HK1033784A - Mobile station assisted timing synchronization in a cdma communication system - Google Patents

Description

Mobile station assisted timing synchronization in a CDMA communication system

Technical Field

The present invention relates to communication systems. More particularly, the present invention relates to a novel and improved method and apparatus for synchronizing base stations using signals transmitted from mobile stations that are simultaneously communicating with a synchronized base station.

Description of the Related Art

The use of Code Division Multiple Access (CDMA) modulation techniques is but one of several methods for facilitating communications in which a large number of system users are present. While other techniques are known, such as Time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA), and AM modulation techniques such as amplitude voltage controlled single sideband (avssb), CDMA has significant advantages over these other modulation techniques. The use of CDMA techniques in multiple access communication systems is described in U.S. patent No. 4,901,307 (entitled "extended disc multiple access communication system using satellite or terrestrial repeaters") and U.S. patent No. 5,103,459 (entitled "system and method for generating signal waveforms in a CDMA cellular telephone system"), both of which are assigned to the assignee of the present invention and are incorporated herein by reference. The method of providing CDMA mobile communications IS standardized by the telecommunications industry association in TIA/EIA/IS-95-a (entitled "mobile station-base station compatibility standard for dual mode wideband spread spectrum cellular systems", hereinafter IS-95) in the united states.

In the above-mentioned patents, multiple access techniques are disclosed in which there are a large number of mobile station users, each having a transceiver, communicating via satellite relays or terrestrial base stations (also known as cell sites or cell sites) using Code Division Multiple Access (CDMA) spread spectrum communication signals. By employing CDMA communications, the frequency spectrum may be reused multiple times, thereby allowing system user capacity to increase. Much higher spectral efficiency can be achieved using CDMA techniques than can be achieved using other multiple access techniques.

Methods for synchronously demodulating signals propagating along different propagation paths from one base station and for synchronously demodulating redundant data provided from more than one base station are disclosed in U.S. patent No. 5,109,390 (the' 390 patent), entitled "diversity receiver in a CDMA cellular communication system," assigned to the assignee of the present invention and incorporated herein by reference. In the' 390 patent, the separately demodulated signals are combined to provide an estimate of the transmitted data that has a higher reliability than the data demodulated by either path or from either base station.

Handover is generally divided into two types-hard handover and soft handover. In hard handoff, when a mobile station disconnects an originating base station and enters a destination base station, the mobile station disconnects its communication link with the originating base station and then establishes a new communication link with the destination base station. In soft handoff, the mobile station completes the communication link with the destination base station before breaking the communication link with the originating base station. Thus, in soft handoff, the mobile station is redundantly communicated with the originating base station and the destination base station for some period of time.

Soft handoff is less likely to drop a call than hard handoff. Further, as the mobile station moves near the coverage boundary of the base station, it may make repeated handover requests in response to small changes in the environment. The problem known as ping-pong (ping-pong) is also greatly alleviated by soft handover. In U.S. patent No. 5,101,501 (entitled "method and system for providing soft handoff in communications in a CDMA cellular telephone system," assigned to the assignee of the present invention and incorporated herein by reference), a process for performing soft handoff is described in detail.

An improved soft handoff technique is disclosed in U.S. patent No. 5,267,261 (entitled mobile assisted soft handoff in a CDMA cellular communication system), assigned to the assignee of the present invention and incorporated herein by reference. In the system of the' 261 patent, soft handoff processing is improved by measuring, at the mobile station, the strength of the "pilot" signal transmitted by each base station. These pilot strength measurements assist in the soft handoff process by facilitating the identification of viable base station handoff candidates.

The base station candidates may be divided into four groups. The first group, referred to as the active group, includes the base stations that are currently communicating with the mobile station. The second group, called candidates, includes base stations whose signals are determined to be of sufficient strength to be useful to the mobile station but are not currently in use. When their measured pilot energy exceeds a predetermined threshold T_ADDThe base station is added to the candidate. The third group is a group of base stations located in the vicinity of the mobile station (and not included in the active group or candidates). Further, the fourth group is the remaining group of all other base stations.

In IS-95, a base station candidate IS characterized by the phase offset of the pseudo-noise (PN) sequence of its pilot channel. When the mobile station searches to determine the strength of the pilot signal from the candidate base station, it performs a correlation operation in which the filtered received signal is associated with a set of PN offset hypotheses. Methods and apparatus for performing correlation operations are described in detail in co-pending U.S. patent application No. 08/687, 694 (filed 7/26 1996 entitled "method and apparatus for performing search acquisition in a CDMA communication system," assigned to the assignee of the present invention and incorporated herein by reference).

The propagation delay between the base station and the mobile station is unknown. This unknown delay produces an unknown shift in the PN code. The search process attempts to determine this unknown shift in the PN code. To do so, the mobile station shifts the output of its searcher PN code generator in time. The range of search translation is referred to as the search window. The search window is centered on the PN shift hypothesis. The base station transmits a message to the mobile station indicating the PN offset of the base station pilot by its actual proximity (in-transmit proximity). The mobile station centers its search window on the PN offset hypothesis.

The appropriate size of the search window depends on several factors, including the priority of the pilot, the speed of the search processor, and the expected delay spread of the multipath arrival. The CDMA standard (IS-95) defines three search window parameters. The search for pilots in the current and candidate sets is governed by a search window "a". The neighbor set pilots are searched for within window "N" and the remaining sets are searched for within window "R". The searcher window size is provided in Table 1 below, where the chips are。

Windowing is a compromise between search speed and the possibility of losing strong paths outside the search window.

The base station transmits to the mobile station a PN hypothesis specifying that the mobile station should search against its own PN offset. For example, the originating base station may instruct the mobile station to search for a pilot 128 PN chips prior to its own PN offset. The mobile station responds by setting its searcher demodulator 128 chips prior to the output chip cycle and searching for the pilot using a search window centered at the specified offset. Once the mobile station is commanded to search for PN hypotheses to determine the resources available to perform a handoff, the PN offset of the destination base station pilot should be very close in time to the directional offset. The speed of the search near the base station boundary is important because delays in completing the required process can result in lost calls.

In CDMA systems in the united states, base station synchronization is achieved by providing each base station with a Global Positioning Satellite (GPS) receiver. However, there are cases where the base station cannot receive the GPS signal. For example, in subways and tunnels, GPS signals are attenuated to the point of prohibiting them from being used to time synchronize base stations or micro base stations. The present invention provides a method and system for providing timing synchronization in situations where a small portion of the network is capable of receiving a centralized timing signal and obtaining timing therefrom, and a portion of the base stations are not capable of receiving the centralized timing signal.

Summary of The Invention

The present invention is a novel and improved method and apparatus for synchronizing base stations that are unable to receive a centralized timing signal in a network in which some of the base stations are able to receive the centralized timing signal. The reference base station performs timing synchronization by receiving a centralized timing signal or other means. In an example embodiment, the reference base stations employ Global Positioning Satellite (GPS) receivers for synchronization. The slave base stations do not have the ability to synchronize because, for example, they cannot receive a centralized timing signal.

In the present invention, the slave base station acquires synchronization with the reference base station through a message transmitted or received from the mobile station between the reference base station and the slave base station. First, the round trip delay between the mobile station and the reference base station is measured by the reference base station. The slave base station then searches until it acquires a signal, called a reverse link signal, transmitted by the mobile station. In response to acquiring the reverse link signal, the slave base station adjusts its timing so that the mobile station can acquire its signal, referred to as the forward link signal. This step is not necessary if the timing error in the slave base station is not very severe.

Once the mobile station acquires the signal from the slave base station, the mobile station measures and reports the difference between the time required for the signal to travel from the reference base station to the mobile station and the time required for the signal to travel from the slave base station to the mobile station. The final measurement required is a measurement by the slave base station of the time difference between the time at which the slave base station receives the reverse link signal from the mobile station and the time required for the slave base station to transmit the signal to the mobile station.

A series of calculations, described in detail herein, are performed on the measured time values to determine the time difference between the slave base station and the reference base station. Adjustments of the slave base station timing are performed based on these calculations. It should be noted that in the preferred embodiment, all of the above measurements are performed during normal operation of the IS-95 CDMA communication system.

Brief Description of Drawings

The features, objects, and advantages of the present invention will become apparent from the following detailed description when taken in conjunction with the accompanying drawings in which like reference characters designate the same parts throughout the figures thereof:

fig. 1 is a block diagram showing a network structure of a wireless communication system including a reference base station and a slave base station;

FIG. 2 is a diagram illustrating various transmissions between a mobile station, a synchronous base station, and an asynchronous base station and corresponding time intervals;

FIG. 3 is a flow chart illustrating a method for synchronizing base stations that are unable to receive a centralized timing signal;

FIG. 4 is a block diagram of a mobile station of the present invention;

fig. 5 is a block diagram of a searcher in a mobile station of the present invention;

fig. 6 is a block diagram of a traffic channel modulator of the mobile station of the present invention;

FIG. 7 is a block diagram of a base station of the present invention;

fig. 8 is a block diagram of a transmission system of a base station of the present invention; and

fig. 9 is a block diagram of a receiver system of a base station of the present invention.

Detailed description of the preferred embodiments

I. Overview of timing error calculation

Referring to fig. 1, a mobile station 60 communicates with a reference base station 62 while it is substantially within the coverage area depicted by a base station coverage boundary 61. The reference base station 62 is synchronized with the rest of the network using a central timing system, such as the Global Positioning System (GPS). In contrast, the slave base station 44 is not synchronized to the central time system by an independent means, such as GPS available to the base station 62. Base station controller 66 routes calls from the Public Switched Telephone Network (PSTN) to base stations 62 or 64 using T1 or other means. In addition, frequency synchronization is provided to the slave base station 64 through the T1 line.

For short periods of time, the method known in the art may be applied, providing acceptable accuracy to the frequency synchronization via the T1 line. However, these problems are common in methods that rely on the T1 line to provide frequency information. These problems lead to timing errors that can be corrected using the present invention. The intermittent correction of phase according to the invention allows the use of less accurate frequency sources when required, due to the relationship between phase and frequency.

Referring to fig. 2, the use of transmissions and corresponding time intervals for synchronizing the synchronization timing of the transmitting base station 64 with the reference base station 62 is shown. Signal path 500 illustrates the transmission of a forward link signal from the reference base station 62 to the mobile station 60. The time interval during which such transmission occurs is designated τ₁. At the mobile station 60, the start of a frame transmission on the reverse link is time aligned with the start of a frame arrival on the forward link. This time alignment IS standardized in IS-95 and incorporated into hardware designed to fit it. It will therefore be appreciated that methods and apparatus for performing such alignment are known in the art.

Transmission 502 depicts the transmission of a reverse link frame from the mobile station 60 to the reference base station 62. The time (τ) required for signal 500 to travel from base station 62 to mobile station 60₁) Equal to the time (also τ) that signal 502 takes from base station 62 to mobile station 60₁). Since base station 62 knows the time at which it sent signal 500 and knows the time at which it received signal 502, base station 62 can calculate round tripsTravel delay time (RTD)₁) It is the calculation of the time error (tau)₀’-τ₀) A desired first value.

Signal path 504 is a reverse link signal sent from mobile station 60 to base transmission station 64 along a different propagation path. The time required for the signal 504 to travel from the mobile station 60 to the base station 64 is designated τ₂. The time required for the reverse link signal 504 to reach the base station 64 is designated as T₂. The time required for forward link signal 506 to travel from base station 64 to mobile station 60 is equal to τ₂. In addition, the slave base station 64 can detect the difference between the time it takes to receive the reverse link signal from the mobile station 60 and the time it takes to transmit its forward link signal to the mobile station 60. The time difference is designated as RTD₂. These times are known to be subject to a time error (τ)₀’-τ₀) And (4) calculating. The calculation of the time error τ is described below₀The method of' above.

First, it can be seen from fig. 2:

T₂=τ₁+τ₂and (1)

τ_l+△T=T₀’+T₂(2) By operating the terms of equations (1) and (2), the following formula is obtained:

T₂+△T=T₀’+2·τ₂ (3)

2·τ₂=T₂-T₀' +. DELTA T (4) to simplify notation, a new variable RTD is assigned₂Is defined as:

RTD₂=T₂-T₀' (5) As can be seen from FIG. 2:

(6)

(7) therefore, the temperature of the molten metal is controlled,

and (8)

By substitution, the time error (T) is visible₀’-T₀) Equal to:

(9)

(10)

(11)

(12)

once the base station 64 knows its timing error (T)₀’-T₀) It adjusts its timing so that it is synchronized with the timing of the base station 62. These measurements are subject to errors, and therefore, in a preferred embodiment, multiple measurements are taken to ensure accuracy of the timing correction.

Now, a method and apparatus for measuring each required time value in equation (12) will be described.

Measuring Round Trip Delay (RTD)₁)

Fig. 3 is a flow chart illustrating a method of synchronizing the timing of the transmitting base station 64 with the reference base station 62 of the present invention. In step 300, the synchronization method begins with the mobile station communicating with the reference base station 62 and within range to perform communication with the slave base station 64. In step 302, the measurement signal is transmitted from the reference base station 62 Round Trip Delay (RTD) to the rover station 60 and from the rover station 60 back to the reference base station 62₁). This is done by aligning the frame boundaries of frames received by the mobile station 60 with the frame boundaries of frames transmitted by the mobile station 60. Methods and apparatus for providing such alignment are known in the art. Thus, the Round Trip Delay (RTD) is measured₁) As the time difference between when the reference base station 62 starts transmitting frames and when the reference base station 62 starts receiving frames from the mobile station 60.

Referring to fig. 4, forward link frames of data from a reference base station 62 are received at antenna 2 and provided through duplexer 3 to a receiver (RCVR) 4. Receiver 4 down-converts, filters, and amplifies the received signal and provides it to searcher 50 and TRAFFIC demodulator (TRAFFIC DEMODS) 54. The searcher 50 searches for pilot channels based on a neighbor table provided by the reference base station 62. The neighbor table is provided as a signaling signal on the traffic channel from the reference base station 62. A signal is provided to the control processor 55 indicating the start of a received frame from the reference base station 62. Control processor 55 generates and provides a time alignment signal to traffic modulator 58 that aligns the start of a frame transmitted from mobile station 60 with the start of a frame received at mobile station 60.

The data frames from the user of the mobile station 60 are provided to a traffic modulator 58, where the traffic modulator 58 aligns the frames transmitted by the transmitter (TMTR)56 with the frames received by the mobile station 60 from the reference base station 62. The reverse link frames are upconverted, filtered and amplified by a transmitter 56 and then provided through a duplexer 3 to be transmitted through an antenna 2.

Acquisition of mobile station by subordinate base station

Fig. 6 shows a traffic channel modulator 58 of a mobile station 60. The data frame is provided to the frame formatter 200. In the exemplary embodiment, frame formatter 200 generates and adds a set of Cyclic Redundancy (CRC) detection bits and also generates a set of tail bits. In an exemplary embodiment, frame formatter 200 follows the frame format protocol standardized in IS-95 and described in detail in U.S. patent No. 5,600,754, entitled "method and System for arranging vocoder data for masking errors induced by the transmit channel," assigned to the assignee of the present invention and incorporated herein by reference.

The formatted data frames are provided to an encoder 202, which encodes the data for error correction and detection. In an example embodiment, encoder 202 is a convolutional encoder. The encoded data symbols are provided to an interleaver 204, which rearranges the symbols according to a predetermined interleaving format. In the exemplary embodiment, walsh mapper 206 receives eight code symbols and maps the groups of symbols to 64-chip walsh sequences. Walsh symbols are provided to spreading means 208 which spreads the walsh symbols according to a long spreading code. Long PN code generator 210 generates a pseudo-noise (PN) sequence that spreads the data and distinguishes the data from reverse link transmission data from other nearby subscribing stations.

In an example embodiment, data is transmitted according to a Quadrature Phase Shift Keying (QPSK) modulation format in which short PN sequences spread the I and Q channels. Spread data is provided to spreading means 214 and 216, which are based on respective PN generators (PN)_IAnd PN_Q)212, and 218 performs a second spreading operation on the data.

In step 304, the slave base station 64 acquires the reverse link signal transmitted by the mobile station 60. The base station controller 66 transmits data to the slave base station 64 indicating the PN code offset that the mobile station 62 uses to spread its reverse link cycle. In response to the signal from the base station controller 66, the slave base station 64 searches for the mobile station 66 centered on the PN offset specified by the signal from the base station controller 66.

In the exemplary search embodiment, the slave base station 64 loads (bank load) its enforcer long code PN generator 106 and its short code PN generators 108 and 110 (shown in fig. 9) in groups based on signals from the base station controller 66. The searcher process of the slave base station 64 is described in detail herein.

Fig. 7 shows the arrangement of the slave base station 64. In the slave base station 64, a signal representing the PN of the mobile station 60 is received from the base station controller 60. The message is provided by the control processor 100. In response, control processor 100 calculates a window search range centered at a particular PN offset. Control processor 100 provides the search parameters to searcher 101 and parametric base station 64 searches for signals transmitted by mobile station 60 in response to those parameters. The signal received by antenna 102 of slave base station 64 is provided to receiver 104, where receiver 104 downconverts, filters, and amplifies the received signal and provides it to searcher 101. In addition, the received signal is provided to a traffic demodulator 105, wherein the demodulator 105 demodulates reverse link traffic data and provides the data to the base station controller 60. The base station controller 66 in turn provides it to the PSTN.

Fig. 9 shows the searcher 101 in detail. Demodulation of reverse link signals is described in detail in co-pending U.S. patent application No. 08/372, 632 (filed on 13/1 1995 entitled "cell-site demodulator architecture for spread spectrum multiple access communication systems") and co-pending U.S. patent application No. 08/316, 177 (30/9/1994 entitled "multipath search processor for spread spectrum multiple access communication systems"), both of which are assigned to the assignee of the present invention and incorporated herein by reference. An estimate of the PN offset for the mobile station 60 is provided from the base station controller 66 to the control processor 100. In response to the PN offset estimates provided by the base station controller 60, the control processor 100 generates initial long PN sequence hypotheses as well as initial short PN sequence hypotheses for searching by the slave base stations 64. In an example embodiment, control processor 100 loads the shift registers of PN generators 106, 108, and 110 in groups.

The signal is received by the antenna 102, down-converted, filtered, and amplified and passed to the correlator 116. Correlator 116 correlates the received signal with the combined long and short PN sequence hypotheses. In the exemplary embodiment, by multiplying the short PN hypotheses generated by PN generators 108 and 110 by the long PN sequence provided by PN generator 106. One combined PN sequence hypothesis is used to despread the I channel and the other hypothesis is used to despread the Q signal of the received QPSK signal.

Processing to Fast Hadamard Transform (FHT)The two PN despread signals are provided by the two PN despreaders 118 and 120. The design and operation of a fast HADAMARD transform processor is described in detail in co-pending U.S. patent application No. 08/173, 460 (filed on 12/22/1993 under the title "method and apparatus for performing fast HADAMARD transforms", assigned to the assignee of the present invention and incorporated herein by reference). FHT processors 118 and 120 correlate the despread signals with all possible Walsh symbols to energy computing device (I)²+Q²)122 provide the resulting amplitude matrix. The energy calculation means 122 calculates the energy of each element of the amplitude matrix and provides an energy value to the maximum detector 124 which selects the maximum energy correlation. The maximum correlation energy is provided to accumulator 126, wherein accumulator 126 accumulates the energy for a plurality of walsh symbols and determines whether mobile station 60 can be acquired under the PN offset based on the accumulated energy.

Initial timing adjustment by a slave base station

Once the mobile station 60 is acquired, the slave base station 64 adjusts its timing so that the mobile station 60 can successfully acquire its forward link transmission in block 306. The slave base station 64 calculates the initial timing adjustment by determining the difference between the PN offset at which it acquires the reverse link signal from the mobile station 60 and the PN offset used by the reference base station 62 to receive the reverse link signal from the mobile station 60. Using this PN offset difference, the slave base station 64 adjusts the timing of its pilot signal in such a way that it falls within the search window of the mobile station 60 when the mobile station 60 searches for its pilot signal.

Acquisition of subordinate base stations by mobile stations

In searching for the mobile station signal, the slave base station 64 is required to have some time indication. In the preferred embodiment, another synchronization method is used to keep the time error of the slave base station 64 at or below 1 ms. There are some methods in which a slave base station 44 that cannot receive GPS signals is activated in order to keep the time at a lower accuracy. One possible way to achieve a degree of initial synchronization is to manually set the time of the originating base station 64 at some interval. The second method is to use the WWV receiver to set the time, an implementation of which is known in the prior art. Unlike GPS signals, WWV centralized timing signals are sent at very low frequencies and can penetrate channels and subways. However, WWV receivers cannot provide the degree of time synchronization required for CDMA communications.

In the exemplary embodiment, the slave base station 64 adjusts its timing, and the slave base station 64 adjusts its timing based on the assumption that the mobile station 60 is directly in the vicinity of the slave base station 64. Initial timing adjustments are then made on the assumption that there is no propagation extension between the slave base station 64 and the mobile station 60. The slave base station 64 then temporally forwards its PN sequence generators 72 and 74, thereby accounting for the increasing propagation delay between the slave base station 64 and the mobile station 60. Once the mobile station 60 acquires the pilot channel of the slave base station 64, the final timing adjustment for the slave base station 64 is performed based on the above calculations using the normal procedure.

The pilot channels of different base stations are distinguished from each other by the phase of their PN generators, as IS known in the art and standardized in IS-95. The reference base station 62 commands the mobile station 60 to search for the slave base station 64 through the neighbor table. The reference base station 62 uses signaling data to indicate that the pilots of the slave base stations 64 are available under a PN phase offset, which is described with respect to the received PN offset of the reference base station 62. The message is demodulated and decoded by a traffic demodulator 54 and provided to the searcher 50. In response, the searcher 50 performs a search centered on the PN phase offset for the PN phase specified in the signal from the reference base station 62.

Typically, the pilot signal is generated by a linear feedback shift register, the method of which is described in detail in the above-mentioned patent. To obtain the pilot signal from the generating base station 64, the mobile station 60 must synchronize to the received signal from the generating base station 64 in terms of phase φ and frequency ω. The purpose of the searcher operation is to find the phase phi of the received signal. As described above, utilizing a T1 link from the base station controller 66 may provide relatively accurate frequency synchronization to the generating base station 64, as is known in the art. The method used by the mobile station to find the phase of the received signal is to test a set of phase hypotheses, called search windows, and determine if an offset hypothesis is correct.

Fig. 5 shows the mobile station searcher 50 in detail. The spread spectrum signal is received at antenna 2. The purpose of the apparatus is to achieve synchronization between the pseudo-random noise (PN) sequence generated by the PN sequence generator 20 and the received spread spectrum signal spread by the same PN sequence of unknown phase transmitted by the slave base station 64. In the exemplary embodiment, pilot signal generator 76 (fig. 7) and PN generator 20 are maximum length shift registers that generate PN code sequences for spreading and despreading the pilot signal, respectively. Thus, the operation of obtaining synchronization between the code for despreading the received pilot signal and the PN spreading code of the received pilot signal includes determining the time offset of the shift register.

The spread spectrum signal is provided by the antenna 2 to the receiver 4. The receiver 4 down-converts, filters and amplifies the signal and provides the signal to a despreading element 6. The despreading element 6 multiplies the received signal by the PN code generated by the PN generator 20. Due to the random noise-like nature of the PN code, the product of the PN code and the received signal should be virtually zero except for the synchronization point.

The searcher controller 18 provides bias hypotheses to the PN generator 20. The bias hypothesis is determined based on the signal transmitted by the reference base station 62 to the mobile station 60. In the exemplary embodiment, the received signal is modulated by Quadrature Phase Shift Keying (QPSK) such that PN generator 20 provides to despreading element 6 a PN sequence for the I modulation component and a separate sequence for the Q modulation component. The despreading element 6 multiplies the PN sequence with its corresponding modulation component and provides the product of the two output components to coherent accumulators 8 and 10.

The coherent accumulators 8 and 10 sum the products over the length of the product sequence. Coherent accumulators 8 and 10 are responsive to signals from searcher controller 18 to reset, lock and set the addition period (summation period). The product-sum is supplied from adders 8 and 10 to squaring device 14. The squaring device 14 squares each sum and adds the squares.

The sum of squares is provided by squaring means 12 to non-coherent combiner 14. The non-coherent combiner 14 determines an energy value from the output of the squaring device 12. The non-coherent accumulator 14 acts to cancel out the effect of frequency differences between the base station transmit clock and the mobile station receive clock and facilitates detection statistics in fading environments. The non-coherent accumulator 14 provides an energy signal to a comparison device 16. The comparison means 16 compares the energy value with a predetermined threshold provided by the searcher controller means 18. Then, each comparison result is fed back to the searcher controller 18. The results fed back to the searcher controller 18 include the correlation energy caused in the measurement and the PN offset.

In the present invention, the searcher controller 18 outputs the PN phase at which it is synchronized with the base station 64. This PN offset is used to calculate the time error, as will be described in more detail below.

In the exemplary embodiment, when the mobile station 60 acquires the slave base station 64, it calculates the difference between the time it receives the signal from the slave base station 64 and the time it receives the signal from the reference 62 base station. The value is provided to a message generator 52 which generates a message representing the difference. On the reverse link, messages are sent as signaling data to the reference base station 62 and the slave base station 64 which send the messages back to the base station controller 66.

Measuring delay between transmitting forward link signal from slave base station and receiving reverse link signal at slave base station

In step 311, the slave base station 64 measures the time (T) at which the slave base station receives the reverse link signal from the mobile station 60₂) And the slave base station transmitting its forward link signal to the mobile station 60 (T)₁) The difference between them. The slave base station 64 stores the PN offset as it transmits its forward link signal and calculates the time difference RTD upon detection of the reverse link signal from the mobile station 60₂. In the exemplary embodiment, the calculated time difference is provided by the slave base station 64 to the base station controller 66, and the calculation of the timing adjustment is made at the base station 66. Those skilled in the art will understandThe invention can be easily extended to the case of performing calculations at the base station or the mobile station.

Timing adjustment of slave base stations

The base station controller 66 performs the calculations described in equation (12) and sends an indication of the required timing adjustment to the slave base station 64. Referring to fig. 7, the timing adjustment signal is received by the slave base station 64 at the control processor 100. The control processor 100 generates and provides control signals to the timing adjustment processor 99. Timing adjustment processor 99 generates a signal that changes the timing of timing source 98 by an amount specified in the signal from base station controller 66.

Time transfer when not in soft handoff

The above adjustment procedure is valid in case the mobile station 60 is in soft handover, i.e. when the mobile station has established a link with the reference base station 62 and the slave base station 64. Establishing links with the reference base station and the slave base stations allows the reference base station 62 to determine the RTD₁And slave base station 64 to determine RTD₂. According to RTD₁And RTD₂Can predict the time error T₀'. However, according to one embodiment of the present invention, the slave base station 64 may be synchronized with the reference base station 62 when the mobile station 60 is not in communication with the reference base station 62 and the slave base station 64, as follows.

Assuming that the mobile station 60 is in communication with the reference base station 62, the RTD may be determined as described above₁The value of (c). In addition, the mobile station 60 and the reference base station 64 preferably communicate through a base station controller 66. The long PN code is known to the base station 62, which the mobile station 60 uses to spread its reverse link transmissions to the reference base station 62. In accordance with the present invention, the reference base station 62 propagates the long PN code through the base station controller 66 to the slave base station 64. In addition, the reference base station 62 uses the communication link through the base station controller 66 to determine the RTD₁The list of values is transmitted to the slave base stations 62, each value being associated with a long PN code that is used by one of the mobile stations 60 to spread the reverse link transmitted by the mobile station 60 communicating with the reference base station 62. It should be understood that each mobile station60 with a special long PN code and RTD₁The values are correlated. The slave base station 64 then uses the long PN code information to attempt to receive one or more reverse link transmissions from the mobile station 60. Since the mobile station 60 is not in a soft handoff state, the signal received by the slave base station 64 from the mobile station 60 is weak. Thus, the slave base station 64 typically needs to accumulate a large number of PN chips in order to detect the mobile station 60 served by the reference base station 62.

The slave base station 64 searches for one mobile station 60 at a time based on the long PN code received by the slave base station 64 from the reference base station 62. Thus, if after a reasonable amount of time the slave base station 64 successfully detects a reverse link transmission from the first base station 60, the slave base station 64 begins searching for a reverse link transmission from the second mobile station 60. According to one embodiment of the present invention, the reference base station 62 assists in determining which mobile station 60 the base station 64 is most likely to detect. This is preferably done by determining the distance of the mobile station 60 from the reference base station 62. In addition, information on the sector where each mobile station 60 transmits is used. That is, if the mobile station is a relatively large distance from the reference station (e.g., as specified by the signal obtained when performing the power control algorithm), and the mobile station 60 is in a sector adjacent to the slave base station 64, then it is more likely that the mobile station 60 will be detected by the slave base station 64. It should be appreciated that the amount of time required for the slave base station 64 to detect a mobile station is reduced by the reference base station 62 assisting in determining which mobile stations 60 the slave base station 64 is likely to detect.

Once the slave base station 64 acquires a transmission from the mobile station on the reverse link, the slave base station 64 determines the time of arrival T of the reverse link transmission₂And obtain a pair of symbols gamma₂τ of₂Estimate of (delay from the mobile station 60 to the slave base station 64). Slave base station 64 estimates T₀’=T₂-(γ₂+τ₁)=T₂-(γ₂+ RTD 1/2). It should be understood that gamma is not measured directly₂. If the location of the mobile station 60 is known, γ can be estimated from the distance between the mobile station 60 and the slave base station 64₂This is because the location of the slave base station is known. If the mobile station 60 bitIf the position is unknown, then gamma can be estimated empirically from a table of values or from a database₂. That is, the path loss estimate γ that can be used between the mobile station 60 and the slave base station 64₂. By measuring the amount of power transmitted and received at the slave base station 64, the path loss can be determined. Alternatively, the path loss between the mobile station 60 and the slave base station 64 may be determined using the received signal strength from the mobile station 60 (such as a pilot signal transmitted by the slave base station 64 and received by the mobile station 60). In such embodiments of the invention, the mobile station 60 transmits an indication of the strength of the received signal to the slave base station via the reverse link.

Time error equal to gamma₂Value of minus τ₂. Therefore, the time transfer accuracy and γ₂Is directly related to the accuracy of. The estimate is typically accurate to less than the cell radius. I.e. at gamma₂Is estimated and gamma is₂Is smaller than the cell radius. Thus, for a cell with a radius of K miles, due to γ₂The resulting timing error is approximately 5K mus.

Whether gamma or not₂Such a time transfer method may provide better timing than may be provided by a number of other devices, such as by a backhaul device (backhaul). Thus, estimating γ according to the invention described above₂The size of the search window may be reduced and the window may be guaranteed not to be exceeded. The present invention also provides timing that is accurate enough that the received signals from the two base stations do not arrive with the same pilot PN phase, resulting in distinguishing pilots from different origins.

It should be noted that if the mobile station 60 communicates with the slave base station 64, rather than the reference base station 64, additional steps may be used. In this case, it is necessary to estimate τ₁Instead of τ₂。

IX. initialization of subordinate base stations

The above adjustment steps are valid in case the initial base station system time is relatively close to the reference base station system time. However, in some cases, the difference between the reference base station system time and the reference base station system time is so large as to invalidate its procedure. For example, when a slave base station first becomes in operation, the system time must be initialized. The slave base station system time may be any value without an external reference. In another example, the slave base station system time may accumulate a large amount of error (i.e., a large deviation from the reference base station system time) due to an oscillator that keeps the system time drifting relative to a reference used by the reference base station when there is no mobile station in the area between the reference base station and the slave base station for a relatively long period of time. In this case, the following initialization steps are provided according to the present invention.

When the slave base station 64 is first powered on, the slave base station 64 may not be properly timed since no time transfer occurs between the slave base station 64 and any external timing reference (such as a GPS signal source or the external base station 62). Thus, according to one embodiment of the present invention, when power is first supplied to the slave base station 64, the forward link transmission from the slave base station is not initiated. Assuming that a more accurate device is not available, it is preferable to use a backhaul device to obtain the initial timing. The slave base station 64 has a reasonable estimate of the proper timing that is sufficient to allow the slave base station 64 to obtain the timing by the reverse link method described in section VIII. Once this is done, the slave base station 64 initiates forward link transmissions at low power. If the mobile station 60 is in a soft handoff region, then the mobile station 60 reports the presence of the new pilot and may deliver time using the more accurate soft handoff method of the present invention, as described above. Once this is done, the forward link power of that base station may be increased to a normal operating power suitable for the slave base station 64.

Claims (1)

1. A method for synchronizing a first base station with a reference base station, comprising the steps of:

measuring a round trip delay interval transmitted from the reference base station to a mobile station in communication with the reference base station and from the mobile station back to the reference base station;

transmitting information from the reference base station to the first base station to assist the first base station in receiving communications from the mobile station;

receiving at the first base station communications transmitted by the mobile station and noting a time of reception;

determining, at the first base station, a delay estimate occurring between transmission by the mobile station and reception by the first base station;

calculating a timing correction value based on the estimate of the delay, the time of receipt at the first base station of a transmission from the mobile station to the first base station, and the measured round trip delay interval.