WO1998054916A1

WO1998054916A1 - Method and apparatus for performing mobile station handoff in a code division multiple access communication system

Info

Publication number: WO1998054916A1
Application number: PCT/US1998/002430
Authority: WO
Inventors: Amitava Ghosh
Original assignee: Motorola Inc.
Priority date: 1997-05-30
Filing date: 1998-02-10
Publication date: 1998-12-03

Abstract

A mobile station (106) in a code division multiple access (CDMA) communication system is instructed to transmit a temporary message which is used to aid in inter-frequency hard handoff. A source base-station (102) transmits to the mobile station (106) a power-up message which includes a power level relative to the nominal power of the mobile station (106) and frequency at which the mobile station (106) is to transmit the temporary message. Target base-stations (102) receive the temporary message and report characteristics of the temporary message to the source base-station (102) for handoff determination.

Description

METHOD AND APPARATUS FOR PERFORMING MOBILE

STAΉON HANDOFF IN A CODE DIVISION

MULTIPLE ACCESS COMMUNICATION SYSTEM

Field of the Invention

The present invention relates, in general, to communication systems and, more particularly, to handoff in such communication systems.

Background of the Invention

One problem associated with a Code Division Multiple Access (CDMA) communication system is how to handle inter-system hard handoff. FIG. 1 generally depicts the problem. As shown in FIG. 1, a border 100 separates a first system A and a second system B, where each system A and B operate at different frequencies F and F2. In this embodiment, system A and system B are systems of different metropolitan trading areas (MTAs). A mobile station 106 moving from system A to system B requires a hard handoff (handoff from one frequency to another) from frequency F to frequency F2. As shown in FIG. 1, system B is another CDMA cellular communication system at a different frequency compared to system A.

One solution to the above problem has been to place a beacon signal transmitter 109 along the border. Generally, the beacon signal transmitter 109 is used to alert the mobile station 106 that a handoff is necessary so that the mobile station 106 can take appropriate action. To implement the beacon signal transmitter 109, a transmitter capable of transmitting a pilot signal at the same frequency Fi as the pilot signal of system A is installed at some location within system B. In this solution, the pilot signal transmitted by the beacon signal transmitter 109 should be in the neighbor set of the system A base-station 102. As is clear from this description, the solution of the beacon signal transmitter 109 is undesirable since it requires operators to purchase additional transmitter equipment which increases the overall cost of the CDMA communication system.

Thus, a need exists for an improved method and apparatus for performing mobile station handoff in a CDMA communication system in a reliable and cost effective manner.

Brief Description of the Drawings

FIG. 1 generally illustrates the problem associated with hard handoff in a CDMA cellular communication system.

FIG. 2 generally depicts a transmitter of a mobile station in

CDMA communication with a receiver of a base station in a manner which may beneficially implement the present invention. FIG. 3 generally depicts a transmitter of a base station in CDMA communication with a receiver of a mobile station in a manner which may beneficially implement the present invention.

FIG. 4 generally depicts the structure of a temporary message transmitted to aid in handoff in accordance with the invention. FIG. 5 generally depicts, in flow diagram form, the steps performed to initiate inter-frequency hard handoff in accordance with the invention.

Detailed Description of the Preferred Embodiment

A mobile station in a code division multiple access (CDMA) communication system is instructed to transmit a temporary message which is used to aid in inter-frequency hard handoff. A source base- station transmits to the mobile station a power-up message which includes a power level relative to the nominal power of the mobile station and frequency at which the mobile station is to transmit the temporary message. Target base-stations receive the temporary message and report characteristics of the temporary message to the source base- station for handoff determination. To aid in the explanation of the present invention, the operation of the transmitters and receivers implemented in both a base-station 102 and a mobile station 106 are beneficial. FIG. 2 generally depicts a transmitter 200 of the mobile station 106 in CDMA communication with a receiver 203 of the base station 102 in a manner which may beneficially implement the present invention. In the encoding portion 201 of the communication system, traffic channel data bits 202 originate from a microprocessor (μP) 205, and are input to an encoder 204 at a particular bit rate (e.g., 9.6 kilobit/second). The μP 205 is coupled to a block designated related functions 207, where functions including call processing, link establishment, and other general functions related to establishing and maintaining cellular communication are performed. The traffic channel data bits 202 can include either voice converted to data by a vocoder, pure data, or a combination of the two types of data. Encoder 204 encodes the traffic channel data bits 202 into data symbols 206 at a fixed encoding rate (1/r) with an encoding algorithm which facilitates subsequent maximum likelihood decoding of the data symbols into data bits (e.g., convolutional or block coding algorithms). For example, encoder 204 encodes traffic channel data bits 202 (e.g., 192 input data bits that were received at a rate of 9.6 kilobits /second) at a fixed encoding rate of one data bit to three data symbols (i.e., 1/3) such that the encoder 204 outputs data symbols 206 (e.g., 576 data symbols output at a 28.8 kilo symbols /second rate).

The data symbols 206 are then input into an interleaver 208. Interleaver 208 organizes the data symbols 206 into blocks (i.e., frames) and block interleaves the input data symbols 206 at the symbol level. In the interleaver 208, the data symbols are individually input into a matrix which defines a predetermined size block of data symbols. The data symbols are input into locations within the matrix so that the matrix is filled in a column by column manner. The data symbols are individually output from locations within the matrix so that the matrix is emptied in a row by row manner. Typically, the matrix is a square matrix having a number of rows equal to the number of columns; however, other matrix forms can be chosen to increase the output interleaving distance between the consecutively input non-interleaved data symbols. The interleaved data symbols 110 are output by the interleaver 208 at the same data symbol rate that they were input (e.g., 28.8 kilo symbols /second). The predetermined size of the block of data symbols defined by the matrix is derived from the maximum number of data symbols which can be transmitted at a coded bit rate within a predetermined length transmission block. For example, if data symbols 206 are output from the encoder 204 at a 28.8 kilo symbols /second rate, and if the predetermined length of the transmission block is 20 milliseconds, then the predetermined size of the block of data symbols is 28.8 kilo symbols /second times 20 milliseconds (ms) which equals 576 data symbols which defines a 18 by 32 matrix.

The encoded, interleaved data symbols 210 is output from encoding portion 201 of the communication system and input to a transmitting portion 216 of the communication system. The data symbols 210 are prepared for transmission over a communication channel by a modulator 217. Subsequently, the modulated signal is provided to an antenna 218 for transmission over the digital radio channel 108.

The modulator 217 prepares the data symbols 210 for direct sequence code divided spread-spectrum transmission by deriving a sequence of fixed length codes from the encoded, interleaved data symbols 210 in a spreading process. For example, the data symbols within the stream of reference-coded data symbols 210 may be spread to a unique fixed length code such that a group of six data symbols is represented by a single 64 bit length code. The codes representing the group of six data symbols preferably are combined to form a single 64 bit length code. As a result of this spreading process, the modulator 217 which received the encoded, interleaved data symbols 210 at a fixed rate (e.g., 28.8 kilo symbols /second) now has a spread sequence of 64 bit length codes having a higher fixed symbol rate (e.g., 307.2 kilo symbols /second). It will be appreciated by those skilled in the art that the data symbols within the stream of encoded, interleaved data bits 210 may be spread according to numerous other algorithms into a sequence of larger length codes without departing from the scope and spirit of the present invention. The spread sequence is further prepared for direct sequence code divided spread-spectrum transmission by further spreading the spread sequence with a long spreading code (e.g., PN code). The spreading code is a user specific sequence of symbols or unique user code which is output at a fixed chip rate (e.g., 1.228 Megachips/ second). In addition to providing an identification as to which user sent the encoded traffic channel data bits 202 over the digital radio channel 108, the unique user code enhances the security of the communication in the communication channel by scrambling the encoded traffic channel data bits 202. In addition, the user code spread encoded data bits (i.e., data symbols) are used to bi-phase modulate a sinusoid by driving the phase controls of the sinusoid. The sinusoid output signal is bandpass filtered, translated to an RF frequency, amplified, filtered and radiated by an antenna 218 to complete transmission of the traffic channel data bits 202 in a digital radio channel 108 with Binary Phase Shift Keyed (BPSK) modulation.

A receiving portion 222 of the base station receiver 203 receives the transmitted spread-spectrum signal from over the digital radio channel 108 through antenna 224. The received signal is sampled into data samples by despreader and sampler 226. Subsequently, the data samples 242 are output to the decoding portion 254 of the communication system.

The despreader and sampler 226 preferably BPSK samples the received spread-spectrum signal by filtering, demodulating, translating from the RF frequencies, and sampling at a predetermined rate (e.g., 1.2288 Megasamples /second). Subsequently, the BPSK sampled signal is despread by correlating the received sampled signals with the long spreading code. The resulting despread sampled signal 228 is sampled at a predetermined rate and output to a non-coherent detector 240 (e.g., 307.2 kilo samples /second so that a sequence of four samples of the received spread-spectrum signal is despread and /or represented by a single data sample) for later non-coherent detection of data samples 242.

As will be appreciated by those skilled in the art, multiple receiving portions 222 through 223 and antennae 224 through 225, respectively, can be used to achieve space diversity. The Nth receiver portion would operate in substantially the same manner to retrieve data samples from the received spread-spectrum signal in digital radio channel 108 as the above described receiving portion 222. The outputs 242 through 252 of the N receiving portions preferably are input to a summer 250 which diversity combines the input data samples into a composite stream of coherently detected data samples 260.

The individual data samples 260 which form soft decision data are then input into a decoding portion 254 including a deinterleaver 262 which deinterleaves the input soft decision data 260 at the individual data level. In the deinterleaver 262, the soft decision data 260 are individually input into a matrix which defines a predetermined size block of soft decision data. The soft decision data are input into locations within the matrix so that the matrix is filled in a row by row manner. The deinterleaved soft decision data 264 are individually output from locations within the matrix so that the matrix is emptied in a column by column manner. The deinterleaved soft decision data 264 are output by the deinterleaver 262 at the same rate that they were input (e.g., 28.8 kilometrics/ second).

The predetermined size of the block of soft decision data defined by the matrix is derived from the maximum rate of sampling data samples from the spread-spectrum signal received within the predetermined length transmission block.

The deinterleaved soft decision data 264, are input to a decoder 266 which uses maximum likelihood decoding techniques to generate estimated traffic channel data bits 268. The maximum likelihood decoding techniques may be augmented by using an algorithm which is substantially similar to a Viterbi decoding algorithm. The decoder 266 uses a group of the individual soft decision data 264 to form a set of soft decision transition metrics for use at each particular time state of the maximum likelihood sequence estimation decoder 266. The number of soft decision data 264 in the group used to form each set of soft decision transition metrics corresponds to the number of data symbols 206 at the output of the convolutional encoder 204 generated from each input data bit 202. The number of soft decision transition metrics in each set is equal to two raised to the power of the number of soft decision data 264 in each group. For example, when a 1/3 convolutional encoder is used in the transmitter, three data symbols 106 are generated from each input data bit 202. Thus, decoder 266 uses groups of three individual soft decision data 264 to form eight soft decision transition metrics for use at each time state in the maximum likelihood sequence estimation decoder 266. The estimated traffic channel data bits 268 are generated at a rate related to the rate that the soft decision data 264 are input to the decoder 266 and the fixed rate used to originally encode the input data bits 202 (e.g., if the soft decision data are input at 28.8 kilometrics /second and the original encoding rate was 1/3 then estimated traffic channel data bits 268 are output at a rate of 9600 bits /second).

The estimated traffic channel data bits 268 are input into a μP 270, which is similar to μP 207. As in the case of μP 207, the μP 270 is coupled to a block designated related functions 272, this block also performing functions including call processing, link establishment, and other general functions related to establishing and maintaining cellular communication. The μP 270 is also coupled to an interface 274, which allows the receiver 203 of the base station 102 to communicate with the CBSC 114.

FIG. 3 generally depicts a transmitter 300 of the base station 102 in CDMA communication with a receiver 303 of the mobile station 106 in a manner which may beneficially implement the present invention. In the encoding portion 301 of the communication system, traffic channel data bits 302 are output from a μP 305, and are input to an encoder 304 at a particular bit rate (e.g., 9.6 kilobit/ second). The μP 305 is coupled to a block designated related functions 307, which performs similar cellular-related functions as blocks 207 and 272 of FIG. 2. The μP 305 is also coupled to an interface 309 which allows the transmitter 300 of base station 102 to communicate with the CBSC 114.

The traffic channel data bits 302 can include either voice converted to data by a vocoder, pure data, or a combination of the two types of data. Encoder 304 encodes the traffic channel data bits 302 into data symbols 306 at a fixed encoding rate (1/r) with an encoding algorithm which facilitates subsequent maximum likelihood decoding of the data symbols into data bits (e.g., convolutional or block coding algorithms). For example, encoder 304 encodes traffic channel data bits 302 (e.g., 192 input data bits that were received at a rate of 9.6 kilobits /second) at a fixed encoding rate of one data bit to two data symbols (i.e., 1/2) such that the encoder 304 outputs data symbols 306 (e.g., 384 data symbols output at a 19.2 kilo symbols /second rate).

The data symbols 306 are then input into an interleaver 308. Interleaver 308 organizes the data symbols 306 into blocks (i.e., frames) and block interleaves the input data symbols 306 at the symbol level. In the interleaver 308, the data symbols are individually input into a matrix which defines a predetermined size block of data symbols. The data symbols are input into locations within the matrix so that the matrix is filled in a column by column manner. The data symbols are individually output from locations within the matrix so that the matrix is emptied in a row by row manner. Typically, the matrix is a square matrix having a number of rows equal to the number of columns; however, other matrix forms can be chosen to increase the output interleaving distance between the consecutively input non-interleaved data symbols. The interleaved data symbols 310 are output by the interleaver 308 at the same data symbol rate that they were input (e.g., 19.2 kilo symbols /second). The predetermined size of the block of data symbols defined by the matrix is derived from the maximum number of data symbols which can be transmitted at a coded bit rate within a predetermined length transmission block. For example, if data symbols 306 are output from the encoder 304 at a 19.2 kilo symbols /second rate, and if the predetermined length of the transmission block is 20 milliseconds, then the predetermined size of the block of data symbols is 19.2 kilo symbols /second times 20 milliseconds (ms) which equals 384 data symbols which defines a 18 by 32 matrix. The encoded, interleaved data symbols 310 are output from encoding portion 301 of the communication system and input to a transmitting portion 316 of the communication system. The data symbols 310 are prepared for transmission over a communication channel by a modulator 317. Subsequently, the modulated signal is provided to an antenna 318 for transmission over the digital radio channel 108.

The modulator 317 prepares the data symbols 310 for direct sequence code divided spread-spectrum transmission by performing data scrambling on the encoded, interleaved data symbols 310. Data scrambling is accomplished by performing the modulo-2 addition of the interleaver output symbols 310 with the binary value of a long code pseudo-noise PN chip that is valid at the start of the transmission period for that symbol. This pseudo-noise PN sequence is the equivalent of the long code operating at 1.2288 MHz clock rate, where only the first output of every 64 is used for the data scrambling (i.e., at a 19200 sample per second rate).

After scrambling, a sequence of fixed length codes from the scrambled data symbols are derived in a spreading process. For example, each data symbol within the stream of scrambled data symbols may preferably be spread to a unique fixed length code such that each data symbol is represented by a single 64 bit length code. The code representing the data symbol preferably is modulo-2 added to the respective data symbol. As a result of this spreading process, the modulator 317 which received the encoded, interleaved data symbols 310 at a fixed rate (e.g., 19.2 kilo symbols /second) now has a spread sequence of 64 bit length codes having a higher fixed symbol rate (e.g., 1228.8 kilo symbols /second). It will be appreciated by those skilled in the art that the data symbols within the stream of encoded, interleaved data bits 310 may be spread according to numerous other algorithms into a sequence of larger length codes without departing from the scope and spirit of the present invention.

The spread sequence is further prepared for direct sequence code divided spread-spectrum transmission by further spreading the spread sequence with a long spreading code (e.g., PN code). The spreading code is a user specific sequence of symbols or unique user code which is output at a fixed chip rate (e.g., 1.2288 Megachips /second). In addition to providing an identification as to which user sent the encoded traffic channel data bits 302 over the digital radio channel 308, the unique user code enhances the security of the communication in the communication channel by scrambling the encoded traffic channel data bits 302. In addition, the user code spread encoded data bits (i.e., data symbols) are used to bi-phase modulate a sinusoid by driving the phase controls of the sinusoid. The sinusoid output signal is bandpass filtered, translated to an RF frequency, amplified, filtered and radiated by an antenna 318 to complete transmission of the traffic channel data bits 302 in a digital radio channel 108 with BPSK modulation.

A receiving portion 322 of the mobile station receiver 303 receives the transmitted spread-spectrum signal from over the digital radio channel 108 through antenna 324. The received signal is sampled into data samples by despreader and sampler 326. Subsequently, the data samples 342 are output to the decoding portion 354 of the communication system.

The despreader and sampler 326 preferably BPSK samples the received spread-spectrum signal by filtering, demodulating, translating from the RF frequencies, and sampling at a predetermined rate (e.g., 1.2288 Megasamples /second). Subsequently, the BPSK sampled signal is despread by correlating the received sampled signals with the long spreading code. The resulting despread sampled signal 328 is sampled at a predetermined rate and output to a non-coherent detector 340 (e.g., 19.2 kilo samples /second so that a sequence of 64 samples of the received spread-spectrum signal is despread and /or represented by a single data sample) for non-coherent detection of data samples 342.

As will be appreciated by those skilled in the art, multiple receiving portions 322 through 323 and antennae 324 through 325, respectively, can be used to achieve space diversity. The Nth receiver portion would operate in substantially the same manner to retrieve data samples from the received spread-spectrum signal in digital radio channel 320 as the above described receiving portion 322. The outputs 342 through 352 of the N receiving portions preferably are input to a summer 350 which diversity combines the input data samples into a composite stream of coherently detected data samples 360.

The individual data samples 360 which form soft decision data are then input into a decoding portion 354 including a deinterleaver 362 which deinterleaves the input soft decision data 360 at the individual data level. In the deinterleaver 362, the soft decision data 360 are individually input into a matrix which defines a predetermined size block of soft decision data. The soft decision data are input into locations within the matrix so that the matrix is filled in a row by row manner. The deinterleaved soft decision data 364 are individually output from locations within the matrix so that the matrix is emptied in a column by column manner. The deinterleaved soft decision data 364 are output by the deinterleaver 362 at the same rate that they were input (e.g., 19.2 kilometrics /second). The predetermined size of the block of soft decision data defined by the matrix is derived from the maximum rate of sampling data samples from the spread-spectrum signal received within the predetermined length transmission block.

The deinterleaved soft decision data 364, are input to a decoder 366 which uses maximum likelihood decoding techniques to generate estimated traffic channel data bits 368. The maximum likelihood decoding techniques may be augmented by using an algorithm which is substantially similar to a Viterbi decoding algorithm. The decoder 366 uses a group of the individual soft decision data 364 to form a set of soft decision transition metrics for use at each particular time state of the maximum likelihood sequence estimation decoder 366. The number of soft decision data 364 in the group used to form each set of soft decision transition metrics corresponds to the number of data symbols 306 at the output of the convolutional encoder 304 generated from each input data bit 302. The number of soft decision transition metrics in each set is equal to two raised to the power of the number of soft decision data 364 in each group. For example, when a 1/2 convolutional encoder is used in the transmitter, two data symbols 306 are generated from each input data bit 302. Thus, decoder 366 uses groups of two individual soft decision data 364 to form four soft decision transition metrics for use at each time state in the maximum likelihood sequence estimation decoder 366. The estimated traffic channel data bits 368 are generated at a rate related to the rate that the soft decision data 364 are input to the decoder 366 and the fixed rate used to originally encode the input data bits 302 (e.g., if the soft decision data are input at 19.2 kilometrics /second and the original encoding rate was 1/2 then estimated traffic channel data bits 368 are output at a rate of 9600 bits/second). The estimated traffic channel data bits 368 are input into a μP 370 which interprets the estimated traffic channel data bits 368 and other fields transmitted in the digital radio channel 108. The μP 370 is coupled to related functions 372 which performs cellular-related functions similar to those performed by blocks 207, 272 and 307.

FIG. 4 generally depicts the structure of a temporary message transmitted to aid in handoff in accordance with the invention. In this approach, handoff is performed by allowing base-station 104 to make quality measurements of mobile station 106. As shown in FIG. 4, the mobile station 106 is shown transmitting only for a portion of time within a predetermined number of frames. As shown in FIG. 4, the parameter MSG_SIZE is included in the power-up message and instructs the mobile station to review a predetermined number of frames for a frame where information is to be transmitted at a rate less than full rate. It should be noted that the parameter names mentioned herein are generic in nature and thus their use is not intended to limit the scope of the invention. Continuing, in the preferred embodiment, the predetermined number of frames reviewed by the mobile station 106 and represented by MSG_SIZE is 10 frames of information. As can be seen in FIG. 4, the temporary message is transmitted during the 3^rd frame (where a frame corresponds to 16 power control groups or PCGs) within the MSG_SIZE parameter, which means that the mobile station determined that an 1/8 rate frame would normally be transmitted during that frame. The particular frame in which the temporary message is transmitted is variable (as shown by its location within MSG_SIZE period 2. As also shown in FIG. 4, a parameter MSG_PERIOD is transmitted to the mobile station 106 to indicate the general overall repetition rate for transmission of the temporary message.

As also shown in FIG. 4, the parameters INC_PWR and PWR_STEP are also included in the power-up message. For the first time (shown as MSG_SIZE period 1 in FIG. 4) the mobile station 106 is instructed to transmit the temporary message, the parameter INC_PWR is used to inform the mobile station 106 the power above the nominal power of the mobile station 106 the mobile station is to transmit. For subsequent transmissions (shown as MSG_SIZE period 2 in FIG. 4), the mobile station uses parameter PWR_STEP to adjust the power level of the temporary message relative to the power level of the prior transmission (in this example, the level represented by INC_PWR). In this manner, the mobile station 106 transmits the temporary message at a power level above the nominal power level of the mobile station 106 in accordance with the invention. FIG. 5 generally depicts, in flow diagram form, the steps performed to initiate inter-frequency (Fi to F2) handoff (commonly referred to as "hard handoff") in accordance with the invention. To begin, a target base-station 104 (or a plurality of target base-stations) decides at step 503 whether the mobile station 106 is a candidate for hard handoff as is well known in the art. If the mobile station 106 is determined to be a candidate for hard handoff, the source base-station 102 sends at step 506 a power-up message to the mobile station 106. In the preferred embodiment, the power-up message is known a priori and includes the power relative to the nominal power of the mobile station and frequency at which the mobile station 106 is to transmit the temporary message. Also in this embodiment, the frequency at which the mobile station 106 is to transmit is the second frequency F2 as shown in FIG. 1, since the mobile station 106 is currently being serviced on frequency Fi. In response to receiving the power-up message at the mobile station 106, the mobile station 106 will transmit at step 509 the temporary message at full rate during a time period when the mobile station should otherwise transmit at a rate less than full rate and will also transmit the temporary message at a power greater than the nominal power of the mobile station. In the preferred embodiment, the mobile station 106 determines when the temporary message is to be transmitted by determining when a frame of information is to be next transmitted at a rate less than full rate, and transmits the temporary message at full rate during that time period. For this implementation, the rates less than full rate include 1/2, 1 /4 or 1/8 rate. By choosing the time period to transmit the temporary message when less than full rate frames would normally be transmitted, minimal degradation in speech quality occurs since a reduced amount of reverse link (mobile station 106 to base-station 102, 104) information is lost. As one skilled in the art will appreciate, the least amount of reverse link information is lost when the mobile station 106 transmits the temporary message during a 1/8 rate frame.

To maintain speech quality in both the forward and the reverse link, the mobile station 106 can be instructed to transmit the temporary message during a time period when the source base-station 102 is transmitting frames of information to the mobile station at a rate less than full rate. In this embodiment, the time period information is provided to the mobile station 106 by the source base-station 102 via puncturing the frames of information with information associated with the time period. In this situation, the mobile station 106 is made aware of when the source base-station 102 will transmit at a rate less than full rate, and can coordinate the transmission of the temporary message accordingly.

If the call quality, as determined by the source base-station 102, is found to be deteriorating rapidly (for example, due to a rapid increase in frame error rate or FER), the mobile station 106 is instructed by the source base-station 102 to send a temporary message immediately instead of determining when a lower rate frame is to be transmitted as described above. In this manner, initiation of hard handoff can be forced to occur almost instantly when call quality is found to be degrading, which leads to fewer dropped calls caused by poor call quality.

After the mobile station 106 transmits the temporary message, the target base-station 104 (or a plurality of target base-stations) receives the temporary message on F2 at step 512 and determines characteristics related to the temporary message. In the preferred embodiment, the characteristics related to the temporary message include an estimate of the received energy of the temporary message, the range of the mobile station 106, an estimate of the signal strength of the temporary message, an estimate of the signal quality of the temporary message, etc. The characteristics related to the temporary message are then forwarded to the source base-station 102. Based on the characteristics, the source base-station 102 effects handoff of the mobile station from the source base-station to the target base-station 102 in accordance with the invention. If a plurality of target base-stations receive the temporary message transmitted by the mobile station 106, then handoff is performed from the source base-station 102 to a chosen one of the plurality of target base-stations in accordance with the invention.

While the invention has been particularly shown and described with reference to a particular embodiment, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention. The corresponding structures, materials, acts and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or acts for performing the functions in combination with other claimed elements as specifically claimed. What I claim is:

Claims

1. A method of performing mobile station handoff in a code division multiple access (CDMA) communication system, the CDMA communication system implementing variable rate transmission, the method comprising the steps of:

transmitting, from a source base-station, a power-up message to the mobile station indicating to the mobile station a frequency at which the mobile station is to transmit a temporary message; receiving, at the mobile station, the power-up message; transmitting, at the mobile station, the temporary message during a time period when the mobile station should transmit at a rate less than full rate; and receiving, at a target base-station, the temporary message transmitted by the mobile station to effect handoff of the mobile station from the source base-station to the target base-station.

2. The method of claim 1, wherein the mobile station handoff is performed from a CDMA communication system having a CDMA base-station at a first frequency to a CDMA communication system having a CDMA base-station at a second frequency.

3. The method of claim 1, wherein the mobile station determines when the temporary message is to be transmitted by determining when a frame of information is to be next transmitted at a rate of either 1/2, 1/4 or 1/8 rate.

4. The method of claim 1, wherein the step of transmitting the temporary message further comprises the step of transmitting the temporary message at full rate during a time period when the mobile station should otherwise transmit at a rate less than full rate.

5. The method of claim 1, wherein the handoff of the mobile station is effected based on characteristics related to the temporary message which include an estimate of the received energy of the temporary message, the range of the mobile station, an estimate of the signal strength of the temporary message and an estimate of the signal quality of the temporary message.

6. A mobile station compatible with a code division multiple access (CDMA) communication system, the mobile station requiring handoff from a CDMA communication system at a first frequency to a CDMA communication system at a second frequency, the mobile station comprising:

means for receiving a power-up message indicating to the mobile station a frequency at which the mobile station is to transmit a temporary message; and means for transmitting the temporary message at full rate during a time period when the mobile station should otherwise transmit at a rate less than full rate, wherein the transmission of the temporary message is at a power greater than a nominal power of the mobile station.

7. The mobile station of claim 6, wherein the mobile station determines when the temporary message is to be transmitted by determining when a frame of information is to be next transmitted at a rate of either 1/2, 1/4 or 1/8 rate.

8. A code division multiple access (CDMA) communication system for performing mobile station handoff, the CDMA communication system implementing variable rate transmission, the CDMA communication system comprising:

a transmitter, at a source base-station, for transmitting a power- up message to the mobile station indicating to the mobile station a frequency at w^╧ähich the mobile station is to transmit a temporary message; a receiver, at the mobile station, for receiving the power-up message; a transmitter, at the mobile station, for transmitting the temporary message at full rate during a time period when the mobile station should otherwise transmit at a rate less than full rate, wherein the transmission of the temporary message is at a power greater than a nominal power of the mobile station; and a receiver, at a target base-station, for receiving the temporary message transmitted by the mobile station to effect handoff of the mobile station from the source base-station to the target base-station.

9. The CDMA communication system of claim 8, wherein the power at which the mobile station transmits the temporary message relative to the nominal power of the mobile station is specified in the power-up message.

10. The CDMA communication system of claim 8, wherein the transmitter for transmitting the temporary message transmits the temporary message during a time period when the source base-station is transmitting frames of information to the mobile station at a rate less than full rate, the time period provided to the mobile station via puncturing the frames of information with information associated with the time period.