HK1108794A - Communication system employing multiple handoff criteria - Google Patents

Description

Communication system using multiple handover criteria

The present application is a divisional application of an invention patent application entitled "communication system using multiple handover criteria" filed on 6.2.2004 and having application number 02815417.7 by the applicant.

Technical Field

The present invention relates generally to data communications, and more particularly to a novel and improved method and apparatus for a communication system using multiple handover criteria.

Background

Wireless communication systems are widely deployed to provide various types of communication such as voice, data, and so on. These systems may be based on Code Division Multiple Access (CDMA), Time Division Multiple Access (TDMA), or some other modulation technique. CDMA systems offer certain advantages over other types of systems, including increased system capacity.

The CDMA System may be designed to support one or more CDMA standards, such as (1) the "TIA/EIA-95-BMobject State-Base State Compatibility Standard for Dual-ModeWideband Spread Spectrum Cellular System" (IS-95 Standard), (2) the "TIA/EIA-98-Cryptommended Minimum Standard for Dual-ModeWideband Spread Spectrum Cellular State" (IS-98 Standard), (3) the Standard provided by the "3 Generation Parershipproject" (3GPP) including the set of Nos.3G TS25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214(W-CDMA Standard), (4) the Standard provided by the "3 Generation Parershipproject" (3GPP) including the set of Nos.3G TS 25.25.2000, 3G TS25.211, 3G TS 25.3G TS 25.212, 3G TS 25.213, and 3G TS 25.214(W-CDMA Standard), (4) the set of the "3 Generation ParneParneParneParneParneP" (2) the set of Nos.5-SporUsry System including the set of Layer subsystem "(" 3G Specification ") including the set of Specification" 3G 25.45 " "C.S0005-A Upper Layer (Layer 3) Signaling Standard for cdma2000 Spread Spectrum Systems" and "C.S0024 cdma2000 High Rate packet Data Air Interface Specification" (cdma2000 standard) and (5) some other standards. These named standards are incorporated herein by reference. A system that implements the high rate packet data specification of the cdma2000 standard is referred to herein as a High Data Rate (HDR) system. HDR systems are described in "CDMA 2000 High Rate Packet Data Air Interface Specification" of TIA/EIA-IS-856, which IS incorporated herein by reference. The proposed wireless system also provides a combination of HDR and low data rate services (such as voice and fax services) using a single air interface.

In view of the growing demand for wireless data applications, the need for very efficient wireless data communication systems is becoming increasingly important. There are many differences between voice and data services. An important difference between voice services and data services is that the former have strict and fixed delay requirements. Typically, the overall one-way delay of a speech frame must be less than 100 msec. In contrast, data latency may be a variable parameter used to optimize a data communication system. In particular, an efficient error correction coding technique that allows a larger delay than that of a voice service may be used. An exemplary efficient encoding scheme FOR the data is disclosed in U.S. patent application Ser. No. 08743688, entitled "SOFT DECISION FOR DECODING CONvulatory ALLY ENCODED ODEDRORDS", filed 11/6/1996, assigned to the assignee of the present invention and incorporated herein by reference.

A second important difference between voice services and data services is that the former requires a fixed and common service level for all users. Generally, for digital systems providing voice services, this means a fixed and equal transmission rate for all users and a maximum tolerable error rate value for speech frames. In contrast, for data services, the GOS can be different for each user and can be a parameter for increasing the overall efficiency of the data communication system. The GOS for a user's data communication system is generally defined as the total delay that occurs within the transmission of a predetermined amount of data, hereinafter referred to as a data packet.

A third important difference between voice services and data services is that the former require a reliable communication link, which in the exemplary CDMA communication system is provided by a soft handoff. Soft handoff results in repeated transmissions from two or more base stations to improve reliability. A variety of soft handoff techniques are known in the art, and specific techniques are detailed below. Since data is transmitted at a high rate, the impact of soft handoff on system capacity is severe. In addition, the erroneously received packet may be retransmitted. For data services, the transmit power used to support soft handoff is often more efficiently used to transmit additional data. An exemplary System using soft handoff is disclosed in U.S. patent No. 5056109 entitled "Method and Apparatus for Controlling Transmission Power a CDMA Cellular Mobile Telephone System", filed 1991, 8.10, assigned to the assignee of the present invention and incorporated herein by reference.

The most important parameters that measure the quality and efficiency of a data communication system are the transmission delay required to transmit a data packet and the average throughput rate of the system. The effect of transmission delay on data communication is different from its effect on voice communication, but it is an important metric for measuring the quality of a data communication system. The average throughput rate is a measure of the effectiveness of the data transmission capabilities of the communication system. Factors that measure the quality and effectiveness of a data service to a particular user as the user accesses the communication channel include the maximum or minimum throughput rate to the user, and the access frequency granted to the user. These factors are related to the GOS to which the user is provided.

In a CDMA system, the signal-to-noise-and-interference ratio (C/I) for any given user is a function of the user's location within the coverage area. The C/I obtained by a mobile station for any given user on a particular link from a base station determines the information rate that the link can support. Given the particular modulation and error correction method used for transmission, a given level of performance can be achieved at the corresponding C/I level. The C/I obtained for any given user is a function of path loss, which is a function of r for terrestrial cellular systems³To r⁵Where r is the distance to the emission source. In addition, path loss is affected by random variations of man-made or natural obstacles on the radio wave path. The best performance defined in terms of the maximum received C/I value is obtained when the mobile station is served by the best base station.

A system similar to the HDR system is disclosed in the appended U.S. patent application Ser. No. 08/963386 (hereinafter referred to as the 386 application), entitled "METHOD AND APPATUS FOR HIGHER RATE PACKETDATA TRANSMISSION", filed on 1997, 11/3, assigned to the assignee of the present invention AND incorporated herein by reference. In such systems, the above-described characteristics of data communication, other than voice communication, are used to provide efficient high-speed wireless data transmission. These systems are summarized below.

Each mobile station communicates with one or more base stations and monitors control channels for the duration of the communication with the base stations. The control channel may be used by the base station to transmit small amounts of data, paging messages to a particular mobile station, and broadcast messages to all mobile stations. The paging message informs the mobile station that the base station has data to send to the mobile station.

Upon receiving a paging message from one or more base stations, the mobile station measures the signal-to-noise-plus-interference ratio (C/I) of the forward link signal (e.g., forward link pilot signal) and uses the C/I measurement to select the best base station. The C/I may be measured at the mobile station using a variety of known techniques, such as measuring a pilot signal, a broadcast channel, or any known signal from a base station. At each slot, the mobile station sends a transmission request for the highest data rate that can be reliably supported with the measured C/I to the selected base station on a dedicated Data Request (DRC) channel. The selected base station transmits data in data packets at a data rate that does not exceed the data rate requested by the mobile station on the DRC channel. By transmitting from the best base station at each time slot, improved throughput and transmission delay can be achieved.

The selected base station transmits to the mobile station at the data rate requested by the mobile station at the peak transmit power for the duration of one or more time slots. In CDMA voice communication systems, such as IS-95, the base station operates at a predetermined offset (e.g., 3 dB) in available transmit power due to variations in usage. Thus, the average transmit power is half of the total transmit power. However, in data systems such as the HDR described in the 386 application, data transmission is always scheduled and does not have to be compensated from the available peak transmit power.

The ability to transmit from different base stations in different time slots enables the data communication system to adapt quickly to changes in the operating environment. In addition, the ability to transmit data packets on non-contiguous time slots by using sequence numbers to identify data units within the data packets is possible. Complexity is introduced to achieve this, however, since base stations often independently select rates and other base stations often cannot read DRCs, other base stations cannot know the amount of data transmitted in any given time slot. This task is often accomplished by ACK and NACK (described in detail below). In addition to handling the transmission sequence number (packet ID), a transmission ID speed-up rate may be required. On the backhaul, the base station can send a message including the packet ID and the amount of data sent within the packet.

The system increases flexibility by forwarding data packets from a central controller to a particular mobile station to all base stations that are members of the mobile station's active set. The active set is a set of base stations selected from the mobile station's neighborhood, typically based on the quality of the signal received at the mobile station. In these systems, the data transmission may come from any base station in the mobile station's active set at each time slot. Due to the complex requirements of maintaining data queues and the associated backhaul traffic within many base stations, the frequency with which a mobile station selects a new base station for transmission is limited to minimize these effects. For example, a mobile may be required to remain at a particular base station for a given number of time slots, for a certain duration, or for a particular amount of data to be transmitted.

By queuing forward link traffic data to multiple base stations (typically included in the mobile station's active set), one or more of these base stations can transmit data to the mobile station with minimal processing delay. The total capacity of the system can be increased by reducing the forward link transmit power to any particular mobile station. Therefore, the system capacity is optimized by reducing the base station to mobile station transmit power in the active set and causing only a subset of the active set to be transmitted to the mobile station such that the required minimum C/I is received at the mobile station. A technique of queuing data to various base stations and then causing a subset of the base stations to transmit depending on the particular mobile station environment is known as a standby handoff. A description of armed handoff is disclosed in appended U.S. patent application Ser. No. 08/925518, entitled "METHOD AND APPARATUS FOR CHANGING FOR TRAFFICCHANNEL POWER ALLOCATION", filed 1997, 8/9, assigned to the assignee of the present invention AND incorporated herein by reference.

As described above, data systems such as HDR and described in the' 386 application maximize throughput by scheduling the entire forward link channel for transmission to a particular user. The use limitation of the standby switch is that only a single base station of the active set transmits in each time slot. Thus, the power required for soft handoff is turned to be used for data transmission. While this technique can optimize overall system capacity, it can be problematic if it is desired to provide a particular user with a minimum GOS. For example, in a system in use, there is a geographic environment in which the maximum C/I value received by a mobile station from any one base station is only allowed to the mobile station at a relatively low data transmission rate. This situation may occur at the physical location of the base station and various natural or man-made obstacles in the transmission path. The GOS desired by the user can be realized by allocating additional medium access time (i.e., allocating additional slots) to the user. In this case, the service to the base station may severely reduce the capacity of that part of the network. In extreme cases, even a continuous allocation of the entire forward link from any one base station may not provide sufficient throughput for the GOS requirements desired by the user. In these cases, enabling more than one base station to transmit forward link data simultaneously (i.e., soft handoff) may provide additional C/I to obtain the desired GOS to a particular mobile station while increasing the total capacity available to other users.

A retransmission mechanism may be used for data units received in error. In a system such as that described in the HDR and 386 applications, each data packet includes a predetermined number of data units, each identified by a sequence number. Upon receiving one or more data units in error, the mobile station sends a Negative Acknowledgement (NACK) on the reverse link data channel indicating the sequence number of the missing data unit retransmitted from the base station. The base station receives the NACK message and retransmits the erroneously received data unit.

Notably, enabling different base stations to transmit portions of forward link data in respective time slots and the benefits of retransmission mechanisms require coordination of maintaining queues in the multiple base stations included in the active set. As an example, assume that a first base station is selected to transmit a first portion of data from its queue to a mobile station. After transmitting the data, the first base station knows to transmit a second portion of the data in its queue next, excluding requests for retransmission of the first portion. It is further assumed that after the first portion of data is transmitted from the first base station again, the mobile environment changes such that the second base station is selected for successive transmissions. Coordination must be made so that the second base station knows to transmit the second part of the data instead of the first part. This coordination involves some complexity and network traffic over the backhaul network (i.e., the network between the base station controller and the various base stations). Backhaul network traffic increases as more base stations are introduced into the active set because the coordination information also increases proportionally and forward link traffic data is sent for storage in additional base station queues. In addition, it is clear that as data systems achieve higher data rates, the data transmission required over the backhaul network also increases significantly.

The base station candidates in the active set of the mobile station may be selected using a variety of techniques. One such technique is disclosed in U.S. Pat. No. 6151502 (hereinafter the' 502 patent), entitled "METHOD AND DAPPARUTUS FOR PERFORMING SOFT HANDOFF IN A WIRELESS COMMUNICATIONSYSTEM", assigned to the assignee of the present invention and incorporated herein by reference. Using this technique, a base station should be added to the active set if the received pilot signal is above a predetermined add threshold and removed from the active set if the pilot signal is below a predetermined drop threshold. Alternatively, a base station is added to the active set if the additional energy of the base station (e.g., measured using pilot signals) and the energy of base stations already in the active set exceed predetermined thresholds. With this technique, a small number of base stations whose transmitted energy includes the total received energy at the mobile station are not added to the active set.

Soft handoff techniques and active set selection techniques are known in the art to apply to sectors within a base station as well as to the base station itself. However, the backhaul network congestion conditions described above do not apply to sectors within a base station. This is because multiple sectors within a base station share a common queue of forward link data. Thus, when a sector of a base station is added to the active set already containing another sector of the base station, additional forward link data need not be sent over the backhaul network. In addition, coordinating data transmitted from the base station's queues in the base station's sector causes less complexity than coordinating data transmissions between base stations.

As described above, wireless data users require high-speed data transmission and may have certain minimum grade of service (GOS) requirements. There is therefore a need in the art for a communication system that efficiently manages backhaul network traffic and optimizes overall system capacity while providing maximum quality of service to all users.

Disclosure of Invention

Embodiments disclosed herein address the need for a communication system that manages the grade of service (GOS) requirements of all users while efficiently managing backhaul network traffic and optimizing overall system capacity. In an aspect, armed handoff is used to transmit forward link data from a single base station to a particular mobile station while meeting certain minimum requirements, such as minimum data throughput, minimum data rate, and GOS requirements. If these criteria are not met, soft or softer handoff is used to transmit forward link data from one or more base stations to a particular mobile station until the conditions for armed handoff are met. In another aspect, the test for adding a base station sector to the active set varies depending on whether other sectors of the base station are already in the active set. The test is not stringent when the sector is not the first sector of the base station to join the active set, and is more stringent when it is the first sector of the base station to join the active set. These aspects have the benefit of providing a method of optimizing system capacity while providing varying desired GOS levels while reducing complexity and minimizing traffic on the backhaul network. Various other aspects of the invention are also shown.

The present invention provides methods and systems that implement various aspects, embodiments, and features of the present invention, as described in more 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 illustrates an example data communication system;

FIG. 2 is a block diagram illustrating the basic subsystems of an exemplary data communication system;

FIG. 3 is a flow chart describing steps for joining one or more base stations to the active set;

FIG. 4 is a flowchart describing steps for removing one or more base stations from the active set;

fig. 5 is a flow chart depicting the use of multiple handover criteria in an example embodiment.

Detailed Description

Fig. 1 illustrates an exemplary data communication system of the present invention. Each cell 2 (referenced 2a-2g) is accordingly served by a corresponding base station 4 (referenced 4a-4 g). A plurality of mobile stations 6 (labeled 6a-6j) are dispersed throughout the data communication system. In the exemplary embodiment, each mobile station 6 is capable of communicating with one or more base stations 4 on the reverse link, depending on whether the mobile station 6 is in soft handoff. Reverse link communications are not shown in fig. 1 for simplicity of illustration.

The term mobile station is used throughout to include any access terminal such as a fixed wireless handset, handheld computer, unmanned data terminal, etc. The term base station is used throughout to include any access point. Commonly found in modern communication systems are sectorized base stations. Such base stations include more than one sector, each sector covering a unique portion of a cell (typically with overlap). The method of distinguishing base stations can be applied to distinguish sectors (even within the same cell). In a CDMA system, a unique pseudo-noise cover sequence is used to distinguish each sector of each base station. As disclosed herein, there are some differences in the words sector and base station that the present invention utilizes, as will be indicated in the following description. Thus, unless otherwise indicated, those skilled in the art will appreciate that "base station" as used herein may be interchanged with "base station sector" without departing from the scope of the invention.

In HDR-based systems, to increase system capacity, each base station 4 typically transmits to a single mobile station 6 with its entire power allocation, thus removing interference from multiple forward link transmissions in the cell. A plurality of mobile stations 6 within a cell are served by sharing base station transmission resources in time. The smallest increment allocated to a particular mobile station 6 is called a slot. To further maximize capacity, each mobile station 6 communicates on the forward link with at most one base station 4 in a time slot, thus maintaining the proportion of one base station serving one mobile station. For example, base station 4a transmits data only to mobile station 6a, base station 4b transmits data only to mobile station 6b, and base station 4g transmits data only to mobile station 6g on the forward link during time slot n. In fig. 1, a solid line with arrows indicates data transmission from the base station 4 to the mobile station 6. The dashed line with arrows indicates that the mobile station 6 is receiving a pilot signal but has no data transmission from the base station 4.

Mobile stations 6, particularly those located at cell boundaries, can receive pilot signals from multiple base stations 4. For example, mobile station 6j can receive signals from base stations 4a and 4b, and mobile station 6e can receive signals from three base stations 4b, 4d, and 4 e. If the pilot channel is above the predetermined threshold, the mobile station 6 can request that the base station 4 be added to the active set of the mobile station 6. In one embodiment of the invention, a higher threshold is used to join a new base station to the active set, and a lower threshold is used to join a new sector of a base station already in the active set to the active set. As described above, the transmitted forward link data may be queued in multiple base stations in the active set. In general, a sector of one of these base stations 4, which is included in the active set of mobile station 6, may transmit on the forward link in any given time slot.

In certain circumstances, however, it may be preferable to allow more than one base station 4 to transmit to mobile station 6 in a single time slot (i.e., soft handoff of the forward link). For example, consider mobile station 6h (located on the boundary of cells 2c and 2 d). The C/I received by mobile station 6h from any one base station may not be sufficient to provide communications with sufficient GOS, possibly due to natural or man-made obstructions between mobile station 6h and base stations 4C and 4 d. Communication may be possible, but the supported data rate is low. If the mobile station 6h remains in the depicted position, its throughput will stay at a lower level. The result may be that the mobile user is not satisfied with the quality and delay of the data service used. Additionally, the mobile network provider may wish to increase revenue and/or increase its competitiveness by providing agreed service assurance (GOS) or quality of service (QOS) levels, which is not possible with the mobile station 6h just described. In addition, GOS or QOS guarantees may be achieved by allocating a higher proportion of the available timeslots from either base station 4c or 4d (albeit at a lower transmission rate).

Soft handoff on the forward link, along with the diversity reception capabilities of the mobile station, enables higher data transmission rates. The increased data rate may be achieved through redundant transmissions from the base station or through transmit diversity of multiple streams. In some cases, system capacity may be improved if GOS requirements can be achieved by using fewer timeslots for forward link soft handoff. In this case, base stations 4c and 4d transmit forward link data to mobile station 6h simultaneously in a single time slot, as shown by the double-dashed line. In one embodiment, mobile stations 4c and 4d may transmit to mobile station 6h only in one time slot at the same time. In another embodiment, base stations 4c and 4d may allocate a portion of their forward link power for simultaneous transmission to mobile station 6h, with the remaining power being transmitted to mobile stations 6c and 6d in the same time slot.

In addition to meeting GOS requirements or capacity limitations as described above, forward link soft handoff is used when the system is not fully loaded. A mobile station within a geographic area of limited maximum data rate, such as mobile station 6h, may have a higher throughput regardless of the minimum GOS of mobile station 6h when the neighboring cell is not fully loaded. In many instances, when the system is not fully loaded (i.e., not every base station transmits to a unique user at maximum power per time slot), the overall throughput may still be increased regardless of whether any user within the system receives data at less than the maximum rate (degraded C/I due to path loss). In these cases, forward link soft handoff may be able to increase the data rate of that user without affecting the data rates of other users. Thus increasing the capacity of the system.

Fig. 2 is a block diagram illustrating the basic subsystems of the data communication system of the present invention. The base station controller 10 interfaces with a packet network interface 24 (which may be one or more packet data serving nodes, or PDSNs). A Public Switched Telephone Network (PSTN)30 and all base stations 4 within the data communications network (only one base station 4 is shown in figure 2 for simplicity). Base station controller 10 coordinates communication between mobile station 6 and other devices connected to packet network interface 24 and PSTN 30 within the data communication system. PSTN 30 interfaces with users through a standard telephone network (not shown in fig. 2).

The base station controller 10 comprises a number of selector elements 14 (only one shown for simplicity in fig. 2). A selector element 14 is assigned to control communication between one or more base stations 4 and a mobile station 6. To assign element 14 to mobile station 6, call control processor 16 is notified that mobile station 6 needs to be paged. Call controller processor 16 then directs base station 4 to page mobile station 6.

Data source 20 contains data to be transmitted to mobile station 6. Data source 20 provides data to packet network interface 24. The packet network interface 24 receives data and routes the data to the selector element 14. The selector element 14 sends data to each base station 4 that is in communication with the mobile station 6. The link between the base station controller 10 and all base stations 4 is referred to as the aforementioned backhaul network. Each base station 4 maintains a queue containing data to be transmitted to mobile stations 6.

In an example embodiment, on the forward link, a data packet refers to a predetermined amount of data independent of the data rate. The data packet is formatted and encoded with other control and coding bits. If data transmission occurs over multiple Walsh channels, the encoded packets are demultiplexed into parallel streams, each stream transmitted over one Walsh channel.

Data is then transmitted from the data queue 40 to the channel element 42 in the form of data packets. For each data packet, channel element 42 inserts the necessary control fields. The data packet, control field, frame check sequence bits, and code tail bits comprise the formatted packet. Channel element 42 then encodes the one or more formatted packets and interleaves (i.e., reorders) the symbols within the encoded packets. Next, the interleaved packet is scrambled with a scrambling sequence, covered with a Wal sh cover code, and spread with long PN codes and short PNI and PNQ codes. The spread data is quadrature modulated, filtered and amplified by a transmitter within the RF unit 44. The forward link signals are transmitted over the air via an antenna 46 on a forward link 50.

As described above, the base station 4 typically includes a plurality of sectors (not shown). Those skilled in the art will appreciate that the data queue 40 can be used for transmissions on any one sector of a base station, and thus adding one sector of a base station 4 to the active set already containing one sector of a base station 4 does not require additional signaling and data transmissions on the backhaul network (connections between the base station controller 10 and the plurality of base stations 4).

At mobile station 6, the forward link signal is received by antenna 60 and routed to a receiver within a front end 62. The receiver filters, amplifies, quadrature demodulates, and quantizes the signal. The digitized signal is provided to a demodulator (DEMOD)64 where it is despread with the long PN code and the short PNI and PNQ codes, decovered with the Walsh cover and descrambled with the same scrambling sequence. The demodulated data is provided to a decoder 66 which performs the inverse of the signal processing functions performed at the base station 4, particularly the de-interleaving, decoding and frame check functions. The decoded data is provided to a data sink 68. The hardware as described above supports data, messaging, voice, audio, and other communication transmissions over the forward link.

System control and scheduling functions may be accomplished by a number of implementations. The location of the channel scheduler 48 depends on whether a centralized or distributed control/scheduling process is required. For distributed processing, for example, channel scheduler 48 may be located within each base station 4. Conversely, for centralized processing, the channel scheduler 48 may be located within the base station controller 10 and designed to coordinate the transmission of data by multiple base stations 4. Other implementations of the functionality described herein are also contemplated and are within the scope of the present invention.

As shown in fig. 1, mobile stations 6 are dispersed throughout the data communication system and communicate with zero or one base station 4 on the forward link, and more than one base station 4 if in a soft handoff situation. In the exemplary embodiment, channel scheduler 48 coordinates forward link data transmissions for one base station 4. In the exemplary embodiment, channel scheduler 48 is coupled to data queue 40 and channel element 42 within base station 4 and receives the queue size (which indicates the amount of data to be transmitted to mobile station 6) and the Data Request Channel (DRC) message from mobile station 6. The channel scheduler 48 schedules high rate data transmissions so that the system goals of maximum data throughput and minimum transmission delay can be achieved.

In the exemplary embodiment, the data transmission portion is scheduled based on the communication link quality. An exemplary communication system FOR selecting a transmission rate based on link quality is disclosed in U.S. patent application serial No. 08/741320, entitled "METHOD AND APPARATUS FOR PROVIDING HIGH SPEED DATA communications in a CELLULAR ENVIRONMENT", filed 1996, 9/11/1996, assigned to the assignee of the present invention, AND incorporated herein by reference. In the present invention, the scheduling of data communication may be based on additional considerations, such as the GOS of the user, the queue size, the type of data, the amount of delay already experienced, and the error rate of the data transmission. These considerations are described in detail in U.S. patent application No. 08798951, entitled "METHOD AND APPARATUS FOR FORWARD LINK RATECHEDULING", filed 1997, month 2, day 11, AND U.S. patent No. 5923650, both assigned to the assignee of the present invention AND incorporated herein by reference. Other factors may also be considered in scheduling the data transmission and are within the scope of the present invention.

The data communication system of the present invention supports data and message transmission on the reverse link. Within mobile station 6, controller 76 handles data or message transmission by routing the data or message to encoder 72. The controller 76 may be programmed within a microcontroller, microprocessor, Digital Signal Processing (DSP) chip, or ASIC to implement the functions described above.

In an exemplary embodiment, the encoder 72 encodes messages in accordance with the blank and burst signaling data format described in the aforementioned U.S. patent No. 5504773. Encoder 72 then generates and appends a set of CRC bits, appends a set of code tail bits, encodes the data and appended bits, and reorders the symbols within the encoded data. The interleaved data is provided to a Modulator (MOD) 74.

The modulator 74 can be implemented in a number of embodiments. In the exemplary embodiment, the interleaved data is covered with a Walsh code, spread with a long PN code, and further spread with a short PN code. The spread data is provided to a transmitter within front end 62. The transmitter modulates, filters, amplifies, and transmits the reverse link signal over the air via antenna 46 on reverse link 52.

In the exemplary embodiment, mobile station 6 spreads the reverse link data according to a long PN code. Each reverse link channel is defined in terms of a time offset of a common long PN sequence. At two different offsets, the resulting modulation sequences are uncorrelated. The offset of mobile station 6 IS determined based on the unique numerical identification of mobile station 6, which in the exemplary embodiment of IS-95 IS the mobile station specific identification number of mobile station 6. Thus, each mobile station 6 transmits on an uncorrelated reverse link channel determined from its unique electronic serial number. In another embodiment, the reverse link channel may be defined by a unique, uncorrelated code in addition to the reverse link channel generated by the offset of the common PN code.

At base station 4, the reverse link signal is received by antenna 46 and provided to RF unit 44. The RF unit 44 filters, amplifies, demodulates, and quantizes the signal and provides a digitized signal to the channel element 42. Channel element 42 despreads the digitized signal with the short PN code and the long PN code. Channel element 42 also performs Walsh code decovering and pilot and DRC extraction. Channel element 42 then reorders the demodulated data, decodes the deinterleaved data and performs a CRC check function. The decoded data (e.g., data or message) is provided to the selector element 14. The selector element 14 routes the data and messages to the appropriate destination. The channel element 42 also forwards a quality indicator, which indicates the condition of the received data packet, to the selector element 14.

As described above, in the present invention, the mobile station 6 may communicate with one or more base stations 4 simultaneously. The action taken by mobile station 6 depends on whether mobile station 6 is in soft handoff. These two cases will be discussed separately below.

Consider the first case where mobile station 6 is not in soft handoff (i.e., it is only communicating with base station 4). Referring to fig. 2, data to a particular mobile station 6 is provided to a selector element 14 that has been assigned to control communications with that mobile station 6. The selector element 14 forwards the data to a data queue 40 within the base station 4. The base station 4 queues the data and transmits a paging message on the control channel. The base station 4 then monitors the reverse link DRC channel for DRC messages from the base station 6. If no signal is detected on the DRC channel, the base station 4 can retransmit the paging message until the DRC message is detected. After a predetermined number of retransmissions, the base station 4 can suspend processing or resume the call with the mobile station 6.

In the exemplary embodiment, mobile station 6 transmits the requested data rate in the form of a DRC message to base station 4 on the DRC channel. In further embodiments, mobile station 6 transmits a forward link channel quality indication (e.g., a C/I measurement) to base station 4. In the exemplary embodiment, the DRC message is transmitted in the first half of each slot. The base station 4 then decodes the DRC message with the remaining half slot and configures the hardware for data transmission in the next successive slot (if that slot is available for transmission to the mobile station 6). If the next successive slot is not available, the base station 4 waits for the next available slot and continues to monitor for a new DRC message on the DRC channel.

In a first embodiment, the base station 4 transmits at the requested data rate. This embodiment enables the mobile station 6 to make an important decision to select the data rate. The benefit of always transmitting at the requested data rate is that the mobile station can predict which data rate. The mobile station 6 therefore demodulates and decodes the traffic channel only in accordance with the requested data rate. The base station 6 need not transmit a message to the mobile station 6 indicating the data rate used by the base station 4.

In the first embodiment, upon receiving the paging message, mobile station 6 successively attempts to demodulate the data at the requested data rate. Mobile station 6 demodulates the forward traffic channel and provides soft-decision symbols to the decoder. The decoder decodes the symbols and performs a frame check on the decoded packet to determine whether the packet was received correctly. The frame check may indicate a packet error if the packet was received in error or if the packet was sent to another mobile station 6. Also in the first embodiment, the mobile station 6 demodulates data on a slot-by-slot basis. In an exemplary embodiment, mobile station 6 can determine whether it is a data transmission sent to it based on a preamble included in each transmitted data packet, as described below. Thus, if it is determined that a transmission is being sent to another mobile station 6, the mobile station 6 can abort the decoding process. In both cases, mobile station 6 will transmit a Negative Acknowledgement (NACK) message to base station 4 to confirm the incorrect receipt of the data unit. Upon receiving the NACK message, the erroneously received data unit is retransmitted.

The transmission of the NACK message may be implemented in a manner similar to the transmission of Error Indicator Bits (EIB) in CDMA systems. THE implementation and use OF EIB TRANSMISSION is disclosed in U.S. Pat. No. 5568483, entitled "METHOD AND APPARATUS FOR THE FORMATION OF DATA FOR TRANSMISSION", assigned to THE assignee OF THE present invention and incorporated herein by reference. Additionally, a NACK may be sent with the message.

In a second embodiment, the data rate is determined by the base station 4 and the input from the mobile station 6. Mobile station 6 performs C/I measurements and transmits link quality indications (e.g., C/I measurements) to base station 4. The base station 4 can adjust the requested data rate, such as the queue size and the available transmit power, according to the resources available to the base station 4. The adjusted data rate may be transmitted to mobile station 6 at the adjusted data rate prior to or concurrently with the data transmission, or may be indicated at the time of encoding of the data packet. In the first case, where the mobile station 6 receives the adjusted data rate prior to data transmission, the mobile station 6 demodulates and decodes the received packet in the manner described in the first embodiment. In a second embodiment, in which the adjusted data rate is transmitted to mobile station 6 along with the data transmission, mobile station 6 can demodulate the forward traffic channel and store the demodulated data. Upon receiving the adjusted data rate, mobile station 6 decodes the data according to the adjusted data rate. In the third case, where the adjusted data rate is indicated within the encoded data packet, mobile station 6 demodulates and decodes all candidate rates and determines the transmission rate a posteriori for selection of the decoded data. METHODs AND APPARATUS FOR achieving RATE determination are disclosed in detail in U.S. Pat. No. 5751725 entitled "METHOD AND APPARATUS FOR DETERMINING THE RATE OF RECEIVED DATA IN A VARIABLE RATE COMMUNICATION SYSTEM" AND U.S. Pat. No. 6175590 entitled "METHOD AND APPARATUS FOR DETERMININGTHE RATE OF RECEIVED DATA IN A VARIABLE RATE COMMUNICATION SYSTEM", both OF which are assigned to the assignee OF the present invention AND incorporated herein by reference. For all of the cases described above, if the frame check result is negative, mobile station 6 transmits a NACK message as described above.

The discussion hereafter is based on the first embodiment, wherein the mobile station 6 transmits a DRC message to the base station 4, the message indicating the requested data rate, unless otherwise indicated. However, the invention described herein is equally applicable to the second embodiment, in which the mobile station 6 transmits a link quality indication to the base station 4.

Now consider the second case, where the mobile station 6 is in soft handover, i.e. it communicates with multiple base stations 4 simultaneously. As described above, in certain situations, mobile station 6 may only receive data from a single base station 4 on the forward link, even in soft handoff situations, taking into account capacity. Regardless, as noted above, GOS and/or capacity considerations may require soft handoff on the forward link under certain conditions. On the reverse link, more than one base station 4 can receive, demodulate, and combine data from one mobile station 6 without affecting capacity, because mobile station 6 can transmit from any location where it is located regardless of how many base stations 4 are monitoring their transmissions. In effect, when multiple base stations receive and combine the reverse link signals, soft handoff is used on the reverse link to transmit at a lower power level (thus reducing interference and increasing capacity).

There are various techniques for determining when a forward link soft handoff should be used rather than a standby handoff. In one embodiment, the mobile station monitors the received C/I values from the available base stations and sends a DRC request for a soft handoff of the front-line link at the appropriate time. This can be determined along with statistics of the C/I value data rates received in the past and GOS requirements. The GOS may contain the minimum criteria for all users as described above. In addition, some users may require higher GOS requirements than the minimum standard. The service may be measured in terms of a minimum transmission rate in any one time slot, an average throughput over some particular time interval, a received C/I or other statistical measure of received C/I, data rate and/or throughput. Regardless of how GOS is defined, a particular mobile station 6 may determine that current conditions warrant a request to transmit forward link data from more than one base station in a single time slot. Based on the received C/I (including combinations thereof) from the available base stations, the mobile can indicate which active set candidates should be included in the forward link soft handover transmission.

In another embodiment, the base station 4 or the base station controller 10 can determine when a soft handoff is appropriate. In an embodiment, as base station 4 may be used to determine the forward link data rate based on mobile-provided link measurements (as described above), the same link quality measurements, along with past link quality measurements and/or previously provided data rates, can be used to determine whether a suitable GOS is provided. If not, the forward link soft handoff is normal. In another embodiment, the base station 4 can determine that the soft handoff is normal based on the currently requested data rate on the DRC along with the previously transmitted transmission rate. In another case, the base station controller 10 determines when to use a forward link soft handoff based on overall capacity optimization and measurements of either the base station or the mobile station as just described.

Various methods may be used to determine whether to initiate a forward link soft handoff. First, if the best received C/I from a single base station is below a threshold, then a soft handover may be requested whenever it increases the received C/I above the threshold. This may be limited to two-way soft handoff or it may cause any number of base stations and/or sectors to be in soft handoff. Second, soft handoff requests may be limited to situations where initiating a soft handoff increases the C/I sufficiently to support one of a subset of higher data rates. For example, in some systems, there are a discrete number of data rates available, which may be requested on the DRC based on the received C/I. If the improvement introduced by soft handover is not sufficient to result in a higher data rate, there is no benefit to offset the capacity degradation that may occur. Similarly, soft handoff may be limited to situations where more than one increased data rate improvement may be obtained. Third, the system load may be factored into consideration. The margin improvement required in the received C/I or data rate under different loading conditions can be adjusted to make the forward link soft handoff request somewhat more free. This third method can be used in the base station controller or mobile station 6 as described with appropriate messages indicating the current load in the system (or more precisely, given the active set and/or neighbor set of the mobile station 6).

The mobile station 6 can receive pilot signals from a plurality of base stations 4 simultaneously. If the C/I measurement of base station 4 is above a predetermined threshold, base station 4 can be added to the active set of mobile station 6. As described above, the threshold for adding a base station to the active set may be higher than base stations not already in the active set. A lower threshold may be used when adding an additional sector of base station 4 to the active set that already contains at least one sector from that base station, taking into account backhaul network congestion and the maintenance of data queue 40.

In general, none of the thresholds just described must be a fixed threshold. As described in the' 502 patent, if a sector or base station is to be added to the active set, another technique for adding a base station or sector is to calculate the margin improvement within the received C/I. To illustrate, if the active set contains only a single base station with a strong received C/I, it is found that another strong signal from the new base station may not be able to include that base station in the active set because the relative improvement in received C/I is small. In contrast, a mobile station whose active set contains several weaker base stations measured in terms of received C/I may add a newly discovered base station with moderate strength to its active set because, although the strength of the new signal is moderate, the relative improvement in received C/I may exceed the boundary threshold required to include it into the set.

Referring to fig. 2, the selector element 14 is assigned to control communication with the mobile station 6, which forwards data to all base stations 4 in the active set of the mobile station 6. All base stations 4 receiving the data from the selector element 14 transmit paging messages to the mobile station 6 on their respective control channels. In response, mobile station 6 performs two functions. First, the mobile station 6 selects the best base station 4 based on a set of parameters including the best C/I measurements. The mobile station 6 then selects the data rate corresponding to the C/I measurement and sends a DRC message to the selected base station 4. In addition, if conditions permit, mobile station 6 can determine whether soft handoff is normal and which base station should be included (using the techniques described above). Mobile station 6 can direct the DRC message transmission to a particular base station 4 by covering the DRC message with the Walsh cover assigned to that particular base station 4. Second, mobile station 6 attempts to demodulate the forward link signal based on the requested data rate at each successive time slot. A soft handoff request in a DRC message to a particular base station 4 can be sent to the base station controller for initiating a soft handoff.

After sending the paging message, all base stations 4 in the active set monitor the DRC channel for DRC messages from the mobile station 6. Also, since the DRC message is covered with the Walsh code, the selected base station 4 assigned the same Walsh cover can decover the DRC message. Upon receiving the DRC message, the selected base station 4 will transmit data to the mobile station 6 in the next available slot.

In the exemplary embodiment, base station 4 transmits data in the form of packets containing a plurality of data units to mobile station 6 at the requested data rate. If the data unit is not correctly received by mobile station 6, a NACK message is sent on the reverse link to all base stations 4 in the active set. In the exemplary embodiment, the NACK message is demodulated and decoded by the base station 4 and forwarded to the selector element 14 for processing. In processing the NACK message, the data unit is retransmitted using the procedure described above. In the exemplary embodiment, the selector element 14 combines the signals received from all base stations 4 into one NACK message and sends the NACK message to all base stations 4 in the active set.

In an example embodiment, mobile station 6 can detect changes in the best C/I measurements and dynamically request data transmissions from different base stations 4 at each time slot to improve efficiency. When using a standby soft handoff, data transmission is from only one base station 4 in any given time slot, so other base stations 4 in the active set may not know which data units, if any, are sent to mobile station 6. In the exemplary embodiment, the transmitting base station 4 informs the selector element 14 about the data transmission. The selector element 14 then sends a message to all base stations 4 in the active set. In the exemplary embodiment, the transmitted data is assumed to have been correctly received by mobile station 6. Thus, if mobile station 6 requests a data transmission from a different base station 4 in the active set, the new base station 4 transmits the remaining data units. In the exemplary embodiment, the new base station 4 transmits according to the most recent transmission update from the selector element 14. Alternatively, the new base station 4 uses a prediction scheme to select the next transmitted data unit based on metrics such as the average transmission rate and previous updates from the selector element 14. These mechanisms minimize the possibility of repeated retransmissions of the same data unit by multiple base stations 4 in different time slots, thus mitigating the efficiency penalty. The base station 4 can retransmit the data units received in error out of order when assigned a unique sequence number. In an example embodiment, if a hole (or unsent data unit) occurs (e.g. as a result of a soft handover from one base station 4 to another base station 4), the missing data unit is considered as being received in error. Mobile station 6 sends a NACK message for the missing data units and these data units are retransmitted.

In the exemplary embodiment, each base station 4 in the active set maintains a separate data queue 40 containing data to be transmitted to mobile station 6. The selected base station 4 transmits the data present in its data queue 40 in sequence, except for the retransmission of the erroneously received data unit and the signaling message. In the exemplary embodiment, the transmitted data units are deleted in queue 40 after transmission.

Fig. 3 is a flow chart describing steps for joining one or more base stations to an active set according to an example embodiment. Within block 300, the signal quality (such as C/I) received at the mobile station is measured for each available base station. The set of available base stations to search (sometimes referred to as the neighbor and candidate sets) can be transmitted from one or more base stations to the mobile station, or may include base stations searched by the mobile station. Assume that there are N candidate base stations. Each will be tested for inclusion in the active set in a loop that begins at block 310, setting i to 1.

C/I (C/I) received at the mobile station from the ith base station_i) And compared to a first threshold (threshold 1) in decision block 320. If C/I_iAbove threshold 1, base station i is added to the active set in block 350. Flow then proceeds to block 360 to determine whether additional candidates need to be tested. If C/I_iIf the threshold 1 is not exceeded, processing proceeds to block 330 to determine if another sector of base station i is already included in the active set. If so, then C/I_iAnd compared to a second (generally lower) threshold (threshold 2) at decision block 340. If the second threshold is exceeded, processing proceeds to block 350 to join base station i to the active set. As described above, adding additional sectors to the active set for which the base station already exists has relatively low requirements for queue maintenance and backhaul network traffic, so a lower threshold for these sectors is appropriate. If there are no other sectors of base station i in the active set at decision block 330 or if the threshold 2 is not exceeded at decision block 340, processing proceeds to block 360.

At block 360, i is incremented by one, and if i > N in decision block 370, the entire candidate set has been tested, and flow proceeds to block 380. If i is less than or equal to N, additional candidates remain to be tested and the loop is repeated by proceeding to block 320, as described above. In block 380, a message is sent to one or more base stations and to a base station controller to indicate the new status of the active set. Various methods are known to establish and send such messages.

It is noted that any number of threshold determination processes may be applied to threshold 1 and threshold 2. For example, a simple static threshold may be used to determine both thresholds. Alternatively, the relative static threshold may be determined based on system variables, such as the overall load or relative GOS of the mobile station in question. As described above, a threshold may be calculated for each new candidate such that the added benefit of adding the candidate exceeds some incremental advantage threshold. A variety of procedures for determining the threshold value are contemplated and within the scope of the invention.

Fig. 4 is a flow chart describing steps for removing one or more base stations from an active set according to an example embodiment. This is similar to the process described with reference to fig. 3 for joining a base station. In block 400, the signal quality (such as C/I) received at the mobile station is measured for each base station in the active set. Assume that the active set contains N base stations. Each will be tested in a loop to determine whether it is to remain in the active set, which is initiated by setting i to 1 in block 410.

C/I (C/I) received at the mobile station from the ith base station_i) And compared to a second threshold (threshold 2) in decision block 420. If C/I_iIf threshold 2 is not exceeded, then base station i is removed from the active set in block 450. Flow then proceeds to block 460 to determine whether additional candidates need to be tested. If C/I_iBeyond threshold 2, processing proceeds to block 430 where C/I_iCompared with a first threshold (generally higher than a second threshold) -threshold 1. If threshold 1 is not exceeded, processing proceeds to block 440 to determine if base station i has another sector in the active set. If not, processing proceeds to block 450 to remove base station i from the active set. If threshold 1 is exceeded at decision block 430 or another sector of base station I already exists in the active set at decision block 440, base station I need not be removed from the active set. Processing proceeds to block 460. (it is noted that it may be desirable to add an additional step (not shown) to ensure that two base station sectors do not remain in the active set at all times (when neither exceeds the first, i.e., higher, threshold)).

At block 460, i is incremented by one, and if i > N within decision block 470, the entire candidate set has been tested, and flow proceeds to block 480. If i is less than or equal to N, additional candidates remain to be tested and the loop is repeated by proceeding to block 420, as described above. In block 480, a message is sent to one or more base stations and to a base station controller to indicate the new status of the active set. Various methods are known for creating and sending such messages.

It is noted that threshold 1 and threshold 2 described for fig. 4 may be the same as the corresponding thresholds described in connection with fig. 3. However, this is also not essential — the removal threshold may be different from the addition threshold. The various techniques described in fig. 3 for determining the threshold may be applied to the threshold described in fig. 4.

Fig. 5 is a flow chart describing the use of multiple handover criteria according to an example embodiment. In block 500, the received C/I from each base station in the active set is measured at the mobile station. In decision block 510, the maximum C/I is compared to a threshold. As described above, other forward link quality measurements may be substituted for C/I. Various methods of determining the threshold are disclosed above. For example, a minimum threshold may be set such that a corresponding minimum data rate can be supported given the C/I measurement. The threshold may be set differently in different mobile stations (if there are different GOS requirements between them). The threshold may be changed based on the average throughput received by the mobile station in the past. For example, a lower C/I and therefore a lower data rate may be acceptable for a given mobile station at a given time, but may need to be increased if the average throughput is not acceptable for a given GOS.

If the maximum C/I is above the threshold at decision block 510, then a standby soft handoff may be performed. Only a single base station with the largest C/I is required to transmit an acceptable data rate. In block 520, a DRC message for a standby soft handoff is prepared to identify the base station. The message may also indicate the data rate or give a measure of the forward link quality, as described above. The message is transmitted to the base station in block 530 and may specifically be transmitted to the base station using the Walsh covering technique described above. The base station receives the message at block 540 and transmits to the mobile station in the next time slot. Notably, the base station may continue to monitor DRC messages from the mobile as the mobile environment changes.

If the maximum C/I does not exceed the threshold at decision block 510, a forward link soft handoff is required to obtain the C/I threshold. At block 550, one or more additional base stations whose combined transmitted C/I exceeds a minimum threshold are identified. If the active set base stations are sorted in descending order of received C/I, one approach is to test the combined C/I of the base stations that join in order until a threshold is reached. Numerous other methods are contemplated and are within the scope of the invention.

At block 560, a DRC message including a soft handover of the associated base station is prepared. At block 570, the message is sent through one or more base stations to a base station controller. In block 580, the base station controller sets the forward link for the soft handoff to transmit to the mobile station in the next available time slot.

It is noted that in all embodiments disclosed above, method steps may be interchanged without departing from the scope of the invention.

Those of skill in the art would understand that information and signals may be represented using any of a variety of different technologies and techniques. For example, data, instructions, commands, information, signals, bits, symbols, and chips that may be referenced throughout the above description may be represented by voltages, circuits, electromagnetic waves, magnetic fields or particles, optical fields or particles, or any combination thereof.

Those of skill would further appreciate that the various illustrative logical blocks, modules, and algorithm steps described in connection with the embodiments disclosed herein may be implemented as electronic hardware, computer software, or combinations of both. To clearly illustrate this interchangeability of hardware and software, various illustrative components, blocks, modules, circuits, and steps have been described above generally in terms of their functionality. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. Skilled artisans may implement the described functionality for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the present invention.

The various illustrative logical blocks, modules, and circuits described in connection with the embodiments disclosed herein may be implemented or performed with: a general purpose processor, a Digital Signal Processor (DSP), an Application Specific Integrated Circuit (ASIC), a Field Programmable Gate Array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof. A general purpose processor is preferably a microprocessor, but in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine. A processor may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration.

The steps of a method or algorithm described in connection with the embodiments disclosed herein may be embodied directly in hardware, in a software module executed by a processor, or in a combination of the two. A software module may reside in RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, hard disk, a removable disk, a CD-ROM, or any other form of storage medium known in the art. An exemplary storage medium is coupled to the processor such the processor can read information from, and write information to, the storage medium. In the alternative, the storage medium may be integral to the processor. The processor and the storage medium may reside in an application specific integrated circuit, ASIC. The ASIC may reside in a user terminal. In addition, the processor and the storage medium may reside as discrete components in a user terminal.

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 (6)

1. A method of high speed data transmission from one or more sectorized base stations to a mobile station, said method comprising:

measuring forward link signal quality from one or more sectors of one or more base stations;

adding one or more sectors of one or more base stations to the active set when the forward link signal quality from each sector exceeds a higher threshold; and

one or more sectors of the one or more base stations are added to the active set when the forward link signal quality from each sector exceeds a lower threshold and at least another sector of the base stations of that sector is in the active set.

2. The method of claim 1, further comprising:

transmitting from the sector with the highest measured forward link signal quality to the mobile station in the timeslot when said measured forward link signal quality meets a certain criterion; and

when the highest measured forward link signal quality does not meet certain criteria, transmissions are made to the mobile station from one or more sectors in soft handoff during the time slot.

3. The method of claim 1 wherein said upper threshold is determined based on adding a minimum margin improvement factor to the forward link signal quality from the sector currently in the active set.

4. The method of claim 1 wherein the lower threshold is determined based on adding a minimum margin improvement factor to the forward link signal quality from the sector currently in the active set.

5. The method of claim 1, further comprising removing a sector from an active set when said forward link signal from said sector falls below a removal threshold.

6. The method of claim 5, wherein the removal threshold is a lower threshold when more than one sector from a base station is in the active set and a higher threshold when the sector is the only sector for which the base station is in the active set.