. If the coverage area of the base station is sufficiently small, the time uncertainty due to signal propagation delay is also small (e.g., a few chips or less). For a W-CDMA system, the time difference between the common channel frame and the dedicated channel frame can be determined (e.g., relative to a reference base station) and all other base stations can be associated with the same time difference between their common and dedicated channel frames.

The above described scheme may be suitable for various applications such as hard and soft handoff, positioning, and other possible applications. The present invention further provides methods, terminals, base stations and devices implementing various aspects, embodiments and features of the invention, as described in further detail below.

Drawings

The features, nature, and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings in which like reference characters identify correspondingly throughout and wherein:

FIG. 1 is a schematic diagram of a wireless communication system supporting multiple users and capable of implementing various aspects of the invention;

FIGS. 2A through 2D are schematic diagrams illustrating three different synchronous and asynchronous system configurations;

FIGS. 3A through 3C are flow diagrams of a process for determining time differences for a plurality of base stations according to three different embodiments of the present invention; and

fig. 4 is a simplified block diagram of an embodiment of a base station and a terminal.

Detailed Description

Fig. 1 is a diagram of a wireless communication system 100 that supports multiple users and is capable of implementing various aspects of the invention. The system 100 includes a plurality of base stations 104 that provide coverage for a number of geographic areas 102. A base station is also commonly referred to as a Base Transceiver System (BTS) and typically combines the base station and its coverage area to form a cell. System 100 may be designed to implement one or more CDMA standards such as IS-95, W-CDMA, CDMA2000, and others, or combinations thereof.

As shown in fig. 1, the various terminals 106 are dispersed throughout the system. In one embodiment, each terminal 106 may communicate with one or more base stations 104 on the downlink and uplink at any given moment, depending on whether the terminal is active and whether it is in soft handoff. The downlink (i.e., forward link) refers to transmission from the base station to the terminal, and the uplink (i.e., reverse link) refers to transmission from the terminal to the base station.

As shown in fig. 1, base station 104a transmits to terminal 106a over the downlink, base station 104b transmits to terminals 106b, 106c, and 106i, base station 104c transmits to terminals 106d, 106e, and 106f, and so on. In fig. 1, a solid line with arrows indicates user-designated data transmission from the base station to the terminal. The dashed line with arrows indicates that the terminal is receiving pilot and other signaling from the base station, but is not receiving user-specified data transmissions. As shown in fig. 1, terminals 106b, 106f, 106g, and 106i are in soft handoff and each of these terminals is communicating with multiple base stations simultaneously. For simplicity, uplink communications are not shown in fig. 1.

Each terminal is associated with an active set that includes a list of one or more "active" base stations with which the terminal is in communication. These active base stations may transmit radio frames simultaneously to the terminal, and the transmissions from each active base station are referred to as a radio link in W-CDMA terminology. One of the base stations in the active set is designated as a reference base station. For example, the terminal may designate the base station with the strongest received signal as the reference base station, or the system may indicate which is the reference base station in a common message or a dedicated message.

In accordance with the W-CDMA standard, the base stations in the system may be operated such that they are all synchronized with each other, or they may be operated such that they are asynchronous with each other. The choice of synchronous or asynchronous operation depends on the way the network operator operates the system. The W-CDMA system may also be operated such that some base stations are synchronized and others are not. Various possible base station configurations in the system are described below.

Fig. 2A is a diagram illustrating a first system configuration (S1) in which multiple base stations (e.g., three in this example) are operated synchronously with time-aligned frame start and numbering. For this configuration, radio frames on the common channel of the base station (i.e., common channel frames) start at approximately the same time of each frame (e.g., at t)_n，t_n+1And so on). The common channel is a channel used to transmit information to all channels, and generally includes a paging channel, a broadcast channel, etc. Synchronization between base stations may be represented by the time relationship between the base station's common channel frames, which is nearly constant in time, except that there may be minor fluctuations around a nominal value. The System Frame Number (SFN) value of the common channel frame at any given time instance is the same for all three base stations in this configuration.

Fig. 2B is a diagram illustrating a second system configuration (S2) in which multiple base stations are operated synchronously, also starting with time-aligned frames, but with non-aligned frame numbering. In this configuration, the common channel frames from the base station start at approximately the same time. However, the SFN value for the common channel frame at any given time instance may not be the same for all base stations.

Fig. 2C is a diagram illustrating a third system configuration (S3) in which multiple base stations may be operated synchronously, but with non-aligned frame start and numbering. In this configuration, the common channel frames from the base station do not start at the same time but are offset from each other by some (constant) value. Thus, the SFN value of the common channel frame at any given time instance may not be the same for all base stations.

As used herein, "synchronous configuration" includes any configuration in which the system knows the relative timing differences between base stations to a certain required accuracy. The base stations may or may not operate according to different asynchronous clocks. However, if the system employs some means to determine the relative timing differences between base stations (e.g., by explicit measurements, or by implicit measurements if the base stations are known in advance to be synchronized), then the base stations can be considered to be operating in a synchronized configuration.

Fig. 2D is a diagram illustrating a fourth system configuration (a1) in which multiple base stations are operated asynchronously. In this configuration, the base stations are not synchronized and the time relationship between the common channel frames of these base stations drifts over time. It is typical that the common channels of each base station are aligned with each other but not with the common channels of the other base stations. The long-term average of this drift may be zero or some non-zero value (i.e., the time difference between base stations may continuously increase or decrease). Because of asynchronous operation, the common channels of these base stations cannot start at the same time (except by overlapping). Furthermore, the SFN value of the common channel frame at any given time instance is unlikely to be the same for all base stations.

For soft handoff in an asynchronous configuration as shown in fig. 2D, the transmissions from multiple base stations are not synchronous, and user-specific radio frames for a given terminal may begin to be transmitted by the base stations at different times (unless they are time-compensated). Furthermore, the propagation time of the transmission from each base station may be unique and depends on the distance between the base station and the terminal. Thus, it is possible for a terminal to receive user-specific transmissions from different base stations at different times (again, unless time-compensated). (for W-CDMA systems, only user-specific data transmissions are time compensated, and transmissions on the common channel are not compensated).

For the asynchronous configuration shown in fig. 2D, the radio frames (1158, 1159.) received from base station 2 are offset in time by Δ T relative to the radio frames (202, 203.) received from base station 1_1，2Wherein can be based onWhether the start of a given frame from base station 2 is earlier or later than the start of another given frame from base station 1, Δ T_1,2And may take positive or negative values. Similarly, the radio frame (3102, 3103.) received from base station 3 is offset by Δ T relative to the radio frame received from base station 1_1，3. The time difference or offset, Δ T, cannot be defined using a specific relationship_1,2And Δ T_1,3They may further vary from frame to frame. In general, for asynchronous system configurations, the time difference Δ T_X,YAny (random) value may be taken because (1) the base station transmits asynchronously without a deterministic time relationship, and (2) the propagation time from the base station to the terminal is a variable and depends in part on the location of the terminal.

For some functions it is useful or necessary to know the arrival times of (common) transmissions from multiple base stations. The time difference between the transmissions received from the various base stations may then be calculated using the times of arrival of the signals measured at the terminals. The time difference may then be used for various functions, such as for hard and soft handoffs.

The soft handoff process requires evaluation of one or more new candidate base stations for inclusion in the active set of the terminal. To facilitate soft handover in a W-CDMA system, a terminal (i.e., User Equipment (UE) in W-CDMA terminology) reports to the communication system (i.e., UMTS radio access network (UTRAN) in W-CDMA terminology) time difference measurements and signal quality measurements for each new candidate base station included in the active set of the terminal. The signal quality measurements may be used to determine whether the candidate base station is included in the active set of terminals. The time difference measurement can be used to adjust the timing of data transmission to the terminal, as described below.

The hard handoff process requires replacing the current active set of the terminal with a potentially new, disconnected active set on the same or a different frequency. Even if the new active set consists of a single base station, the system determines the relative time difference between the common and dedicated frames for all members of the new active set. The reference base station of the new active set is typically indicated in a message sent to the terminal for hard handoff.

Typically, the time difference between the new candidate base station and the reference base station is measured. The reference base station is a specific base station designated by the terminal or system as being in the active set. If the terminal is not already in active communication with the system (i.e. not already on the dedicated channel), the reference base station is the one on which the terminal is currently "camping", i.e. the measurement required before the terminal receives its broadcast channel from the base station and forwards the set dedicated channel to it, which is normally done directly in a handover.

For each new candidate base station selected from the active set included in the terminal (whether a hard or soft handoff), the system may command the new base station to compensate its timing for the terminal so that the radio frames transmitted by this new base station on the Dedicated Physical Channel (DPCH) will arrive at the terminal at approximately the same time as the other base stations in the terminal's active set (i.e., the current active base station) transmitted the radio frames on their corresponding DPCHs. Essentially, the timing of user-specific radio frames on the DPCH from each active base station of the terminal is offset relative to the timing of radio frames on the common channel of the base station so that the radio frames on the DPCH of all active base stations can get the same arrival time.

The timing compensation performed at the active base stations causes the start of radio frames received at the terminal from these base stations to be approximately aligned with a particular time window (e.g., the window may span several chips). With timing compensation, a user-specific radio frame on a DPCH (i.e., dedicated channel) from all active base stations is approximately aligned, even though their common channel frames may be received at different time instances due to different transmission times and different propagation delays. In this way, the terminal can process multiple signal instances from all transmitting base stations within a small defined window (e.g., 256 chips). If the time difference between the downlink DPCH and the downlink common channel of a particular candidate base station cannot be determined, then the system may not add the candidate base station to the active set of the terminal according to the current W-CDMA standard.

The time difference between each candidate base station for handover and the reference base station is specific to the terminal. In general, a coarse time difference measurement (e.g., one chip or worse resolution) is sufficient for handoff.

The time difference between two base stations may be measured or estimated from various types of transmissions from these base stations. The W-CDMA standard defines a (logical) Broadcast Control Channel (BCCH) mapped to a (transport) Broadcast Channel (BCH) which in turn is mapped to a (physical) primary common control channel (P-CCPCH). The broadcast control channel is a higher layer channel used to broadcast messages to terminals in the system. The broadcast message is encoded in a20 millisecond (msec or ms) transport block, which is then transmitted in a (10 msec) radio frame on the P-CCPCH. In W-CDMA, 20 ms is the interleaver size range, which is also referred to as the Transmission Time Interval (TTI). Since the transport block is 20 ms long, the number included in each radio frame is not a true SFN value but derived from SFNPrime, i.e. for the first 10 ms frame of the 20 ms TTI, SFN is SFNPrime, and for the last 10 ms frame of the 20 ms TTI, SFN is SFNPrime + 1. The start of the transmitted radio frames can be determined by processing the SCH and/or CPICH and these frame start times can then be used as the signal arrival times of the base stations. The broadcast channel on the P-CCPCH may be further processed (e.g., demodulated and processed) to reproduce the system frame number of the transmitted common channel frame. Typically, the time difference between two base stations is determined based on the arrival times of the earliest multi-path signals of these base stations.

According to the W-CDMA standard, the time difference between two base stations can be measured by a terminal and reported to the system through various types of messages. The W-CDMA standard defines SFN-SFN measurements, which represent the time offset DeltaT in FIG. 2D_X，Y. The terminal may generate and transmit this measurement to the system so that transmissions from the new base station may be compensated for as part of the handoff process. W-CDMA standard supports multiple types of SFNSFN measurements, as briefly described below.

The observed time difference between the new candidate base station and the reference base station can be reported using "SFN-SFN observed time difference type 1 measurements" (or more simply, SFN-SFN type 1 measurements). This measurement includes both frame-level timing and chip-level timing, which can be obtained by processing the broadcast channel and the P-CCPCH, respectively. The broadcast channel and P-CCPCH are described in further detail in document nos. 3GPP TS 25.133, 25.305 and 25.331, all of which are available from 3GPP organization publications, which are incorporated herein by reference.

To make SFN-SFN type 1 measurements for the new candidate base station, the terminal initially processes the SCH and/or CPICH to reproduce the chip-level time difference between the candidate base station and the reference base station. This chip-level time difference represents the difference between the start of the common channel frames from the two base stations and can be determined from the timing of the pseudo-noise (PN) sequence used to descramble the CPICH. The chip-level time difference has a range of [0..38, 399] chips, which is a full frame.

To obtain frame level timing, the terminal processes (e.g., demodulates and decodes) the broadcast channel from the candidate base station (and the reference base station, if the terminal is not yet known to it) to retrieve the common channel frame number at the particular time instance. For each base station to be reported, the terminal processes a Broadcast Control Channel (BCCH) of 20 ms or more because a Transmission Time Interval (TTI) including SFN information (SFNPrime) is 20 ms long. The terminal then determines the difference in system frame numbers for these base stations.

The observed SFN and chip difference are then combined by taking the SFN difference modulo 256, scaling the modulus result by 38, 400, and adding the scaled value to the chip-level timing difference, where 38, 400 represents the number of chips in a 10 millisecond radio frame. The combined result is a value in the range of [0..256 · 38, 400-1] chips, where 256 represents the maximum value of the SFN difference after modulo 256 operation and is in units of frames. The time difference between the candidate base station and the reference base station can be reported with a resolution of one chip. The measurement of SFN-SFN type 1 is further described in document numbers 3GPP TS 25.133 and 25.331 (section 10.3.7.63).

"time difference of SFN-SFN observed type 2 measurements" (or more simply, SFN-SFN) can also be used

Type 2 measurements) to report the observed time difference between the candidate base station and the reference base station and include only chip level timing. The terminal determines the difference in chip-level timing between these base stations at a higher resolution (e.g., between 1/2 chip to 1/16 chip resolution). The observed chip-level time difference is then represented by a value in the range of [ -1280.. 1280] chips. For SFN-SFN type 2 measurements, the terminal does not need to determine the system frame number of the candidate base station.

To add the candidate base station to the active set of the handover terminal, the terminal may measure the time difference observed between the common channel frames of the candidate base station and the reference base station and report to the system. The observed time difference can be provided to the system via a report message of SFN-SFN1 type measurements. To facilitate the handover procedure, the entire range of SFN-SFN type 1 measurements is provided by the terminals in the report message. The entire range includes the frame level time difference plus the chip level time difference.

The time difference of candidate base stations using SFN-SFN1 type measurements to report a handoff may be somewhat worse than in the optimized cases, especially for system configurations that do not require frame-level timing. For the system configurations S1, S2, and S3 shown in fig. 2A, 2B, and 2C, the base stations are synchronized and the frame-level timing of the base stations is generally known to the system. For these system configurations, only chip-level timing needs to be reported to the system. For system configuration S1, the timing difference is mainly due to different distances to the bs and small inaccuracy of the synchronization time of the bs.

However, for measurements of SFN-SFN type 1, currently defined as W-CDMA standard, the terminal must determine and report both frame-level timing and chip-level timing. To determine frame-level timing, it is necessary to demodulate, decode, and reproduce, by the terminal, the common radio frame on the candidate base station's broadcast channel, which is undesirable for a variety of reasons. First, if the broadcast channel of the candidate base station needs to be restored in order to report and consider a possible handover to the base station, the handover area may be limited to only the area where the broadcast channel can be restored, which may be only a portion of the total area covered by the candidate base station. Second, the processing of the broadcast channel results in additional delay (20 milliseconds or more for each measured base station) that can prolong the handoff process and degrade performance. Therefore, if frame-level timing is not required, it is not desirable to use SFN-SFN type 1 measurements (as currently defined by the W-CDMA standard) to report the time difference of the candidate base station.

In a system configuration where the system does not require frame level timing, only the chip level timing needs to be reported for the time difference. Such chip offsets may be reported using SFN-SFN type 2 measurements as defined by the W-CDMA standard.

However, using SFN-SFN type 2 measurements to report the chip-level timing of the handover may also be undesirable for various scenarios. For SFN-SFN type 2 measurements, the particular resolution is 1/16 chips, while the accuracy requirements range from 1/2 chips (as currently defined by the W-CDMA standard) to 1/16 chips or perhaps better (for future releases of the W-CDMA standard). To achieve a more accurate sub-chip resolution, more complex and/or longer search and acquisition processes may be required.

Furthermore, SFN-SFN 2 type measurements, which were originally intended for positioning, may be used to report the time difference of the handover, which may lead to some undesirable consequences. According to the current W-CDMA standard, SFN-SFN type 2 measurements can only be reported in OTDOA messages. (OTDOA or observed time difference of arrival is a positioning technique used in W-CDMA, which is similar to the E-OTD or enhanced observed time difference positioning technique used in CDMA 2000). Thus, certain OTDOA-related messages can be exchanged as a request for the terminal to send SFN-SFN 2 type measurements. Also, it is possible to support SFN-SFN 2 type measurements only by terminals supporting OTDOA, and not by all terminals employed in the field. Therefore, SFN-SFN 2 type measurements cannot be relied upon to report the chip-level timing of the handover, as some terminals may not support the measurement values.

Aspects of the present invention provide various schemes for achieving time-aligned data transmission from a plurality of base stations to a terminal. To achieve time alignment, the time difference between the arrival times of the downlink signals transmitted from the base stations is determined by observation at the terminals and provided to the system (e.g., UTRAN). The system then uses the timing information to adjust the timing at the base station so that user-specific radio frames transmitted from the base station can all arrive at the terminal within a particular time window. Several of these embodiments are described in detail below, and other embodiments can be implemented and are within the scope of the invention.

In the first time alignment scheme, the time difference between two base stations is divided into two parts, and only the required part is reported. For a W-CDMA system, measurements of SFN-SFN type 1 can be divided into frame-level timing and chip-level timing, as described above. Whenever a request is made to perform and report a measurement of a time difference for a list of one or more base stations, the terminal may measure and report the chip-level timing for each base station in the list. In addition, the terminal can also measure and report frame-level timing if needed, and include this information in the SFN-SFN type 1 measurements (e.g., specified by the system). Otherwise, if frame level timing is not required, the terminal may set the frame level portion to a predetermined value. The predetermined value may be a fixed value that is known (e.g., zero), any arbitrary value that is selected by the terminal and ignored by the system, a value that is obtained or known a priori by various means (e.g., previous measurements of the same base station, transmissions from the system, etc.), or some other value.

As shown in fig. 2A through 2D, the system may be operated according to one or more system configurations. The system may also be operated to operate some base stations synchronously while other base stations are operated asynchronously. For synchronous configurations such as those shown in fig. 2A to 2C, the system is generally aware of frame level timing and the terminal does not need to report when requesting execution and reporting of time difference measurements. For a synchronous base station with a fixed frame level timing known to the system, the terminal does not need to measure the frame level timing.

In one embodiment, information may be provided to specifically identify base stations that do not require frame-level timing. For simplicity, these base stations are all referred to as "synchronous base stations" regardless of whether they actually operate synchronously or not. All other base stations that require frame level timing are referred to as "asynchronous base stations" regardless of whether they are actually operating asynchronously. Using this information, the terminal does not make frame-level time difference measurements when not needed, and selectively ignoring these measurements may provide various benefits as described below.

In one embodiment, the system provides the identity of the synchronous base station to the terminal through a user-specific message. For a W-CDMA system, a "measurement control" message is sent to the terminal each time a measurement of the time difference is to be performed and reported. (a set of "default" measurements is defined in the system information, which may be sent on the common channel, and used as default values unless a measurement control message is received.) the measurement control message includes a list of base stations from which time difference measurements are requested. The list may include the currently active base station and/or neighboring base stations that are potential candidates for handoff. For each base station in the list, the measurement control message may be configured to include an indication of whether the base station requires frame-level timing. In one particular implementation, this indicator is the "read SFN indicator" defined in the W-CDMA standard, which may be set to "true" if frame-level timing is required, and to "false" otherwise. By reading out the SFN indicator for each base station in the reproduction list, the terminal can determine whether the base station needs frame level timing.

In another embodiment, the system provides the identity of the synchronized base station to the terminal via an Information Element (IE) defined in the W-CDMA standard (document No. 3GPP TS 25.331, section 10.3.7.106, entitled "UE Positioning OTDOA neighbor Cell Info"). The information element provides approximate cell timing as well as cell location and precise cell timing. In particular, the information element provides an SFN-SFN for the neighboring cell with a resolution of 1/16 chips and a range of [0..38, 399] chips, and also provides SFN-SFN drift. In general, the terminal may use the information element to reduce the search space, in particular, to estimate which base stations are synchronized. According to the current W-CDMA standard, when operating in dedicated mode, the information element is sent to terminals that are allowed-OTDOA by means of measurement control messages or to all terminals in the cell by means of system information messages and is used to assist the terminals in positioning. In one embodiment, this information may be provided and used to narrow the signals of neighboring cells to facilitate location measurements as well as measurements used for hard and soft handoffs.

In yet another embodiment, the system provides the identity of the synchronization base station through a broadcast message (e.g., a broadcast channel) sent on a common channel. The broadcast message may include a list of synchronized base stations that do not need to report frame level timing. Alternatively, the broadcast message may include a list of asynchronous base stations that need to report frame level timing. In another embodiment, the identity of the synchronous and/or asynchronous base station is transmitted to the terminal over a dedicated channel or some other channel.

In yet another embodiment, the identity of the synchronous and/or asynchronous base station is provided to the terminal a priori before a measurement of the time difference is requested. This information may be provided during call setup, for example, or may be stored in the terminal through some previous communication or transaction.

Upon receiving the identities of the synchronous and/or asynchronous base stations, the terminal knows that the SFN values may not need to be reproduced for some or all of the base stations. For each base station that does not require frame-level timing, the terminal may measure only the chip-level time difference between that base station and the reference base station relative to a common frame boundary and report only the chip-level timing. The frame-level timing can be set to a predetermined value, and if the predetermined value is zero, the value of the reported SFN-SFN type 1 measurement will fall within the reduced range of [0..38, 399] chips, or within a frame.

Fig. 3A is a flow diagram of one process of measuring and reporting time difference measurements according to one embodiment of the invention. This process implements the first time alignment scheme described above. Initially, at step 312, the terminal receives a request to provide a list of time difference measurements applicable to the base station. The system may send this request for a particular function, such as for soft and hard handoff, positioning, etc. This request may also be generated internally by the terminal, e.g. the occurrence of a specific event, the fulfilment of a specific condition, etc. determined periodically according to a timer.

In one embodiment, the request may specifically identify the base station that requires the time difference measurement. In further embodiments, the terminal determines the time difference of the base stations in the identified list that the terminal receives. For this embodiment, a terminal may be deemed to have received a base station if one or more requirements are met, such as received signal quality being greater than or equal to a particular threshold value. The received base station is then included in a list of base stations that are to report the time difference measurement.

The terminal also receives the identity of the base station that does not require frame level timing at step 314. These base stations may simply be denoted as synchronous base stations and all other base stations may then be denoted as asynchronous base stations. The list of base stations requiring time difference measurements may include any number (zero or more) of synchronous base stations and any number (zero or more) of asynchronous base stations. Information identifying synchronous and/or asynchronous base stations may be provided to a terminal by various means including: (1) the request of the time difference measurement value is specially sent to the terminal; (2) broadcasting to the terminal through signaling on a broadcast channel; (3) providing to the terminal during call setup; (4) stored in the terminal by the previous operation; or (5) make the terminal available by some other means.

Upon request, the terminal estimates a chip-level time difference for each base station in the list at step 316. The chip-level time difference for each base station may be determined relative to the timing of a reference base station, which is a particular base station in the active set of the terminal and is known to both the system and the terminal.

A determination is then made at step 318, for each base station in the list, whether frame-level timing is required. This can be done by checking whether the base stations are synchronous or asynchronous. If frame level timing is required, the terminal estimates a frame level time difference of the base station at step 320. This may be done by demodulating and decoding a common channel (e.g., broadcast channel) from the base station to retrieve the system frame number, as described above. A time difference measurement is then formed for each asynchronous base station based on the estimated chip-level and frame-level time differences at step 322. And at step 324, for each synchronized base station for which frame-level timing is not required, a time difference measurement for that base station is formed from the estimated chip-level time difference and a predetermined value (e.g., zero).

The time difference measurements for all base stations in the list (i.e., both synchronous and asynchronous) are then reported to the system at step 326. In one embodiment, the time difference measurements for all base stations are encapsulated in a measurement report message of the SFN-SFN1 type which is then sent to the system. The system receives the report message and adjusts the timing of data transmission from each selected base station to the terminal based on the time difference estimated for the selected base station. The process then terminates.

The first time alignment scheme can be used for all configurations for which the system knows its frame level timing and no measurement and reporting is required. This approach is particularly well suited for synchronous system configurations such as those shown in fig. 2A through 2C. In the system configurations S2 and S3, the true value of the frame-level time difference may be a non-zero value. However, the terminal reports a predetermined value (e.g., zero) for the frame-level portion of the SFN-SFN type 1 measurement. Since the actual frame difference value (if it is a non-zero value) is a constant value known to the system, the potentially erroneously reported value of the frame level portion will not affect the system's ability to add new candidate base stations to the terminal's active set.

The first time alignment scheme provides a number of advantages. First, when the information is not needed, the terminal does not need to demodulate and decode the broadcast channel to reproduce the system frame number of the candidate base station. This improves the above-mentioned disadvantages (i.e., smaller handoff area and additional demodulation delay). Second, by setting the frame-level portion to a predetermined value, the length of the SFN-SFN 1-type measurement message is not affected regardless of whether a "valid" frame-level message is included in the message.

The first time alignment scheme uses measurement report messages of the SFN-SFN1 type as defined by the W-CDMA standard and allows the terminal to report only the chip-level time difference if the frame-level timing is known to the system. The system has the ability to broadcast such messages as currently defined by the W-CDMA standard. With this information, the terminal does not need to process and recover the broadcast channel to reproduce the SFN because the terminal can set the frame-level part of the SFN-SFN1 type measurement to a predetermined value. However, the terminal still sends the SFN-SFN type 1 measurement message as a number encoded with 24 bits (since the maximum range of SFN-SFN type 1 measurement is [0..9, 830, 399] chips), however, this will have a reduced range of [0..38, 399] chips (8 most significant bits set to zero).

In a second time alignment scheme, the terminal determines the time difference between two base stations based on the partial decoding of some base stations received by the terminal. For this scheme, the terminals initially process the downlink signals transmitted from the base station to detect their presence. The terminal further decodes the primary common control channel (P-CCPCH) for a number of base stations that may be selected based on a particular criteria. For example, some base stations with received signal strengths above a certain threshold (i.e., strength sufficient for decoding) may be selected for decoding, starting with the strongest base station received. In one embodiment, if two or more decoded base stations have the same SFN value at a particular time, a synchronous system configuration with time-aligned frame starts can be deduced (i.e., configuration S1 shown in fig. 2A). The terminal may then assume that the other base stations (which received the weaker and undecoded) also have the same SFN value, and may report predetermined values for the frame-level portion of the SFN-SFN type 1 measurements for these "assumed" base stations.

Fig. 3B is a flow diagram of a process of measuring and reporting time difference measurements according to another embodiment of the invention. This process implements a second time alignment scheme. Initially, at step 322, the terminal receives a request to provide time difference measurements for a number of candidate base stations. The terminal then receives and processes downlink signals from the candidate and reference base stations at step 344.

At step 336, the terminal estimates a chip-level time difference for each candidate base station, e.g., in the manner described above. At step 338, the terminal also estimates a frame-level time difference for two or more candidate base stations. A determination is then made at step 340 whether the estimated frame-level timing of the two or more candidate base stations is the same. At step 344, if the frame-level timing is the same, the terminal assumes a synchronous system with time-aligned frame starts and thus sets the frame-level time difference for each remaining candidate base station to a predetermined value. Otherwise, if the frame level timing at step 340 is not the same, then at step 342, the frame level time difference for each of the remaining candidate base stations is estimated.

Then, at step 346, a time difference measurement is formed for each candidate base station based on the estimated chip-level time difference or the estimated frame-level time difference or a predetermined value. The time difference measurements for the candidate base stations are then reported to the system at step 348. The system receives the reported time difference measurements and adjusts the timing of the data transmission from each selected base station to the terminal based on the time difference estimated by the selected base station. Where the process terminates.

The second time alignment scheme may provide a sufficiently accurate time difference measurement for the system configuration S1 shown in fig. 2A (which is more similar to the system configuration used by the network operator than the other system configurations shown in fig. 2B through 2D).

Since there is no guarantee that all base stations are synchronized as long as some base stations are synchronized, nor can there be a guarantee that the time at which the same SFN value is consistently obtained at a particular time, a mechanism can be provided to bypass this scheme and implement some other schemes to provide the required frame-level timing. For example, if the reported measurement does not match the reported distribution (profile) of some base stations, a message may be sent to the terminal. On the other hand, the terminal may also decide later that it is not possible to decode the radio frame from the assumed base station because the timing has been adjusted to the wrong value. In any case, upon receiving an indication that the previously reported time difference measurement is erroneous due to an incorrect assumption, the terminal can perform a full SFN-SFN type 1 measurement and decode the P-CCPCH of the assumed base station to obtain a true frame-level timing.

In a third time alignment scheme, the base station determines the timing of the terminal based on uplink transmissions from the terminal. The reproduced timing information may then be used to adjust the timing of the downlink transmission of the terminal.

In one embodiment, the system may direct some base stations (i.e., neighboring base stations) that are not in the active set of the terminal but are near the terminal to measure uplink transmissions from the terminal (e.g., transmissions on the uplink Dedicated Physical Channel (DPCH)). If neighboring base stations are able to receive uplink transmissions with sufficient strength, they can accurately estimate the time of arrival of the uplink transmission. Based on a priori knowledge of the estimated signal arrival times from the neighboring base stations and the time relationship between the respective active base stations and the common channel frames between the neighboring base stations, the system can determine the correct timing of each neighboring base station, which can be added to the active set of the terminal so that the downlink transmissions from the added base station are correctly time aligned at the terminal.

The third time alignment scheme may be implemented based only on measurements performed at neighboring base stations. Each neighboring base station may be designed to include a receiver processing unit that searches for and processes downlink transmissions from terminals located in neighboring cells. This scheme can be used for both synchronous and asynchronous system configurations.

Fig. 3C is a flowchart of a process of determining a timing of a terminal according to uplink transmission according to still another embodiment of the present invention. This process implements a third time alignment scheme. Initially, at step 372, the candidate base stations receive an uplink transmission from the terminal, and at step 374, each candidate base station estimates the signal arrival time of the received uplink transmission. The system then retrieves the time difference for the active base station (i.e., the base stations in the active set of the terminal) at step 376. In this system, the difference between the common channel frames of S1, S2, and S3 may be known for all synchronous system configurations.

Then, at step 378, the system estimates the time difference for each candidate base station based on the estimated signal arrival time for the candidate base station and the time difference for the active base station. Thereafter, at step 380, one or more candidate base stations may be selected for data transmission to the terminal. In this case, at step 382, the timing of the data transmission from each selected base station to the terminal may be adjusted according to the estimated time difference for the selected base station. By which the process terminates.

The above-described techniques provide various advantages. First, when a terminal is handed off from a first base station to a second base station, both the SFN-SFN time difference measurement and the round-trip delay measurement between these base stations (which is performed by the first base station) allow the second base station to determine where to search for the terminal's uplink transmission. This mechanism is described in a paper entitled "Self-synchronization of CDMA Cellular networks" by Chuck Wheatley (5.1999, Microwave Journal, pages 320-328), which is incorporated herein by reference. Second, the second base station can use the SFN-SFN time difference measurement to time align its downlink transmissions such that the terminal receives its time close to the time of receiving the downlink transmission from the first base station. Other advantages may also be realized by using the techniques described herein.

Fig. 4 is a simplified block diagram of an embodiment of a base station 104 and a terminal 106 capable of implementing various aspects and embodiments of the present invention. For simplicity, only one base station and one terminal are shown in fig. 1. However, when in soft handoff, the terminal 106 may be simultaneously communicating with multiple base stations 104 and may further receive messages from various other neighboring base stations.

On the downlink, at base station 104, user-specific data is signaled to identify synchronous and asynchronous base stations, and a request for a time difference measurement is provided to a Transmit (TX) data processor 412, which formats and codes the data and message according to one or more coding schemes to provide coded data. Each coding scheme may include any combination of Cyclic Redundancy Check (CRC), convolutional, Turbo, block, and other coding or no coding at all. In general, different schemes may be used to encode data and messages, and different message types may also be encoded differently.

The encoded data is then provided to a Modulator (MOD)414 and further processed to generate modulated data. The processing by modulator 414 may include (1) covering the coded data with orthogonal codes (e.g., Orthogonal Variable Spreading Factor (OVSF) codes) to channelize the user-specific data and messages into their respective dedicated and control channels; and (2) scrambling the covered data with the PN sequence assigned to the terminal. The modulated data is then provided to a transmitter unit (TMTR)416 and conditioned (e.g., converted to one or more analog signals, amplified, filtered, and quadrature modulated) to generate a downlink modulated signal suitable for transmission over a wireless link. The downlink modulated signal is then passed through a duplexer (D)418 and transmitted via an antenna 420 to the terminals.

At terminal 106, an antenna 450 receives the downlink modulated signal, passes through a duplexer 452, and provides to a receiver unit (RCVR) 454. Receiver unit 454 conditions (e.g., filters, amplifies, frequency downconverts, and digitizes) the received signal and provides samples. A demodulator (DEMOD)456 then receives and processes the samples to provide recovered symbols. The processing by demodulator 456 includes despreading the samples with a PN sequence that is aligned with the time of arrival of the multipath signal being processed, decovering the despread samples, channelizing the received data and message into their respective dedicated and control channels, and coherently demodulating the decovered data with the reproduced pilot. Demodulator 456 may implement a comb (rake) receiver that processes multiple instances of the received signal and combines symbols from various multipaths belonging to the same base station to provide recovered symbols.

A Receive (RX) data processor 458 then decodes the recovered symbols to recover the user-specific data and messages transmitted on the downlink. The reproduced message may be provided to the controller 470. The processing by demodulator 456 and RX data processor 458 is complementary to that performed by modulator 414 and TX data processor 412, respectively, at base station 104.

The demodulator 456 is further operable, as directed by the controller 470, to determine the signal arrival times of the received base stations (e.g., based on the timing of the terminal-generated PN sequences), and to derive chip-level time differences between the two base stations based on the signal arrival times. Alternatively, demodulator 456 can determine the signal arrival time and provide it to controller 470, and controller 470 can then determine the chip-level time difference. The RX data processor 458 is further operable, under the direction of a controller 470, to reproduce and provide the sfn of the common channel frame for one or more received base stations (e.g., candidate base station and reference base station). The controller 470 may then determine the frame-level time difference, if and as needed.

The controller 470 may receive information about which base station needs and which does not need frame-level timing and may further receive a request for a time difference measurement. Controller 470 then directs demodulator 456 to provide the received chip-level timing information for the base stations, and further directs RX data processor 458 to provide frame-level timing information for certain base stations requiring such information. The controller 470 then forms an SFN-SFN type 1 measurement report message for the received base station.

On the uplink, at terminal 106, the SFN-SFN type 1 measurement report message is provided to TX data processor 464, which then processes the report message according to a defined processing scheme. The processed message is then further processed by a Modulator (MOD)466 and conditioned by a transmitter unit (TMTR)468 to generate an uplink modulated signal, which is then passed through a duplexer (D)452 and transmitted via an antenna 450 to base stations.

At base station 104, the uplink modulated signal is received by an antenna 420, passed through a duplexer 418, and provided to a receiver unit (RCVR) 422. Receiver unit 422 conditions the received signal and provides samples. A demodulator (DEMOD)424 then processes (e.g., despreads, decovers, and demodulates) the samples, and decodes (if necessary) by a RX data processor 426 to reproduce the transmitted report message. The reproduced report message is then provided to a controller 430, which may transmit the report message to a Base Station Controller (BSC) or some other system entity. The signal strength and time difference messages included in the report message may be used to select one or more base stations included in the active set of the terminal and correctly time aligned with the downlink transmissions from the selected base stations.

The units of terminal 106 and base station 104 may be designed to implement various aspects of the present invention, as described above. The elements of the terminal or base station may be implemented with a Digital Signal Processor (DSP), an Application Specific Integrated Circuit (ASIC), a processor, a microprocessor, a controller, a microcontroller, a Field Programmable Gate Array (FPGA), a programmable logic device, other electronic elements, or any combination thereof. Some of the functions and processes described herein may also be implemented in software executing on a processor. For example, estimation of chip-level and frame-level time differences and encapsulation of time difference measurements into SFN-SFN type 1 measurement report messages may be performed by controller 470.

For clarity, various aspects, embodiments, and aspects have been described with particular reference to SFN-SFN type 1 measurements for the W-CDMA standard. Other mechanisms may also be employed to report frame-level timing and chip-level timing. For example, W-CDMA supports reporting with the parameter Tm in chip level timing and reporting with the parameters OFF and COUNT-C-SFN in frame level timing. These parameters are further described in document No. 3GPP TS 25.402, section 5, which is incorporated herein by reference.

The techniques disclosed herein may also be applied to other communication systems in which the time difference may be split into two or more portions having different resolutions and/or associated with different measurement types. In the above, the time difference is divided into a chip-level part and a frame-level part. For some other systems, the time difference may be partitioned into a fine resolution portion and a coarse resolution portion. The time difference may also be used in some other way for some other systems. For each of these cases, the measurement may be performed only for the required portion or portions, and a predetermined value or a default value may be used for the unnecessary portion.

Headings are included herein for reference only and to aid in locating certain sections. These headings are not intended to limit the scope of the concepts described therein under, which concepts may be applied to all other portions of the entire specification.

The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without the use of the inventive faculty. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims (39)

1. A method for determining time differences between a plurality of base stations in a wireless communication system, each of the plurality of base stations operating in a synchronous or asynchronous manner, the method comprising:

receiving a request for a time difference measurement between a plurality of base stations, the time difference measurement comprising a first portion giving a chip-level time difference and a second portion giving a frame-level time difference;

estimating a chip-level time difference for each of a plurality of base stations based on a downlink signal received from the base station;

determining whether frame level timing is required for each of the plurality of base stations;

if the frame-level timing is not required, determining a time difference measurement value of a base station working synchronously according to the estimated chip-level time difference and a preset value comprising the frame-level time difference;

estimating a frame-level time difference of the asynchronously operating base stations; and

and if the frame-level time sequence is required, determining a time difference measurement value of the asynchronously operating base station according to the estimated chip-level time difference and the estimated frame-level time difference.

2. The method of claim 1, further comprising:

receiving an indication identifying each of the plurality of base stations as operating synchronously.

3. The method of claim 2, wherein the receiving an indication identifying each of the plurality of base stations as operating synchronously is included in the request.

4. The method of claim 2, wherein the receiving the indication identifying each of the plurality of base stations as operating synchronously is based on a read System Frame Number (SFN) indicator defined by a W-CDMA standard.

5. The method of claim 2, wherein the receiving the indication identifying each of the plurality of base stations as operating synchronously is received over a common channel.

6. The method of claim 2, wherein the receiving the indication identifying each of the plurality of base stations as operating synchronously is received over a dedicated channel.

7. The method of claim 1, wherein the estimated chip-level time difference is based on a time shift of a pseudo-noise sequence used to descramble the downlink signal.

8. The method of claim 1, wherein the estimated frame-level time difference is based on recovered radio frames transmitted on a common channel from multiple base stations.

9. The method of claim 1, further comprising:

time difference measurements for the plurality of base stations are encapsulated within a SFN-SFN1 type measurement report D message defined by the W-CDMA standard.

10. The method of claim 1, further comprising:

reporting the determined time difference measurements of synchronously or asynchronously operating base stations.

11. The method of claim 10 wherein said reporting said determined time difference measurement comprises a chip-level time difference reported by a parameter Tm defined by the W-CDMA standard.

12. The method of claim 10 wherein said reporting said determined time difference measurement comprises said frame-level time difference reported by parameters OFF and COUNT-C-SFN defined by the W-CDMA standard.

13. The method of claim 1, further comprising:

aligning the timing of data transmissions from each of the plurality of base stations according to the determined time difference measurements.

14. An apparatus for determining a time difference between a plurality of base stations in a wireless communication system, each of the plurality of base stations operating in a synchronous or asynchronous manner, the apparatus comprising:

means for receiving a request for time difference measurements between the plurality of base stations, the time difference measurements comprising a first portion giving a chip-level time difference and a second portion giving a frame-level time difference;

means for estimating a chip-level time difference for each of the plurality of base stations based on received downlink signals transmitted from the base stations;

means for determining whether each of said plurality of base stations requires a frame level timing sequence;

means for determining a time difference measurement for a base station operating in a synchronous manner based on said estimated chip-level time difference and a predetermined value comprising a frame-level time difference if said frame-level timing is not required;

means for estimating a frame-level time difference for a base station operating in an asynchronous manner; and

means for determining a time difference measurement for a base station operating in an asynchronous manner based on the estimated chip-level time difference and the estimated frame-level time difference if the frame-level timing is required.

15. The apparatus of claim 14, further comprising:

means for receiving an indication identifying each of the plurality of base stations as a synchronized mode of operation.

16. The apparatus as recited in claim 15 wherein said indication identifying each of said plurality of base stations as synchronous operation is included in said request.

17. The apparatus of claim 15, wherein the indication identifying each of the plurality of base stations as a synchronous mode of operation is based on reading an SFN indicator as defined by a W-CDMA standard.

18. The apparatus of claim 15, wherein the indication identifying each of the plurality of base stations as a synchronous mode of operation is received over a common channel.

19. The apparatus of claim 15, wherein the indication identifying each of the plurality of base stations as synchronous operation is received over a dedicated channel.

20. The apparatus of claim 14, wherein the means for estimating the chip-level time difference estimates the chip-level time difference based on a time shift of a pseudo-noise sequence used to descramble a downlink signal.

21. The apparatus of claim 14, wherein the means for estimating the frame-level time difference estimates the frame-level time difference based on recovered radio frames sent on a common channel from the plurality of base stations.

22. The apparatus of claim 14, further comprising:

means for encapsulating the time difference measurements for the plurality of base stations in a SFN-SFN 1-type measurement report message defined by the W-CDMA standard.

23. The apparatus of claim 14, further comprising:

means for reporting the determined time difference measurements of base stations operating in a synchronous or asynchronous manner.

24. The apparatus of claim 14 wherein said means for reporting determined time difference measurements reports chip-level time differences reported by a parameter Tm defined by the W-CDMA standard.

25. The apparatus of claim 14 wherein the means for reporting the determined time difference measurement reports the frame-level time difference reported by parameters OFF and COUNT-C-SFN defined by the W-CDMA standard.

26. The apparatus of claim 14, further comprising:

means for aligning the timing of data transmissions from each of the plurality of base stations according to the determined time difference measurements.

27. A communication system, comprising:

a plurality of base stations, each of which operates in a synchronous manner or an asynchronous manner; and

at least one terminal, comprising:

a receiver to receive a request for time difference measurements between the plurality of base stations, the time difference measurements including a first portion giving a chip-level time difference and a second portion giving a frame-level time difference; and

a processor in communication with a receiver, the processor comprising means for estimating a chip-level time difference for each of the plurality of base stations from received downlink signals transmitted from the base stations, means for determining whether frame-level timing is required for each of the plurality of base stations, means for determining a time difference measurement for a base station operating in a synchronous manner from the estimated chip-level time difference and a predetermined value comprising the frame-level time difference if frame-level timing is not required, means for estimating a frame-level time difference for the base station operating in an asynchronous manner, and means for determining a time difference measurement for the base station operating in an asynchronous manner from the estimated chip-level time difference and the estimated frame-level time difference if frame-level timing is required.

28. The system of claim 27, wherein the terminal receives an indication that identifies each of the plurality of base stations to operate in a synchronized manner.

29. The system of claim 28, wherein the terminal receives an indication included in the request identifying that each of the plurality of base stations is operating in a synchronized manner.

30. The system of claim 28, wherein the indication identifying that each of the plurality of base stations is operating in a synchronous manner is based on reading an SFN indicator defined by a W-CDMA standard.

31. The system of claim 28, wherein the indication identifying that each of the plurality of base stations is operating in a synchronized manner is received over a common channel.

32. The system of claim 28, wherein the indication identifying that each of the plurality of base stations is operating in a synchronized manner is received via a dedicated channel.

33. The system of claim 27, wherein the terminal estimates the chip-level time difference based on a time shift of a pseudo-noise sequence used to descramble the downlink signal.

34. The system of claim 27, wherein the terminal estimates the frame-level time difference based on recovered radio frames on a common channel sent from the plurality of base stations.

35. The system of claim 27 wherein the terminal encapsulates the time difference measurements for the plurality of base stations within a SFN-SFN 1-type measurement report message defined by the W-CDMA standard.

36. The system of claim 27, wherein the terminal reports determined time difference measurements for base stations operating in a synchronous or asynchronous manner.

37. The system of claim 27 wherein said terminal reporting said determined time difference measurement comprises a chip-level time difference reported by a parameter Tm defined by the W-CDMA standard.

38. The system of claim 27, wherein the terminal reporting the determined time difference measurement comprises a frame-level time difference reported over OFF and COUNT-C-SFN defined by a W-CDMA standard.

39. The system of claim 27, wherein the terminal aligns timing of data transmissions from each of the plurality of base stations based on the determined time difference measurements.