AU2006202385A1 - Method for measuring channel quality in a closed loop transmit diversity communication system - Google Patents

Description

P/00/011 Regulation 3.2

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION STANDARD PATENT Invention Title: Method for measuring channel quality in a closed loop transmit diversity communication system The following statement is a full description of this invention, including the best method of performing it known to us: 004809988 O 2 METHOD OF MEASURING CHANNEL QUALITY IN A CLOSED LOOP ;TRANSMIT DIVERSITY COMMUNICATION SYSTEM Field of the invention oO 00 The present invention relates generally to wireless communication systems, and in particular to the determination of channel quality information in a high speed downlink 1-1 packet access communication system. The present invention has particular application in relation to code division multiple access systems, and it will be convenient to describe the invention in relation to that exemplary application.

Background of the invention 0 In existing Code Division Multiple Access (CDMA) systems utilising closed loop transmit diversity, base stations having multiple antennas use an antenna weight coefficient factor to adjust the phase and/or relative amplitude of signals transmitted from each antenna. Such systems, a mobile station computes a set of optimised antenna coefficients that should be applied at the base station antennas to maximise the mobile received signal power. The mobile station then feeds back to the base station a set of antenna control bits for use by the base station in generating the optimised antenna weights. While receiving high speed downlink packet access channels, the mobile station also feeds back a Channel Quality Indication (CQI) for use by the base station in transmissions scheduling and Transport Format (TF) selection, namely the transmission power, transport block size, modulation scheme, number of channels send in a time slot. In prior art closed loop transmit diversity systems, the channel quality information is computationally complex and includes a number of inherent inaccuracies.

It would be desirable to provide a method of determining channel quality information in a high speed downlink packet access communication system with closed loop transmit diversity that reduces the need for hardware and/or software resources within a mobile station forming part of the communication system. It would moreover be advantageous to provide a method for determining channel quality information that improved upon the accuracy of existing methods. It would also be desirable to provide a method for 004809988 O 3 determining channel quality information in a high speed downlink packet access communication system with closed look transmit diversity that ameliorates or S overcomes one or more problems associated with existing channel quality information determining methods, or at least provides a practical alternative thereto.

o00 M 5 Summary of the invention One aspect of the present invention provides a method for determining channel quality information in a high speed packet access communication system with closed loop transmit diversity, the method including the steps of: determining common pilot channel (CPICH) power and variance in each finger of 0 a rake receiver; adjusting the CPICH power in each finger with a closed loop transmit gain adjustment; combining the adjusted CPICH power and variance in all fingers of the rake receiver; determining the CPICH signal-to-noise ratio; determining the high speed downlink shared channel (HS-DSCH) signal-to-noise ratio from the CPICH signal-to-noise ratio; and mapping HS-DSCH signal-to-noise ratio to derive the channel quality information.

The closed loop transmit gain adjustment for each finger may advantageously be calculated from complex channel estimates for Antenna 1 and Antenna 2 and complex antenna weight.

The closed loop transmit gain adjustment AG applied to each finger k may be determined from the following: 004809988 k k- kA ak W 2 1 2 -au a2Q W 2 Q +aIQ a2Q W 21 +aIQ a21 W 2

Q

lGk=1+2 lal +la 2"l where k is the finger index; w is the complex antenna weight; 1 1 a, Ca,(n)and a2 =-a 2 (n)with N=5; and n n a,k(n) and a 2 k(n) denote complex channel estimates for Antenna 1 and Antenna 2.

Another aspect of the invention provides a mobile station for carrying out the abovedescribed methods.

Brief description of the drawings .0 The present invention will now be described with reference to the following drawings, in which the method for determining channel quality information is illustrated in a preferred embodiment. It is to be understood however that the described embodiment is exemplary only, and the invention is not to be understood as being limited to that embodiment only.

In the figures: Figure 1 shows and exemplary wireless communication system including a base station and mobile station Figure 2 shows a flow diagram of exemplary operations for channel quality feedback processing that may be performed by the mobile station of Figure 1 Figure 004809988 3 is a schematic diagram showing various functional components of the mobile station of Figure 1; Figure 4 is a representation of the common pilot channel modulation pattern for the two antennas in the closed loop transmit diversity communication system shown in 00 5 Figure 1; and N Figure 5 is a flow diagram of exemplary operations for determining channel 0 quality information that may be performed by the mobile station of Figure 1.

Detailed description of the embodiments Referring now to Figure 1, there is shown generally the primary elements in a wireless communication system 100 employing closed loop transmit diversity in accordance with the present invention. The wireless communication system 100 includes a base station 110 in communication with the mobile station 120 via a forward downlink channel 130.

Whilst only a single base station 110 and mobile station 120 are shown, the wireless communication system 100 may include multiple base stations and mobile stations.

The wireless communication system 100 may be capable of operating in accordance with any number of known standards, such as the Universal Mobile Telecommunications System (UMTS) standard, the CDMA 2000 standard and their evolutions, which are hereby incorporated by reference in their entirities.

The base station 110 includes two antennas 132 and 134 for transmitting signals on the forward channel 130. As illustrated, the transmitted portion 136 may include conventional components, such as a channel encoder 138 to receive and encode signals to be transmitted, such as control and data signals. Encoded signals from the encoder 138 are received as input by a spreader multiplier 140, which multiples the received signals by selected spreading codes. Copies of the spread signals from the spread multiplier 140 are received as input by weight multipliers 142 and 144 where the signals are multiplied by antenna weights w, and w 2 in order to adjust the phase and/or amplitude of the spread signals. The weighted signals from the weighted multipliers 142 and 144 are combined with the pilot signals by combiners 146 and 148. Each of the combined signals are transmitted to the mobile station 120 via respective one of the 004809988 IND 6 antennas 132 and 134. As illustrated, the mobile station 120 generally includes one or more antennas 150 (only one is shown) a receiver portion 152, channel quality S estimator 154, weight calculator 156 and feedback encoder 158.

As seen in Figure 2, the operations 200 of the communications system 100 shown in oo 5 Figure include the transmission, at step 202, by the base station 110 of a signal or signals to the mobile station 120. The operations 200 may be entered in step 202 with every transmission within a time slot) from the base station 110, or periodically, for example every N time slots, where N may correspond to a Transmission Time Interval (TTI) or maybe otherwise predetermined, for example, depending upon how often 0 feedback is desired. At step 204, the mobile station 120 receives signals transmitted from the base station antennas 132 and 134 via antenna 150, which may be fed to the receiver 152 for processing (by demodulation, decoding etc) of the signals using well known techniques.

At step 206, the mobile station 120 determines the channel quality based on the received signals. For example, the received signals may be fed from the receiver 152 to the channel quality estimator 154 to determine channel quality. The channel quality estimator 154 may calculate the channel quality information using well known measures, such as Signal-to-Noise Ratio (SNR). At step 208, the mobile station 120 calculates antenna weights to be applied at the base station, based on the received signals. For example, the received signals may be fed from the receiver 152 to the antenna weight calculator 156 to calculate the antenna weights. The antenna weights may be a matrix of complex valued signals. As previously described, the antenna weights wl and w 2 are generally calculated in an effort to maximise the strength of the signals received at the mobile station 120, and maybe calculated using well known techniques. Calculation of antenna weights 208 in this particular context is optional and need not be an integral part of the sequence of operations 200.

At step 210, the mobile station generates a feedback message containing Channel Quality Information (CQI) or Antenna Control Information (ACI). For example, the feedback encoder 158 may be generally configured to receive channel quality output 004809988 O 7 S from the channel quality estimator and antenna weights from the antenna weight calculator and to generate the feedback message with the CQI or ACI.

In At step 212, the mobile station transmits the feedback message to the base station 110 and, at step 214, the operations 200 are terminated, for example, prior to repeating the 00oo 5 operations 200 for a subsequent transmission. Whilst not depicted in Figure 1, it will be cl understood that the mobile station 120 also includes a transmitter, which may include S any combination of well known components. The base station 110 may receive the feedback message and process the feedback information at a feedback decoder 160 to S extract the feedback information (CQI or ACI) used by a scheduler 164 to control future 0 transmissions to the mobile station 120. It should be understood that the uplink signal from the mobile station to the base station is not necessarily received by the same two antennas on the base station that transmitted signals to the mobile station.

Figure 3 shows in further detail functional blocks forming part of the channel quality estimator block 154 of the mobile station 120. The receiver 152 demodulates the multipath signal transmitted from antennas 132 and 134. The channel quality estimator 154 includes a despreading finger for each multipath signal component. In Figure 3, two such depreader fingers 300 and 302 are shown. Each of the despreader fingers includes a Common Pilot Channel (CPICH) despreading block, respectively referenced 304 and 306 for correlating the CPICH signal with a spreading code which is time aligned with the delay of each despreader finger. After despreading, the CPICH modulation pattemrns for the two antennas 132 and 134, as shown in Figure 4, are recovered.

The despread CPICH signal is then forwarded to a power and variance calculation block, respectively referenced 308 and 310 after which a closed loop gain adjustment is applied by a gain adjustment block, respectively referenced 312 and 314 in the two despreader fingers 312 and 314. Following gain adjustment, the gain adjusted CPICH power and the CPICH variance in each finger are combined and the signal-to-noise ratio of the High Speed Shared Channel (HS-DSCH) is calculated in the HS-DSCH SNR calculation block 316. The signal-to-noise ratio computed by the block 316 is then relied upon by a Channel Quality Information (CQI) mapping block 318 in order to derive 004809988 IND 8

O

the channel quality information to be included in the feedback signal sent from the mobile station 120 to the base station 100.

In From Figure 3, can be seen that the calculations of channel quality information provided to the base station 110 by the mobile station 120 is carried out per slot by firstly 00 5 determining the CPICH power and variance per finger, and then applying an adjustment to the CPICH power with the closed loop transmit gain per finger. The adjusted CPICH power and CPICH variance for all fingers is then totalled. The total CPICH power and 0 variance is then adjusted for HS-DSCH power offset, prior to using a mapping function to derive the CQi bits transmitted to the base station 110. The steps performed by the mobile station 120 to derive the channel quality information is shown in Figure 5. Each slot, the process 500 shown in Figure 5 is repeated, commencing with initialisation at step 502. At step 504, the power and variance of the CPICH for each finger is calculated by the CPICH power and variance calculation blocks 364 and 366. The power and variance calculations are well known and any one of a number of methods are able to be used.

An example of such a calculation is shown below. The calculations may be performed on a slot basis.

1. For each CPICH slot: Symbols for Antenna 1: X,k(n) (Sk Sk(2n (1) where n from 0 to 4, k is the finger index, Sk(n) is the n-th symbol in a slot from the output of CPICH despreader.

Symbols for Antenna 2:

X

2 k n (2n) S' (2n 004809988

IN

9 where n from 0 to 4, is the slot number from 0 to 14, k is the finger index.

2. Based on X, and X 2 calculate signal powers and variances for each antenna: sigpow, X1 2 n=0 and S4 2 sigpw X 2 k*(n) ann=0 and k 1 4 2 var, k(n) 2 X, (n) 5 n=O 5 n= n=O n=0 3. Total variance per finger is then calculated as the sum of two variances, rk Finger CPICH _Finger var

I

var 2 4. Signal power is calculated over all symbols, SCPICH Finger SigpOW, Sigpow 2 At step 506, the CPICH power is then adjusted using the closed loop transmit gain for each finger. In this step, the Transmit Adaptive Array (TXAA) gain adjustment is firstly calculated for each finger k according to the following: 004809988 k k k k k k ak l 521 W 21 -ai a2Q W 2 Q +aIQ a2Q w 2 1 +alQ 521 W2Q AG =1+2 awhe where k is the finger index. w is the complex antenna weight,

I

a, N n 1 and a2 =-_a2n) N with N=5, a 1 (n)and a 2 k(n)denote complex channel estimates for Antenna 1 and Antenna 2 (namely, the two antennas 132 and 134 shown in Figure 1).

The CPICH power per finger is then adjusted with the calculated closed loop transmit gain adjustment as follows: pk pk AGk CPICH Finger Adj CPICH _Finger At step 506, the total adjusted CPICH power and variance for all fingers is calculated.

Any known method from existing literature can be used to combine the CPICH power and variance for all fingers. An example of such a known method is set out below.

1. From CPICH power per finger per slot obtained in the total CPICH power per slot for all fingers is

K

PCPICH Finger Comb CPICH Finger Adj k=1 (11) where K is the number of fingers 2. From CPICH variance per finger per slot obtained in the total CPICH variance per slot for all fingers is 004809988 \0 11

O

0 CPICH Finger Comb =CPICH -Finger (12)

KI

Swhere K is the number of fingers oo Finally, at step 508, the CPICH signal-to-noise ratio is computed, and the HS-DSCH CN signal-to-noise ratio is then determined from the CPICH signal-to-noise ratio. The HS- 0 CN 5 DSCH signal-to-noise ratio is then used to determine the channel quality information.

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c The HS-DSCH SNR calculation is based on adjusting CPICH SNR with the HS-DSCH power offset and spreading factor gain. The power offset is available from higher layer messages; the spreading gain is fixed for HS-DSCH. The CQI mapping table is used for mapping measured HS-DSCH SNR onto corresponding CQI. The number of entries in 0 the table is 30. The mapping table corresponds to UE category as defined by 3GPP.

Each threshold (one per entry) in the table corresponds to a CQI.

One example of the SNR calculation and CQI mapping is shown below: 1. Calculate average over last N slots for adjusted CPICH variance OCPiCH_Finger Comb and power PICpcn Finger Comb 2. An additional low pass filtering could be applied to the averaged values of CPICH power and variance over several past measurements 3. HS-DSCH power calculation is obtained by adjusting the final CPICH power with the HS-DSCH power offset signalled by higher layers 4. HS-DSCH power and variance is mapped onto CQI by means of a mapping table It will be appreciated by those skilled in the art that the block diagrams herein represent conceptual views of illustrative circuitry embodying the principles of the invention.

Similarly, it will be appreciated that any flow charts, flow diagrams, pseudo code and the like represent various processes which are able to be substantially represented in 004809988 IND 12 0 computer readable medium and so executed by a computer or processor, whether or not such a computer or processor is explicitly shown. Further various functions of the various elements shown in the Figures may be provided through the use of dedicated hardware as well as hardware capable of executing software in association with appropriate software.

00oo The above described arrangement provides a practical manner in which the mobile station is able to generate channel quality information for transmission to a base station that provides better accuracy than any known method of channel quality information generation, and importantly minimises the software processing required for the channel 0 quality information processing. Use of computational resources and energy requirements for the calculation of this information is therefore minimised.

Finally, it is to be appreciated that various modifications and/or additions may be made to the present invention without departing from the spirit or ambit of the claims appended hereto.

Claims (6)

1. A method for determining channel quality information in a high speed packet S access communication system with closed loop transmit diversity, the method including the steps of: oO 00 determining common pilot channel (CPICH) power and variance in each finger of S a rake receiver; Sadjusting the CPICH power in each finger with a closed loop transmit gain adjustment; combining the adjusted CPICH power and variance in all fingers of the rake 0 receiver; determining the CPICH signal-to-noise ratio; determining the high speed downlink shared channel (HS-DSCH) signal-to-noise ratio from the CPICH signal-to-noise ratio; and mapping HS-DSCH signal-to-noise ratio to derive the channel quality information.

2. A method according to claim 1, wherein the closed loop transmit gain adjustment for each finger is calculated from complex channel estimates for Antenna 1 and Antenna 2 and complex antenna weight.

3. A method according to claim 2, wherein the closed loop transmit gain adjustment AGk applied to each finger k is from determined from the following: 004809988 ND 14 0 F AG =1+2 w k k- 2 k- 2 In where oO c k is the finger index; (O w is the complex antenna weight; 1 1 a] and a2 a 2 (n)with N=5; and a, and ak denote complex channel estimates for Antenna 1 and Antenna 2.

4. A mobile station forming part of a high speed packet access communication system with closed loop transmit diversity, the mobile station including circuitry for carrying out a method according to any one of the preceding claims. 0

5. A method for determining channel quality information in a high speed packet access communication system having the steps substantially as hereinbefore described with reference to the accompanying drawings.

6. A mobile station forming part of a high speed packet access communication system substantially as hereinbefore described with reference to the accompanying drawings. Dated 5 June 2006 Freehills Patent Trade Mark Attorneys Patent Attorneys for the Applicant: NEC AUSTRALIA PTY LTD