WO2007030308A2

WO2007030308A2 - Method and apparatus for de-mapping a symbol modulated by a high order quadrature amplitude modulation

Info

Publication number: WO2007030308A2
Application number: PCT/US2006/032643
Authority: WO
Inventors: Qingyuan Dai; Robert Lind Olesen; Chang-Soo Koo; Nirav Shah
Original assignee: Interdigital Technology Corporation
Priority date: 2005-09-08
Filing date: 2006-08-22
Publication date: 2007-03-15
Also published as: WO2007030308A3

Abstract

A method and apparatus for de-mapping a symbol modulated by a high order quadrature amplitude modulation (QAM) are disclosed. A transmitting wireless transmit/receive unit (WTRU) maps N input bits to one of 2N symbols in a 2N-QAM constellation. A receiving WTRU receives a signal and generates a sample of the received signal. A soft bit value of the most significant bit (MSB) is calculated based on a value of the sample. A magnitude of the soft bit value of the MSB is subtracted from a threshold. The threshold is initially set with respect to the QAM order, N. A soft bit value of the next MSB is calculated based on the subtraction results. The calculation and subtraction steps are repeated for the next MSB until soft bit values of all the remaining bits are obtained while diving the threshold by 2 each iteration.

Description

[0001] METHOD AND APPARATUS FOR DE-MAPPING A

SYMBOL MODULATED BY A HIGH ORDER QUADRATURE AMPLITUDE MODULATION

[0002] FIELD OF INVENTION

[0003] The present invention is related a wireless communication system.

More particularly, the present invention is related to a method and apparatus for de-mapping a symbol modulated by a high order quadrature amplitude modulation (QAM).

[0004] BACKGROUND

[0005] Figure 1 is a block diagram of a conventional wireless transmit/receive unit (WTRU) 100 on a transmit side. The WTRU 100 includes a symbol mapper 102 and a transmitter 104. The symbol mapper 102 maps a predetermined number of input bits 101 to a symbol 103 in accordance with a modulation scheme. The symbol 103 is then transmitted by the transmitter 104 over a wireless channel.

[0006] For 256-QAM, eight (8) input bit

re mapped to one of 256 complex QAM symbols, (i.e., the first four (4) input bits,

are mapped to an in-phase (I)-channel symbol and the other four (4) input bits,

are mapped to a quadrature (Q)-channel symbol), X(k) as follows: _{; and}

quation (1)

Equation (2) where k denotes a k-th symbol index, X(Jc) is a transmitted symbol, subscript r denotes a real symbol, (i.e., an I channel symbol), subscript i denotes an imaginary symbol, (i.e., a Q channel symbol), and bo and b4 are the most significant bits (MSBs). The constellation for Gray coded 256-QAM is shown in Table 1 and Figure 2.

Table 1

[0007] Conventional maximum likelihood channel decoding, (such as

Viterbi decoding), requires symbol-to-bit de-mapping of received samples. The conventional maximum likelihood de-mapping schemes have a computational complexity proportional to the order of QAM. This becomes practically impossible when dealing with a high order QAM, such as 256-QAM for an IEEE 802. Hn multiple-input multiple-output (MIMO) system.

[0008] A low complexity de-mapping scheme has been developed for binary phase shift keying (BPSK), quadrature phase shift keying (QPSK), 16 QAM or 64 QAM. The present invention extends a conventional symbol de-mapping method and apparatus for high order QAM, such as 256-QAM.

[0009] SUMMARY

[0010] The present invention is related to a method and apparatus for de- mapping a symbol modulated by a high order QAM. A transmitting WTRU maps N input bits to one of 2^N symbols in a 2^N-QAM constellation. A receiving WTRU receives a signal and generates a sample of the received signal. A soft bit value of the most significant bit (MSB) is calculated based on a value of the sample. A magnitude of the soft bit value of the MSB is subtracted from a threshold. The threshold is initially set with respect to the QAM order, N. A soft bit value of the next MSB is calculated based on the subtraction results. The calculation and subtraction steps are repeated for the next MSB until soft bit values of all the remaining bits are obtained while dividing the initial threshold, (e.g., 4, 8, 16 for 64, 256, 512 QAM, respectively), by 2 each iteration for the next MSB.

[0011] BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 is a block diagram of a conventional WTRU on a transmit side.

[0013] Figure 2 shows a conventional constellation for 256-QAM.

[0014] Figure 3 is a block diagram of a WTRU on a receive side configured in accordance with the present invention.

[0015] Figure 4 shows a bit error rate (BER) versus a signal-to-noise ratio

(SNR) in simulation results for comparing the low complexity de-mapping scheme of the present invention with a conventional maximum likelihood de-mapping scheme.

[0016] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS [0017] The features of the present invention may be incorporated into an integrated circuit (IC) or be configured in a circuit comprising a multitude of interconnecting components.

[0018] The present invention provides low complexity symbol de-mapping method and apparatus for QAM, such as 256-QAM. The present invention may be implemented in a WTRU. The terminology "WTRU" includes but is not limited to a user equipment (UE), a mobile station, a fixed or mobile subscriber unit, a pager, a Node-B, a base station, a site controller, an access point (AP), or any other type of device capable of operating in a wireless environment. [0019] The present invention may be implemented in any wireless communication network including, but not limited to, wideband code division multiple access (WCDMA), WCDMA long term evolution (LTE), IEEE 8O2.xx, orthogonal frequency division multiplexing (OFDM), or any other wireless communication system. The present invention is also applicable to a system using a smart antenna, (e.g., switched beam antenna, phased array antenna, diversity and multiple-input multiple-output (MIMO) antenna, or the like). [0020] Hereinafter, the present invention will be explained with reference to 256 QAM. However, it should be noted that the present invention is applicable to any modulation scheme lower or higher than 256-QAM for the Gray coded constellation. It is assumed that the 256 QAM symbols are transmitted at an average power of 170. Therefore, a proper scaling must be taken into account when the transmit signal is normalized to the average power of 1. [0021] Figure 3 is a block diagram of a WTRU 300 on a receive side configured in accordance with the present invention. The WTRU 300 includes a receiver 302, a first noπnalizer 304, a symbol de-mapper 306 and a second normalizer 308. The receiver 302 receives transmitted signals 301 over a wireless channel and outputs samples 303 of the received signals to the first normalizer 304.

[0022] A frequency domain received sample for OFDM systems, R(k) , at a given time instant is expressed as follows: Equation (3)

where G(A;) is a channel response and

is an additive noise. [0023] The first normalizer 304 performs normalization on the samples 303 using a minimum mean square error (MMSE) solution for generating a normalized sample value 305. The normalization using the MMSE solution is expressed as follows:

Equation (4)

where W(k) is a noise sample after normalization, σ; is a noise variance and * denotes a complex conjugate.

[0024] Whe

(i.e., high signal-to-noise ratio (SNR)), the normalize sam le ma be written as follows:

Equation (5)

Equation (5) is used for the symbol de-mapping in the symbol de-mapper 306 in accordance with the present invention.

[0025] The received I and Q channel samples after normalization are expressed as follows: _{; and}

quation (6)

Equation (7) [0026] The normalized sample value 305 is sent to the symbol de-mapper

306. The symbol de-mapper 306 de-maps the normalized sample value to bits

307, (preferably soft bits), in accordance with the modulation scheme used in the WTRU 100 on the transmit side, which will be explained in detail hereinafter. [0027] Since

are sign bits of X_r (k) and X₁ (k) , the soft- decision values are determined based on the received sample

value Z_r (k) and Z₁ (k) , respectively, as follows:

and Equation (8)

Equation (9)

Similarly, since

. Equation(lO)

ion(12) s the sign bit of , and b₅(k) is the

[0028] Therefore, the soft-decision value o btained

respectively, as follows:

and Equation (13)

Equation^)

[0029] Following the same procedure, the soft-decision values of the remaining bits are obtained as follows:

Equation (15)

Equation (16)

and Equation (17)

Equation (18)

[0030] The soft bits 307 are sent to the second noπnalizer 308 for normalization with an average transmit power. Since it is assumed that the average power is 170, the soft-decision values need to be normalized by

to be mapped to [-1 1] as follows: Equation (19)

[0031] From the Equations above, it can be seen that only

are Gaussian distributed random variables, while all other soft-bits are non- Gaussian. However, simulation results show that all soft-bits are zero mean with different standard deviation.

[0032] Figure 4 shows a BER versus an SNR, (i.e., Es/No), in simulation results for comparing the low complexity de-mapping scheme of the present invention with a conventional maximum likelihood de-mapping scheme. In the simulation, 256-QAM and a convolutional coding rate

re used for additive white Gaussian noise (AWGN). The results show that the two schemes have almost identical BER over wide range of SNR.

[0033] Embodiments.

[0034] 1. A method for symbol-to-bit de-mapping in a receiving WTRU in a wireless communication system including a plurality of WTRUs, wherein a transmitting WTRU maps N input bits to one of 2^N symbols in a 2^N-QAM constellation.

[0035] 2. The method of embodiment 1 comprising the step of generating a sample of a received signal.

[0036] 3. The method of embodiment 2 further comprising the step of calculating a soft bit value of a MSB based on a value of the sample.

[0037] 4. The method of embodiment 3 further comprising the step of subtracting a magnitude of the soft bit value of the MSB from a threshold, the threshold being set to N.

[0038] 5. The method of embodiment 4 further comprising the step of calculating a soft bit value of a next MSB based on the subtraction results.

[0039] 6. The method of embodiment 5 further comprising the step of repeating the subtraction step and the calculation step for the next MSB until soft bit values of all of the remaining bits are obtained while dividing the threshold by 2 for each iteration for the next MSB.

[0040] 7. The method as in any of the embodiments 1-6, wherein N is equal to 8.

[0041] 8. The method as in any of the embodiments 3-7, further comprising the step of normalizing the soft bit values with a square root of an average power of the transmitted symbols.

[0042] 9. The method as in any of the embodiments 1-8, wherein the symbols are transmitted at an average power of 170 for 256 QAM.

[0043] 10. A WTRU for symbol-to-bit de-mapping in a wireless communication system including a plurality of WTRUs, wherein a transmitting

WTRU maps N input bits to one of 2^N symbols in a 2^N-QAM constellation.

[0044] 11. The WTRU of embodiment 10 comprising a receiver for receiving a signal and generating a sample of the received signal. [0045] 12. The WTRU of embodiment 11 comprising a symbol-to-bit de- mapper for calculating a soft bit value of an MSB based on a value of the sample, subtracting a magnitude of the soft bit value of the MSB from a threshold, the threshold being set to N, calculating a soft bit value of a next MSB based on the subtraction results, and repeating subtraction and calculation of the soft bit value for the next MSB until soft bit values of all of the remaining bits are obtained while dividing the threshold by 2 for each iteration for the next MSB. [0046] 13. The WTRU as in any of the embodiments 10-12, wherein N is equal to 8.

[0047] 14. The WTRU as in any of the embodiments 10-13, further comprising a normalizer for normalizing the soft bit values with a square root of an average power of the transmitted symbols.

[0048] 15. The WTRU as in any of the embodiments 10-14, wherein the symbols are transmitted at an average power of 170 for 256 QAM. [0049] 16. The WTRU as in any of the embodiments 10-15, wherein the

WTRU is a mobile station.

[0050] 17. The WTRU as in any of the embodiments 10-15, wherein the

WTRU is a base station.

[0051] 18. An IC for symbol de-mapping in a wireless communication system including a plurality of WTRUs, wherein a transmitting WTRU maps N input bits to one of 2^N symbols in a 2^N-QAM constellation. [0052] 19. The IC of embodiment 18 comprising a receiver for receiving a signal and generating a sample of the received signal.

[0053] 20. The IC of embodiment 19 comprising a symbol-to-bit de- mapper for calculating a soft bit value of an MSB based on a value of the sample, subtracting a magnitude of the soft bit value of the MSB from a threshold, the threshold being set to N, calculating a soft bit value of a next MSB based on the subtraction results, and repeating subtraction and calculation of the soft bit value for the next MSB until soft bit values of all of the remaining bits are obtained while dividing the threshold by 2 for each iteration for the next MSB. [0054] Although the features and elements of the present invention are described in the preferred embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the preferred embodiments or in various combinations with or without other features and elements of the present invention.

Claims

CLAIMS What is claimed is:

1. In a wireless communication system including a plurality of wireless transmit/receive units (WTRUs), wherein a transmitting WTRU maps N input bits to one of 2^N symbols in a 2^N-quadrature amplitude modulation (QAM) constellation, a method for symbol-to-bit de-mapping in a receiving WTRU, the method comprising:

(a) generating a sample of a received signal;

(b) calculating a soft bit value of a most significant bit (MSB) based on a value of the sample;

(c) subtracting a magnitude of the soft bit value of the MSB from a threshold, the threshold being set to N;

(d) calculating a soft bit value of a next MSB based on the results of step (c); and

(e) repeating steps (c) and (d) until soft bit values of all of the remaining bits are obtained while dividing the threshold by 2 for each iteration for the next MSB.

2. The method of claim 1 wherein N is equal to 8.

3. The method of claim 1 further comprising: normalizing the soft bit values with a square root of an average power of the transmitted symbols.

4. The method of claim 3 wherein the symbols are transmitted at an average power of 170 for 256 QAM.

5. In a wireless communication system including a plurality of wireless transmit/receive units (WTRUs), wherein a transmitting WTRU maps N input bits to one of 2^N symbols in a 2^N-quadrature amplitude modulation (QAM) constellation, a WTRU for symbol de-mapping, the WTRU comprising: a receiver for receiving a signal and generating a sample of the received signal; and a symbol-to-bit de-mapper for calculating a soft bit value of a most significant bit (MSB) based on a value of the sample, subtracting a magnitude of the soft bit value of the MSB from a threshold, the threshold being set to N, calculating a soft bit value of a next MSB based on the subtraction results, and repeating subtraction and calculation of the soft bit value for the next MSB until soft bit values of all of the remaining bits are obtained while dividing the threshold by 2 for each iteration for the next MSB.

6. The WTRU of claim 5 wherein N is equal to 8.

7. The WTRU of claim 5 further comprising: a normalizer for normalizing the soft bit values with a square root of an average power of the transmitted symbols.

8. The WTRU of claim 7 wherein the symbols are transmitted at an average power of 170 for 256 QAM.

9. The WTRU of claim 5 wherein the WTRU is a mobile station.

10. The WTRU of claim 5 wherein the WTRU is a base station.

11. In a wireless communication system including a plurality of wireless transmit/receive units (WTRUs), wherein a transmitting WTRU maps N input bits to one of 2^N symbols in a 2^N-quadrature amplitude modulation (QAM) constellation, an integrated circuit (IC) for symbol de-mapping, the IC comprising: a receiver for receiving a signal and generating a sample of the received signal; and a symbol-to-bit de-mapper for calculating a soft bit value of a most significant bit (MSB) based on a value of the sample, subtracting a magnitude of the soft bit value of the MSB from a threshold, the threshold being set to N, calculating a soft bit value of a next MSB based on the subtraction results, and repeating subtraction and calculation of the soft bit value for the next MSB until soft bit values of all of the remaining bits are obtained while dividing the threshold by 2 for each iteration for the next MSB.