WO2004114549A1

WO2004114549A1 - Enhanced data only code division multiple access (cdma) system

Info

Publication number: WO2004114549A1
Application number: PCT/US2004/018388
Authority: WO
Inventors: Frank Zhou; Petteri Luukkanen; R. Thomas Derryberry; Chris Jensen; Liangchi Hsu; Zhouyue Pi; Prashanth Rao; Giridhar D. Mandyam
Original assignee: Nokia Corporation; Nokia, Inc.
Priority date: 2003-06-13
Filing date: 2004-06-10
Publication date: 2004-12-29

Abstract

A system and method for enhancing the High Rate Data Packet (HRPD) system as specified by IS-856. A Reverse Control Channel (RCCH) is punctured into the existing Reverse Traffic Channel (RTC) pilot channel (FIG. 4) allowing the access terminal to request a desired RTC data rate via pilot/RCCH channel (300), where RTC data channel (500) allows increased throughput. Fixed timing for rate request responses (RC 606, 608) and acknowledgment of reverse link frame quality (ARQ 614, 616) provides improved reverse link control. A forward Channel Quality Indicator (CQI) mechanism employs CQI channel (700) to allow the access terminal to report channel quality to the access network. The access network may then transmit the forward link allocation to the access terminal via Forward Rate Indication Channel (F-RICH 834) based upon the reported CQI.

Description

ENHANCED DATA ONLY CODE DIVISION MULTIPLE ACCESS (CDMA)

SYSTEM

FIELD OF THE INVENTION This invention relates in general to a wireless Code Division Multiple Access (CDMA) system, and more particularly, to a High Rate Packet Data (HRPD) CDMA system.

BACKGROUND OF THE INVENTION Code Division Multiple Access (CDMA) was introduced into cellular based, mobile communication systems in the early 1990s with the introduction of the IS- 95 standard. Since then, CDMA technology has been well accepted in the wireless industry and has been widely disseminated reaching literally hundreds of millions of subscribers throughout the world. New third generation (3G) standards, such as cdma2000, continue to develop backward compatible evolutions of the IS-95 system to further improve its voice service capacity while providing higher data rates for data services. As part of this evolution, the cdma2000 High Rate Packet Data (HRPD) system was developed in relation to the 3G harmonization efforts as endorsed by the International Telecommunications Union (ITU). Cdma2000 HRPD is defined by the cdma2000 HRPD Air Interface Specification, Pub. No. 3GPP2 C.S0024 version 4.0, and is incorporated herein by reference.

Cdma2000 HRPD (also known as IS-856) is optimized for wireless, high speed packet data services, such as may be facilitated by the Internet Protocol (IP). Due to the asymmetric characteristics of IP traffic, however, the IS-856 forward link has drawn much attention, where focused efforts have been introduced to optimize the IS-856 forward link throughput. The IS-856 forward link, for example, uses a Time-Division- Multiplexed (TDM) waveform, which eliminates power sharing among active users by allocating full sector power and all code channels to a single user at any instant. Such a power allocation improves efficiency as compared to the Code-Division-Multiplexed (CDM) waveform of the IS-95 forward link, which continuously transmits with a certain fraction of the total sector power that inevitably results in inefficient sector power usage. The efficient usage of sector power resources in IS-856 not only improves cell coverage, but also improves the Signal to Interference and Noise Ratio (SINR) for noise limited users. Due to the TDM waveform of the IS-856 forward link, an access terminal is allocated the full sector power whenever it is served, thus no power adaptation is needed. Rather, rate adaptation is used on the IS-856 forward link, due to the advantage that rate adaptation poses over power adaptation, since wireless packet data systems do not require a guaranteed quality of service.

In general, the highest data rate that can be transmitted to each access terminal in the IS-856 system is a function of the received SINR from the serving sector. In order to achieve the maximum data rate at each time of transmission, each access terminal predicts the channel condition over the next packet for its serving sector. The access terminal then selects the highest data rate that can be reliably decoded based upon the predicted SINR and then informs the serving sector as to that highest data rate using the reverse link feedback channel. Then, whenever the serving sector decides to serve the access terminal, it transmits at the most recently selected rate that was fed back from that particular access terminal.

Generally, the access terminal's channel prediction and subsequent rate selection for high data rates is quite accurate because short packet lengths are employed that exhibit a high degree of correlation with present and past channel states. For lower data rates, however, longer packet lengths are utilized, which decreases the correlation with present and past channel states. Thus, the access terminal is forced to be more conservative with its data rate selection in order to maintain a satisfactory Packet Error Rate (PER). Such conservatism, however, may result in a waste of transmit power if the access terminal has incorrectly predicted the optimal data rate. In order to reduce the potentially wasted transmission power, IS-856 defines a hybrid Automatic Repeat Request (ARQ) mechanism that can terminate the transmission of a multi-slot packet as soon as it can be correctly decoded by the access terminal. To accomplish this, an access terminal attempts to decode a packet whenever it receives a new portion of the packet and informs the access network to stop transmitting when the packet is correctly decoded. Thus, early packet decoding serves to shut down the potentially wasted transmission power on the IS-856 forward link through the use of the hybrid ARQ mechanism. Another mechanism employed by IS-856 utilizes Data Rate Control (DRC) messaging, which allows the access terminal to explicitly select its IS-856 forward link allocation, or Modulation and Coding Scheme (MCS). Based upon the DRC sent by the access terminal, the base station assigns the mobile terminal on the IS-856 forward link using an indication known as the traffic channel preamble that is sent to the access terminal. The access terminal, however, does not receive an indication of its MCS, since the MCS is only assigned if the base station can provide the requested MCS conveyed by the DRC message. A possible problem with this approach, therefore, is that the serving base station is forced to choose between either scheduling the access terminal at its requested MCS, or denying service to the access terminal altogether. Thus, the base station denies service to the access terminal by strictly adhering to the requested MCS, rather than employing a more lenient approach that may serve to reduce the number of service denials given to requesting access terminals by their serving sectors.

Since design efforts have primarily focused on increasing the efficiency of the IS-856 forward link, other problems have arisen due to the fact that deficiencies of the IS-856 reverse link have virtually been ignored. In particular, current IS-856 systems provide the access terminal with too much autonomy in its selection of the reverse data rate to be used, thus potentially limiting the access terminal's peak throughput on the reverse link. Additionally, IS-856 does not provide a fast ARQ mechanism on the reverse link, but rather requires the use of the Radio Link Protocol (RLP) to recover reverse link frame errors. Use of RLP, however, creates reverse link latency, where turnaround times on the order of 200 milliseconds (mS) are common.

Accordingly, there is a need in the communications industry for a system and method that increases the flexibility and throughput of the IS-856 reverse link. The present invention fulfills these and other needs, and offers other advantages over the prior art.

SUMMARY OF THE INVENTION To overcome limitations in the prior art, and to overcome other limitations that will become apparent upon reading and understanding the 'present specification, the present invention discloses a system and method for an enhanced High Rate Packet Data (HRPD) system. In accordance with one embodiment of the invention, a method of establishing a Reverse Traffic Channel (RTC) allocation in a High Rate Packet Data (HRPD) system comprises establishing a set of Modulation and Coding Schemes (MCS) to be employed by the RTC, creating an MCS code for each MCS established, requesting a desired allocation by communicating the MCS code associated with the desired allocation in a reverse link of the HRPD system, and providing a Rate Control (RC) code in a forward link of the HRPD system in response to receiving the desired allocation request.

In accordance with another embodiment of the invention, a method of establishing a Forward Traffic Channel (FTC) allocation in a High Rate Packet Data (HRPD) system comprises communicating a Channel Quality Indicator (CQI) to an access network in a reverse link of the HRPD system in response to channel quality measurements made on a forward link, establishing the FTC allocation in response to receiving the CQI, and communicating the FTC allocation on the forward link.

In accordance with another embodiment of the invention, a High Rate Packet Data (HRPD) system comprises a forward link to provide communications from an access network to an access terminal and a reverse link to provide communications from the access terminal to the access network. The reverse link includes a Reverse Control Channel (RCCH), where the RCCH allows a Modulation and Coding Scheme (MCS) to be requested by an access terminal for the reverse link via an RCCH code. The reverse link also includes a Channel Quality Indication (CQI) channel, where the CQI channel allows the access terminal to communicate a channel quality of the forward link to the access network via a CQI code, which in turn allows a forward link allocation to be made by the access network.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in connection with the embodiments illustrated in the following diagrams.

FIG. 1 illustrates an enhanced IS-856 communication system in accordance with the present invention; FIG. 2 illustrates an enhanced Reverse Traffic Channel (RTC) architecture in accordance -with the present invention; FIG. 3 illustrates a Time Division Multiplexed (TDM) pilot/Rate Control Channel (RCCH) in accordance with the present invention;

FIG. 4 illustrates a TDM pilot/RCCH data slot in accordance with the present invention; FIG. 5 illustrates an enhanced RTC data channel according to the present invention;

FIG. 6 illustrates an enhanced timing relationship between the forward and reverse links in accordance with the principles of the invention;

FIG. 7 illustrates a reverse Channel Quality Indicator (CQI) channel in accordance with the present invention;

FIG. 8 illustrates an enhanced forward channel structure in accordance with the present invention;

FIG. 9 illustrates an RTC rate control method in accordance with the present invention; and FIG. 10 illustrates a Forward Link (FL) allocation method in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION In the following description of the exemplary embodiment, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration various embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized, as structural and operational changes may be made without departing from the scope of the present invention.

Generally, the present invention is directed to possible embodiments for modifications to IS-856 to allow for several enhanced features. In particular, a higher reverse link throughput is contemplated incorporates a dual data channel. Increased reverse channel control is provided to the access terminal by allowing it to request a desired reverse traffic channel allocation. Accordingly, a corresponding forward link dedicated rate control mechanism is employed to provide the access terminal with a quick response to the requested reverse traffic channel allocation within a fixed amount of time. In addition, a mechanism is employed allowing the serving base station to rapidly transmit acknowledgements to the access terminal indicating the quality of the reverse link frames that it receives. For any given frame, there may be a fixed timing relationship between the transmitted frame and the received acknowledgment as seen at the access terminal. The access terminal may also report forward channel quality, to allow the serving base station to choose the best forward link allocation for the access terminal based on its reported forward channel quality. In response, a forward link rate indication channel is employed to allow the base station to signal the newly allocated forward link modulation and coding scheme to the access terminal.

FIG. 1 illustrates enhanced IS-856 communication system 100, whereby access terminal 114 either seeks to establish, or has established, HRPD communications with a base station, or serving sector (not shown), via forward, e.g., base-to-mobile, and reverse, e.g., mobile-to-base, radio links as shown. The forward links consist of the following TDM channels: pilot channel 116; Medium Access Control (MAC) channel 102; control channel 118; and Forward Traffic Channel (FTC) 120. MAC channel 102 consists of the following subchannels: reverse activity channel 128; DRCLock channel 130; and Reverse Power Control (RPC) channel 132.

The forward link carrier is allocated 1.25 MegaHertz (MHz) of bandwidth and is direct sequence spread at the rate of 1.2288 MegaChips per Second (Mcps). The forward link transmission consists of 16 time slots, where each time slot has a length of 2048 chips occupying 1.667 mS. Within each forward link time slot, the pilot, MAC, control, and FTC are time division multiplexed within each half slot, i.e., 1024 chips. A slot during which no traffic or control data is transmitted is referred to as an idle slot, whereby only the pilot and MAC channels are transmitted to reduce interference to other sectors.

The reverse link consists of access channel 122 and Reverse Traffic Channel (RTC) 104 and is used by the access terminal to transmit user-specific traffic or signalling information to the access network. Access channel 122, which further consists of pilot channel 124 and data channel 126, is used by an access terminal, e.g., mobile terminal 114 that is not in a connected state, to send signaling messages to the access network. In a connected state, the access terminal transmits on enhanced RTC 104, which further consists of enhanced pilot channel 106, enhanced MAC channel 108, acknowledgment (ACK) channel 110, and enhanced data channel 112. As can be seen from FIG. 1, several enhancements are contemplated by the present invention primarily in regard to the RTC operation as discussed in more detail below. Similarly, enhancements to the forward link are also contemplated in support of the reverse traffic channel enhancements.

In particular, the reverse MAC channel implemented by conventional IS- 856 incorporates two sub-channels, the Reverse Rate Indicator (RRI) and the Data Rate Control (DRC) channels. The RRI channel is used to indicate the data rate used by the access terminal on the RTC data channel, whereas the DRC channel indicates to the access network the supportable data rate on the FTC and the best serving sector for the forward link. The present invention contemplates alternate solutions for the RRI and DRC channels, e.g., Reverse Control Channel (RCCH) and Channel Quality Indication (CQI) channels, respectively, which help to increase reverse link throughput. The RCCH, for example, aids in the access terminal's selection of an improved throughput RTC data channel in accordance with the present invention. The CQI channel results in reduced coding as compared to the DRC channel, while simultaneously providing the serving sector greater flexibility in choosing the best forward link allocation for the access terminal.

As discussed in more detail below, the present invention facilitates a higher reverse link throughput as compared to the conventional IS-856 standard. Whereas IS-856 offers a maximum nominal data rate of 153.6 kbps, in one embodiment in accordance with the present invention, a maximum nominal data rate of 614.4 kbps is contemplated, whereby a reverse data frame duration of 13 1/3 mS, i.e., 8 slots at 1.667 mS per slot, is allocated. An exemplary facilitation of the data rates supported by the present invention utilizes two data channels having variable combinations of binary phase or quadrature phase Pulse Code Modulation (PCM) formats. In addition, variable code rate and variable codeword repetition factors are also utilized as necessary to implement the desired spectral efficiency.

In one embodiment according to the principles of the present invention, an improved RTC architecture 200 is illustrated in FIG. 2, in which four orthogonal code- division multiplexed channels are depicted: Pilot/RCCH 202; ACK 204; CQI 206; and data 208. The pilot/RCCH, ACK, CQI, and data channel modulation symbols are each spread by an appropriate orthogonal Walsh function to not only distinguish each channel from one another, but also to substantially eliminate multiple access interference. In particular, pilot/RCCH channel 202 is covered by the 16-chip Walsh function number 0 and the ACK channel 204 is covered by the 8-chip Walsh function number 4. Each code symbol of CQI channel 206 is spread by one of the 8-ary Walsh functions and further spread by the 16-chip Walsh function number 8. An alternate embodiment of data channel 208 in accordance with the present invention incorporates two data channels, whereby data channel 0 is covered by the 4-chip Walsh function number 2 and data channel 1 is covered by the 2-chip Walsh function number 1.

Prior to Walsh spreading, ACK channel 204 is Binary Phase Shift Keying (BPSK) modulated in the first half slot, i.e., 1024 chips, of an active slot. Transmissions on the ACK channel only occur if the access terminal detects a data¹ packet directed to it on the FTC. For a forward data packet transmitted from the access network to the access terminal in slot n, for example, a logic 0 bit is transmitted on the ACK channel in slot n+3 if a data packet has been received successfully, otherwise a logic 1 bit is transmitted. The 3 slots of delay allow the access terminal to demodulate and decode the received packet before transmitting on the ACK channel. Specific enhancements to the pilot/RCCH 202, CQI 206, and data 208 channels are discussed in more detail below.

Prior to quadrature spreading 210, pilot/RCCH 202 and ACK 204 channels are combined to form the in-phase component, I', while the CQI 206 and data 208 channels are combined to form the quadrature component, Q'. Quadrature spreading 210 occurs at the chip rate of 1.2288 Mcps and is equivalent to a complex multiply operation of the resultant I' and Q' channels by the PNi and PN_Q Pseudorandom Number (PN) sequences generated by I/Q PN code generator 212. PN sequences PN_Ϊ and PN_Q are obtained from a combination of long and short PN codes, where the in-phase and quadrature phase long code PN sequences satisfy the linear recursion specified by a 42^nd order characteristic polynomial as specified by IS-856. The in-phase and quadrature- phase short code PN sequences satisfy the linear recursion specified by a 15^th order characteristic polynomial also specified by IS-856.

Following the quadrature spreading operation, the I and Q impulses are applied to the inputs of baseband filtering 214 and 216, respectively, which employ a 48- coefficient Finite Impulse Response (FIR) low pass filter to achieve the impulse response as specified by IS-856. The filtered impulses are then modulated with sine wave and cosine wave, I-carrier and Q-carrier, respectively, and then combined to produce the reverse link waveform S(t).

With conventional IS-856, a Reverse Rate Indicator (RRI) channel is used by the access terminal to indicate whether or not data is being transmitted on the RTC data channel and if transmitted, its associated data rate. The RTC data rate is represented by a 3 -bit RRI symbol at the rate of one 3 -bit symbol per 16-slot physical layer packet. Each RRI symbol is encoded into a 7-bit codeword by a simplex encoder and repeated 37 times to produce 259 binary symbols per physical layer packet. The last three symbols are disregarded, i.e., punctured, resulting in 256 binary symbols per physical layer packet. The 256 binary symbols are then time multiplexed with the pilot channel data sequence (all logic zero values) to yield 128 binary symbols per slot. The time division multiplexed pilot and RRI channel sequence is then spread with the 16-chip Walsh function number 0 to yield 256 RRI chips + 1792 pilot chips per slot, which yields an 87.5% pilot signal duty cycle. As discussed above, however, the conventional IS-856 RTC poses significant restrictions on the throughput of the associated data channel. In particular, the effective data rate range for the conventional IS-856 RTC data channel is 9.6 to 153.6 kbps using 16-slot frames. The packets are encoded using either a rate 1/2 or a rate 1/4 parallel turbo code. The code symbols are then bit-reversal interleaved and block repeated to achieve a fixed 307.2 ksps modulation symbol rate. Thus, not only is the RTC data subchannel rate limited to 153.6 kbps, there is little variability in the modulation and coding scheme allowed for the RTC data channel and hence may be capacity limited or even refused service by the serving sector under certain channel conditions.

In accordance with one embodiment of the present invention, therefore, an enhanced reverse control channel, i.e., pilot/RCCH channel 202, is used by the access terminal to indicate a desired RTC data rate. There exists also a corresponding forward link indication within a fixed amount of time, which informs the access terminal as to whether the RTC data rate request was accepted. As exemplified in FIG. 4, the pilot/RCCH channel is time multiplexed so that the RCCH channel is transmitted during the first 512 chips at the beginning of every 1.667 mS slot, leaving the remaining 1536 chips available for pilot information. The pilot information is used primarily as a coherent phase reference so that the receiving sector may perform initial acquisition, phase recovery, timing recovery, channel estimation, power control, and other reverse link pilot functions.

The RCCH channel of the present invention is punctured into the reverse pilot channel, in place of the Reverse Rate Indication (RRI) as specified by conventional IS-856. In particular, the 3-bit RRI symbol as specified by IS-856 is replaced by an exemplary 6-bit RCCH symbol that in one example may be broken down into: a 4 bit RL MCS field; a 1 bit packet sequence number; and a 1 bit Rate Request (RR) field. The 6-bit RCCH symbol may then be encoded into 32 coded symbols per physical layer packet using bi-orthogonal encoder 302 and repeated 8 times by code repetition block 304 to produce 256 coded symbols per physical layer packet. Time division multiplexer 306 may then multiplex the pilot and RCCH bit sequences into 128 symbols per slot, which may then be spread by the 16-chip Walsh function number 0 to yield 512 RCCH chips + 1536 pilot chips per slot for a 75% pilot signal duty cycle per slot as exemplified by FIG. 4. It should be noted that while the pilot signal duty cycle of the present invention is reduced to 75%o from 87.5% as specified by conventional IS-856, the 75% duty cycle has nevertheless been proven adequate for channel estimation, power control, and other reverse link pilot signal functions, through cdma2000 IX evaluations.

As mentioned above, the RCCH symbol may be defined as a 6 bit symbol, consisting partially of a 4-bit MCS field. The MCS field may be used to identify the particular modulation and coding scheme that is to be utilized by the RTC data channel.

In order to discuss the merits of such a system, it is first necessary to explore an exemplary embodiment of data channel 208 as exemplified by data channel 500 of FIG. 5, whereby two data channels, data channel 518 and data channel 520, may be utilized as necessary by demultiplexer 506 once data bits have been encoded and repeated. Table 1 illustrates one embodiment of data channel parameters that may be defined by the associated 4-bit Reverse Link (RL) MCS field.

Variable rate convolutional encoder 502 performs the variable rate coding using rates r = 1/5, 1/4, and 1/3, as can be verified by inspection of Table 1. An Incremental Redundancy (IR) scheme is performed by code repetition block 504 in order

Table 1 to enhance performance in fast fading channels, such that during a first transmission, information bits and parity bits are combined to effect repetition rates between 1 and 6.4 as shown by Table 1. During a second transmission, the IR scheme increases the repetition rates from between 1.2 and 12.8, also as shown by Table 1.

Depending upon the reverse link packet size used for a particular RL MCS implementation, demultiplexer 506 supplies coded symbols to data channel 518 and data channel 520 in one of three configurations: 1.) data channel 518 only; 2.) data channel 520 only; or 3.) a combination of data channels 518 and 520. For reverse link packet sizes between 128 and 2048 bits (corresponding to RL MCS values 0 through 6), configuration 1.) is used where modulation block 510 performs BPSK modulation for reverse link packet sizes between 128 and 1024 and Quadrature Phase Shift Keying (QPSK) modulation for the 1536 and 2048 reverse link packet sizes. Reverse link packet sizes between 3072 and 4096 bits (corresponding to MCS values 7 through 8), requires configuration 2.) and reverse link packet sizes between 6144 and 8192 bits (corresponding to MCS values 9-10) require configuration 3.) to be used. Configuration 3.) calls for demultiplexer 506 to allocate coded symbols between data channels 518 and 520 as necessary. In particular, for reverse link packet sizes of 6144 and 8192 bits, a set of 6 coded symbols is generated by encoder 502 and code repetition block 504. For each set of 6 code symbols, data channel 518 receives the first two coded symbols and data channel 520 receives the last four coded symbols.

Configuration 1.) utilizes the 4-chip Walsh function number 2 to spread the BPSK/QPSK modulated code symbols for data channel 518. Configuration 2.) utilizes the 2-chip Walsh function number 1 to spread the QPSK modulated code symbols for data channel 520, while configuration 3.) utilizes both the 4-chip Walsh function number 2 and the 2-chip Walsh function number 1. Thus, it can be verified through the use of FIG. 5 in combination with Table 1 that: a 4-chip per symbol Walsh covering is used for the 128- 2048 reverse link packet sizes; a 2-chip per symbol Walsh covering is used for the 3072- 4096 reverse link packet sizes; and both a 4-chip per symbol and a 2-chip per symbol . Walsh covering is used for the 6144-8192 reverse link packet sizes.

Once the coded data symbols of data channel 518 and/or data channel 520 have been spread by their respective Walsh functions, they. are subjected to gain control 514 and 516, respectively. The gain utilized by data channel 520 is set equal to the gain utilized by data channel 518 multiplied by a factor of 1.414 to reduce the peak to average ratio of the RTC data waveform. Data channel 518 and data channel 520 are then combined prior to being summed with CQI channel 206 as illustrated in FIG. 2.

Thus, it can be seen that enhanced data channel 112 of FIG. 1 provides a more robust RTC data channel modulation and coding scheme as compared to the coding scheme as specified by conventional IS-856. Selection of RCCH code, code rate, modulation rate, and repetition factor may be optimized to obtain the required spectral efficiency while optimizing reverse link throughput. Pilot/RCCH channel 300 provides the access terminal with a means of either requesting an initial RTC data rate, or a change in the initially negotiated RTC data rate. Additionally, a reverse control mechanism in accordance with the present invention allows the access terminal to quickly receive responses to the RTC data rate requests, so that configuration changes can be quickly implemented to data channel 500 by the access terminal in response to the negotiated configuration. In order to demonstrate the functionality of such a reverse control mechanism, FIG. 6 illustrates an exemplary timing interaction between the RTC data rate requested by the access terminal via pilot/RCCH channel 300 and the corresponding forward link Rate Control (RC) indication as provided by the serving sector. In particular, the RCCH information may be punctured into the pilot channel as illustrated in FIG. 4 and subsequently transmitted in, for example, slots 602 and 604 of reverse link 612. The corresponding forward link 610 rate control mechanism, e.g., RC, of forward link slots 606 and 608, respectively, may then be provided by the serving sector in response to the access terminal's reverse link rate request to signal whether or not the rate request has been accepted.

Thus, for example, if the initial RL MCS requested by the access terminal in slot 602 corresponds to an MCS = 8, as tabulated in Table 1, in one embodiment according to the present invention, the response to such a request exists within the RC field of forward link slot 606. Thus, the response follows the request in a fixed time delay equal to, for example, 3 slots, or 5 mS. If the serving sector responds to the access terminal's request in the affirmative, e.g., RC = 1, then the access terminal configures data channel 500 in accordance with the requested RL MCS of Table 1 and begins transmission in accordance with the negotiated parameters associated with RL MCS = 8. Conversely, if the serving sector responds to the access terminal's request in the negative, e.g., RC = -1, then the access terminal may change its requested RL MCS and re-transmit in slot 604, where the response, e.g., the RC field of forward link slot 608, to the re-transmitted request then follows in the exemplary 3-slot, fixed time window of 5 mS.

It should be noted that the initial RL MCS value requested by the access terminal may be the result of an earlier message received from the access network concerning the maximum data rate that the access terminal may transmit on the reverse link. Using another method, the access network may control the RTC data rate through the transmission of a Reverse Activity (RA) bit within reverse activity sub-channel 128 of MAC channel 102 as illustrated in FIG. 1. According to conventional IS-856 standards, RA channel 128 is used by the reverse link MAC algorithm to control the total interference received in a given sector. In particular, the access terminal receives an RA bit from each sector in its active set, indicating whether the total reverse traffic channel interference received at the sector is above a certain value. If the RA bit indicates that the total reverse traffic channel interference is below a certain value, e.g., RA = +1, then the access terminal is free to increase its RTC data rate accordingly. If, on the other hand, the RA bit indicates that the total reverse traffic channel interference is above a certain value, e.g., RA = -1, then the access terminal is prohibited from increasing its RTC data rate until such time that the logic value of the RA bit toggles to +1.

In one embodiment, the present invention contemplates usage of the RA bit in combination with the RC indication as provided by the serving sector to negotiate a desired RTC data rate. In particular, if the value of the RA bit is equal to the value of the RC indication, then the access terminal is free to adjust the RTC data rate up or down (depending upon whether the value of the RC/RAB bit is +1 or - 1, respectively). On the other hand, if the value of the RC and RAB bits are different, then the RTC data rate is to be left unchanged by the access terminal.

In another embodiment in accordance with the present invention, the serving sector may be able to transmit acknowledgments to the access terminal to indicate the quality of the reverse link frames that the serving sector receives from the access terminal. In so doing, the present invention alleviates an IS-856 deficiency, in that IS-856 does not provide a fast ARQ mechanism on the reverse link, but rather requires the use of the RLP to recover reverse link frame errors due to inadequate received SINR conditions. Use of RLP, however, creates reverse link latency, where turnaround times on the order of 200 mS are common.

Thus, in one embodiment of the present invention, the access terminal transmits the pilot channel data sequence (all logic zero values) punctured by the RCCH code in pilot/RCCH channel 300 as illustrated in FIG. 3. Upon reception of the reverse link waveform S(t) as illustrated in FIG. 2, and after the appropriate down conversion and despreading of the 16-chip Walsh covered pilot signal, the serving sector is able to measure the SINR as received from the access terminal. Depending upon the RL MCS being utilized by the access terminal, the received SINR may, or may not, be adequate to maintain a satisfactory level of PER.

In such an instance, the serving sector may quickly signal the quality of the received SINR as shown, for example, in FIG. 6, where slot 618 represents one of the eight slots in the reverse link 612 frame that contains the pilot signal. The pilot signal of slot 618 may then be derived by the serving sector, where SINR measurements are then taken. The fast ARQ mechanism of the present invention then transmits ARQ slot 614 with an exemplary 3 time slot delay from slot 618, whereby within 5 mS, for example, the access terminal is apprised as to the quality of the reverse link frames received by its serving sector and can make the appropriate modifications if necessary. After any modifications have been made by the access terminal, slot 620 provides the pilot signal for subsequent measurement by the serving sector, whereby the quality of the reverse link frame received is again signalled in a timely fashion by forward link ARQ slot 616.

In another embodiment according to the present invention, a mechanism is contemplated which allows the access terminal to report forward channel quality, as opposed to reporting the highest received SINR, such that the serving sector may then choose the best forward link allocation for the access terminal. In a conventional IS-856 system, a network of base stations known as the active set, maintains radio connection with a given access terminal, such that a pair of assigned FTC and RTC resources are maintained within the access terminal. The access terminal monitors the received SINR of all sectors in its active set and informs the access network as to the identity of the sector having the highest SINR. The access terminal then estimates the Forward Link (FL) MCS that can be reliably decoded based upon a combination of the received SINR and a prediction of the future channel state. Accordingly, the conventional IS-856 network transmits to the access terminal only on the highest SINR sector as reported by the access terminal at the MCS last reported by the access terminal. As discussed above, however, the base station is forced with the choice of either scheduling the access terminal at its reported FL MCS or simply denying access to the access terminal at that instant in time.

In accordance with the present invention, however, the access terminal is allowed to report channel quality via CQI channel 206 of FIG. 2, in the form of a Channel Quality Indicator (CQI), rather than simply reporting a FL MCS. As such, the base station may assign an appropriate FL MCS to the access terminal based upon multiple criteria that is unknown to the access terminal. Such multiple criteria may include, but is not limited to, FL buffer status and cell loading of other cells. In such an instance, therefore, the base station is provided with enhanced flexibility when establishing the FL allocation, which substantially reduces the occurrence of access request denials from the base station to the requesting access terminal. FIG. 7 illustrates a detailed block diagram of exemplary CQI channel 700 that corresponds to CQI channel 206 of FIG. 2. As can be seen, either a 4-bit CQI code is generated as an input to bi-orthogonal encoder 702, or a 1-bit differential CQI code is generated as an input to bit repetition block 706. In the case of a 4-bit CQI code transmission, a 4-bit CQI code is submitted to bi-orthogonal encoder 702, which encodes the 4-bit CQI code using a 16-ary bi-orthogonal code. The coded symbols are then delivered to codeword repetition block 704 having a repetition factor of, for example, 2. Thus, 16 binary symbols per active slot are generated, which are then spread by one of the 8-ary Walsh functions 710, the function number of which is chosen by a 3-bit CQI COVER SYMBOL in order to indicate the desired transmitting sector on the forward link. As an alternate to the 4-bit CQI code, a 1-bit differential code may be transmitted to the serving sector on the forward link, which is then used to increment or decrement an accumulated CQI value in the serving sector. In such an instance, bit repetition block 706 receives the differential CQI field and performs a bit repetition of, for example, 16. Signal point mapping 708 then maps the repeated differential CQI bits from a logic value of 0 to +1 or from a logic value of 1 to -1. The 16 binary symbols per active slot are then spread by one of the 8-ary Walsh functions 710, the function number of which is chosen by the 3-bit CQI COVER SYMBOL in order to indicate the desired serving sector on the forward link. In either case, the 8-ary Walsh coded sequence is gain controlled and then further spread by the 16-chip Walsh function number 8 to ultimately generate the 1.2288 Mcps CQI channel, which is then combined with data channel 208 as exemplified by FIG. 2.

In one embodiment, the base station receives the 4-bit CQI code from the access terminal via CQI channel 700. Such a channel quality estimate may be derived by the access terminal as a function of the SINR measurements that are maintained by the access terminal in relation to its active set. Based upon several parameters that are unknown to the access terminal, such as forward link transfer buffer status or cell loading of adjacent cells, the base station may assign a FL MCS allocation in accordance with Table 2 in response to the access terminal's reported channel quality. The FTC is a shared medium that carries physical layer packets to all active access terminals. The FTC

Table 2

is transmitted to a single user at a time, whereby a preamble sequence is transmitted to indicate the presence and starting point of the physical layer packet as well as to identify the intended receiving terminal.

In one embodiment according to the present invention, forward channel structure 800 of FIG. 8 is exemplified, whereby the conventional IS-856 Q-channel preamble (PN sequence of all zero valued chips) is replaced by Forward Rate Indication Channel (F-RICH) 834. With such an architecture, the FL MCS allocation, as determined by the base station, may be signalled to the access terminal when the access terminal is assigned. The 4-bit FL MCS code is transmitted in quadrature with the existing I-channel preamble in such a way that the access terminal will decode the I-channel preamble successfully before fully decoding its FL MCS allocation successfully. In particular, an access terminal performs multiple decoding to determine the length of its I-channel preamble. Once determined, the I-channel preamble length is then used by the access terminal to decode its FL MCS code sent via F-RICH 834. Thus, the I-channel preamble is ensured to be detected by the access terminal prior to detecting its FL MCS allocation. Table 2 presents the FL MCS codes corresponding to the enhanced FTC allocations in accordance with the present invention, where each physical layer packet 802 is: encoded with turbo encoder 804 at rate r= 1/5 or r=l/3; randomized by scrambler 806 prior to modulation in order to reduce the peak to average ratio of the forward link waveform; permuted with channel interleaver 808; QPSK, 8-ary Phase Shift Keying (8PSK), or 16-ary Quadrature Amplitude Modulation (16QAM) modulated using modulator 810; subjected to sequence repetition or puncturing 812; demultiplexed into 16 streams by demultiplexer 814; spread by Walsh covering 816; gain controlled by gain control 818; and summed to form I and Q Walsh channels 820 and 822, respectively. The I-channel preamble sequence, e.g., I-STREAM 824, consists of all logic 0 symbols 826 that are covered by a bi-orthogonal sequence 828, which is determined by the MACIndex of the desired access terminal as follows:

is the bit-by-bit

complement of the 32-chip Walsh function of order i. The I-channel preamble sequence is repeated by sequence repetition block 830 as necessary depending upon the preamble length as required by the physical layer packet.

As discussed above, the conventional IS-856 Q-channel preamble is generated using a stream of 64 to 1024 logic 0 valued PN chips depending upon the size of the physical layer packet. In accordance with one embodiment of the present invention, however, F-RICH 834 replaces the Q-channel preamble in order to signal FL MCS 832 to the access terminal during assignment. In particular, the FL MCS 832 values tabulated in Table 2 are coded with 16-ary bi-orthogonal code 836 in order to generate 8, binary coded symbols, which are repeated by repetition block 838 as necessary to match the preamble length. For example, a repetition factor of 8 is required when the preamble length is 64 for FL MCS codes 10-15 as listed in Table 2. In operation, the use of full, e.g., 4-bit CQI, followed by differential, e.g., 1-bit CQI, channel quality reporting in accordance with the present invention results in a substantial decrease in the amount of throughput required, as compared to the DRC signalling employed by conventional IS-856. In particular, a conventional IS-856 access terminal operating in a non-soft handoff scenario, requires 2 slots to transmit a 4-bit DRC code. Similarly, for each DRC request thereafter, an additional 2 slots per 4-bit DRC code is required. In a soft handoff scenario, the conventional IS-856 access terminal requires 4 slots to transmit a 4-bit DRC code, and an additional 4 slots for each DRC requested thereafter. With CQI reporting in accordance with the present invention, on the other hand, an initial 4-bit CQI code may be transmitted by the access terminal, via CQI channel 700 of FIG. 7, in a single slot when operating in a non-soft handoff scenario. Once the full, 4-bit CQI has been communicated by the access terminal, the differential CQI may then be implemented thereafter, whereby only a single CQI bit is required per slot. For example, an initial CQI code = "0111" may be communicated by a particular access terminal to its respective base station, so as to report a channel quality that corresponds to "0111 ". In response, the base station may calculate a FL MCS in accordance with Table 2 that best correlates to such a channel quality and then may communicate the FL MCS value via F-RICH channel 834 as exemplified in FIG. 8. Thereafter, the access terminal need only transmit a differential CQI = "0", for example, to indicate that the initially reported channel quality has not changed.

Should the access terminal determine that the channel quality has degraded, however, the access terminal may transmit a differential CQI = "-1 ", for example, so as to signal the base station to adjust the FL MCS accordingly. Once received, the cumulative CQI value is then decreased to, for example, CQI = "0110" by the base station and the new FL MCS is then transmitted to the access terminal via F-RICH channel 834. Alternatively, the channel quality may improve, whereby the access terminal may signal a differential CQI = "+1 ", for example, which causes the cumulative CQI value stored in the base station to increase to CQI = "1000". The FL MCS value is then modified in accordance with the newly reported increased channel quality and then signalled to the access terminal via F-RICH channel 834. In order to more fully describe the advantages of the enhanced methods offered the present invention, the RTC data rate control flow chart 900 of FIG. 9 and the FL MCS allocation flow chart 1000 of FIG. 10 are presented. In relation to RTC data rate control 900 of FIG. 9, a determination is first made by the access terminal in step 902 as to whether the RC or RC/RAB forward link mechanism as discussed in relation to FIG. 6 is to be used. If the RC/RAB mechanism is to be used, then the RA bit of RA channel 128 is first detected by the access terminal as in step 904. The value of RC field 606 or 608 is then determined and compared to the RA bit value as in step 906, to determine whether a rate change is possible, e.g., RA bit = RC field. If a rate change is possible, then the desired RCCH code is transmitted in pilot/RCCH channel 300 by the access terminal to effect the desired rate change as in step 908. If a rate change is not allowed, e.g., RA bit is not equal to RC field, then the RTC rate control process ends as in step 916.

If, on the other hand, the RC algorithm is to be used, then step 908 is traversed, whereby the requested RTC data rate is transmitted by the access terminal in pilot/RCCH channel 300. The associated RC control field is then received as in step 910 to determine whether the requested RTC data rate is accepted by the serving base station. If the RTC data rate is not accepted, as determined in step 912, then the requested RTC data rate may be adjusted as in step 914 and re-transmitted in step 908, or the RTC data rate process may simply end as in step 916. In another embodiment of a method in accordance with the present invention, FL MCS allocation flow chart 1000 is exemplified in FIG. 10, whereby a channel quality determination is made by the access terminal and is then transmitted to the serving base station as in step 1002 using the full, 4-bit CQI as discussed in relation to FIG. 7. The serving base station may then adjust the FL allocation as in step 1006, in response to an affirmative FL MCS adjustment decision made in step 1004. Such a FL MCS decision may be based upon parameters that are unknown to the access terminal, such as for example, forward link transmit buffer status, cell loading of other cells, or other criteria influencing the FL allocation.

Once the FL MCS has been determined by the base station, it is then transmitted in quadrature with the preamble to the access terminal via F-RICH channel 834 of FIG. 8 as in step 1008. Once the transmission has been received by the access terminal, multiple decodes are performed by the access terminal to determine the length of the preamble as in step 1010, which is then used to decode the F-RICH channel as in step 1012.

Subsequent adjustments to the CQI transmitted in step 1002 may be effected by the access terminal through the transmission of differential CQI codes in step 1016 as discussed in relation to FIG. 7. In particular, whether the channel quality has: improved; deteriorated; or not changed, determines the value of differential CQI to be transmitted by the access terminal as in step 1016. If the channel quality has improved, for example, then the access terminal may transmit a CQI value of "+1 ". If, on the other hand, the channel quality has deteriorated, then the access terminal may transmit a CQI value of "-1". A CQI value of "0" may also be transmitted by the access terminal if the channel quality has not changed. If the CQI is adjusted by the access terminal, then the base station may choose to adjust the FL MCS allocated to the access terminal in response to the cumulative CQI value maintained within the base station relative to the 4-bit CQI and 1-bit differential CQI values received. It can be seen, therefore, that a higher reverse link throughput is achieved by the present invention, through incorporation of a dual data channel. Additionally, increased reverse channel control is provided to the access terminal by allowing it to request a desired reverse traffic channel data rate. Accordingly, a corresponding forward link dedicated rate control mechanism is employed by the present invention to provide the access terminal a quick response to the requested reverse traffic channel data rate within a fixed amount of time.

Additionally, the mechanism facilitated by the present invention allows the serving base station to rapidly transmit acknowledgements to the access terminal indicating the quality of the reverse link frames that it receives. For any given frame, there may be a fixed timing relationship between the transmitted frame and the received acknowledgment as seen at the access terminal. In accordance with the present invention, the access terminal may also report forward channel quality, to allow the serving base station to choose the best forward link allocation for the access terminal based on its reported forward channel quality. In response, a forward link rate indication channel is employed to allow the base station to signal the newly allocated forward link modulation and coding scheme to the access terminal. The foregoing description of the various embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. Thus, it is intended that the scope of the invention be limited not with this detailed description, but rather determined from the claims appended hereto.

Claims

WHAT IS CLAIMED IS: 1. A method of establishing a Reverse Traffic Channel (RTC) allocation in a High Rate Packet Data (HRPD) system, comprising: ' establishing a set of Modulation and Coding Schemes (MCS) to be employed by the RTC; creating an MCS code for each MCS established; requesting a desired allocation by communicating the MCS code associated with the desired allocation in a reverse link of the HRPD system; providing a Rate Control (RC) code in a forward link of the HRPD system in response to receiving the desired allocation request; and providing a reverse link quality indication in a forward link of the HRPD system.

2. The method according to Claim 1, wherein establishing the MCS set comprises allocating a reverse link packet size for each MCS.

3. The method according to Claim 2, further comprising allocating a modulation type for each reverse link packet size.

4. The method according to Claim 3, wherein the modulation type includes one of Binary Phase Shift Keying (BPSK) or Quadrature Phase Shift Keying (QPSK).

5. The method according to Claim 2, wherein a plurality of transmissions are allocated for each packet size, each transmission having a different repetition rate.

6. The method according to Claim 5, wherein a variable number of parity bits are added to vary the repetition rate.

7. The method according to Claim 1 , wherein requesting the desired allocation comprises puncturing a pilot channel of the RTC with the MCS code.

8. The method according to Claim 7, wherein puncturing the pilot channel comprises time multiplexing the MCS code with the pilot channel.

9. The method according to Claim 8, wherein the time multiplexing comprises providing a 3 to 1 ratio of pilot information to MCS code information.

10. The method according to Claim 1 , wherein providing the RC code comprises transmitting the RC code after a fixed amount of time has transpired since receiving the desired allocation request.

11. The method according to Claim 10, further comprising analyzing the RC code to determine whether the desired allocation request is accepted.

12. The method according to Claim 11, further comprising: modifying the desired allocation request in response to a declined MCS code; and submitting the modified allocation request.

13. The method according to Claim 1, further comprising providing a Reverse Activity (RA) bit in the forward link of the HRPD system.

14. The method according to Claim 13, further comprising comparing the RA bit and the RC code to determine whether an RTC allocation request is permitted.

15. A method of establishing a Forward Traffic Channel (FTC) allocation in a High Rate Packet Data (HRPD) system, comprising: communicating a Channel Quality Indicator (CQI) to an access network in a reverse link of the HRPD system in response to channel quality measurements made on a forward link; establishing the FTC allocation in response to receiving the CQI; and communicating the FTC allocation on the forward link.

16. The method according to Claim 15, wherein communicating the CQI comprises: providing a first CQI in response to an initial channel quality measurement; providing a second CQI after providing the first CQI, the second CQI representing a change in channel quality relative to the initial channel quality measurement; and providing subsequent CQIs, each subsequent CQI representing a change in channel quality relative to an immediately previous channel quality measurement.

17. The method according to Claim 16, wherein providing the second and subsequent CQIs requires less bandwidth than the bandwidth required in providing the first CQI.

18. The method according to Claim 16, further comprising accumulating the first, second and subsequent CQIs in the access network to form a cumulative CQI.

19. The method according to Claim 18, wherein establishing the FTC allocation is based on the cumulative CQI.

20. The method according to Claim 15, wherein communicating the FTC allocation comprises: establishing a code indicative of the FTC allocation; and combining the code in quadrature with a preamble on the forward link.

21. A High Rate Packet Data (HRPD) system comprising: a forward link to provide communications from an access network to an access terminal; and a reverse link to provide communications from the access terminal to the access network, the reverse link including, a Reverse Control Channel (RCCH), the RCCH allowing a Modulation and Coding Scheme (MCS) to be requested by an access terminal for the reverse link via an RCCH code; and a Channel Quality Indication (CQI) channel, the CQI channel allowing the access terminal to communicate a channel quality of the forward link to the access network via a CQI code.

22. The HRPD system according to Claim 21, wherein the RCCH comprises: a bi-orthogonal encoder coupled to receive the RCCH code and coupled to provide an RCCH symbol in response to the RCCH code; a code repetition block coupled to receive the RCCH symbol and coupled to provide a repeated sequence of the RCCH symbol; and a time division multiplexer coupled to receive a pilot signal and the repeated sequence and coupled to provide the pilot signal time multiplexed with the repeated sequence.

23. The HRPD system according to Claim 21, wherein the CQI channel comprises: a bi-orthogonal encoder coupled to receive a full CQI code and coupled to provide a CQI symbol in response to the full CQI code; and a bit repetition block coupled to receive a differential CQI code and coupled to provide a repeated sequence of the differential CQI code.

24. The HRPD system according to Claim 21 , wherein the reverse link further comprises a dual data channel, the dual data channel comprising: an encoder coupled to receive data and coupled to provide coded data symbols in response to the received data; a first data channel coupled to receive the coded data symbols and coupled to provide a spread stream of modulated coded data symbols having a first gain value; and a second data channel coupled to receive the coded data symbols and coupled to provide a spread stream of modulated coded data symbols having a second gain value.

25. The HRPD system according to Claim 24, wherein the first data channel comprises a programmable modulator having one of Binary Phase Shift Keying (BPSK) or Quadrature Phase Shift Keying (QPSK) capability.

26. The HRPD system according to Claim 24, wherein the second data channel comprises a modulator having a Quadrature Phase Shift Keying (QPSK) capability.

27. The HRPD system according to Claim 24, wherein the second gain value is equal to a scaled value of the first gain value.