Enhanced Radio Access Technologies for Next Generation Mobile Communication phần 4 docx

The distortion of the signalspectrum due to frequency-selective fading is compensated by using one-tap FDE.The equalized subcarrier components are parallel-to-serial P/S converted into t

(a) Transmitter

Frequency Carrier frequency

Received

signal

Time-domain spreading

De-interleaving

& channel decoding

+

Data modulation

Chip shaping

Channel

coding &

interleaving

f c

Figure 5 Transmitter/receiver structure for SC-CDMA with rake combining

spread signal, resulting in the MC-CDMA signal MC-CDMA with SF= 1 isOFDM The GI insertion is necessary to avoid the orthogonality destruction among

Nc subcarriers due to the presence of multipaths with different time delays The

GI length needs to be larger than the maximum time delay difference amongmultipaths At the receiver, after removing the GI, the received signal is decom-posed by Nc-point FFT into Nc subcarrier components The distortion of the signalspectrum due to frequency-selective fading is compensated by using one-tap FDE.The equalized subcarrier components are parallel-to-serial (P/S) converted into thetime-domain spread signal, followed by despreading as in SC-CDMA receiver.FDE can be jointly used with antenna diversity reception for further performanceimprovement in MC-CDMA Among various FDE weights, it was shown thatthe use of minimum mean square error (MMSE) weight provides the best biterror rate (BER) performance This is because the MMSE weight can provide thebest compromise between the noise enhancement and suppression of frequency-selectivity MC-CDMA with MMSE-FDE provides much better BER performancethan SC-CDMA with coherent rake combining Because of this, until recently,research attention was shifted from SC techniques to MC techniques such as MC-

CDMA and OFDM SF= 1 But, as will be shown in this chapter, FDE can

(a) Transmitter

(b) Power spectrum

Frequency Carrier frequency

Insertion

of GI

#0

Time-domain spreading

c(t)

IFFT S/P

Conversion to freq.-domain spread signal

MC-CDMA signal

(c) Receiver

Frequency-domain equalization

Removal

of GI

De-interleaving

& channel decoding Data

demodulation

Recovered data P/S

Time-domain despreading Received

Figure 6 Transmitter/receiver structure for MC-CDMA

also be applied to SC-CDMA with much improved performance compared to rakecombining SC-CDMA is considered again as a promising access technique similar

to MC-CDMA

The application of MMSE-FDE to SC-CDMA can replace the coherent rakecombining with much improved BER performance First, FDE for SC-CDMA isshown However, the residual inter-chip interference (ICI) is present after MMSE-FDE and this will limit the BER performance improvement The ICI cancellationcan be used to reduce the residual ICI and hence improve the BER performance.These are presented here

Transmitter/receiver structure of multicode SC-CDMA with FDE is illustrated in

(b) Receiver

Received data

Integrate and dump

Orthogonal spreading code

Multicode despreading

Code-multiplexing

Orthogonal spreading code Multicode spreading

Figure 7 Multicode SC-CDMA transmitter/receiver structure

transmitter, the uth binary data sequence is transformed into a data modulatedsymbol sequence {dun; n= 0 ∼ Nc/SF− 1}, u = 0 ∼ C − 1, and then spread

by multiplying an orthogonal spreading sequence cut with spreading factor SF.

The resulting C chip sequences are added and further multiplied by a commonscramble sequence cscrt to make the resulting multicode SC-CDMA chip sequencewhite-noise like C is called code-multiplexing order This is called multicodespreading Then, the orthogonal multicode SC-CDMA chip sequence is divided into

a sequence of blocks of Nc chips each and then the last Ngchips of each block arecopied as a cyclic prefix and inserted into the GI placed at the beginning of each

block as shown in Figure 8 The GI-inserted multicode SC-CDMA chip sequence

{ˆst t = −Ng∼ Nc−1} in a block can be expressed, using the equivalent lowpassrepresentation, as

(3) ˆst =

2Ec

Nc subcarrier components {Rk; k= 0 ∼ Nc− 1} (the terminology “subcarrier” isused for explanation purpose although subcarrier modulation is not used) The kthsubcarrier component Rk can be written as

(5)

Rk=

t=0rt exp

Tc HkSk+ k



where Sk, Hk and k are the kth subcarrier components of st, the channelgain and the noise component due to the additive white Gaussian noise (AWGN),respectively Hk corresponds to Hf , t defined by Eq (2), but with f= k/(NcTc;time dependency of the channel gain is dropped since we are assuming very slowfading channel for simplicity

FDE is carried out similar to MC-CDMA Rk is multiplied by the FDE weightwk as

(6)

ˆRk = wkRk

=

2Ec

Tc Sk ˆHk+ ˆk



where ˆHk= wkHk and ˆk = wkk are the equivalent channel gainand the noise component after performing FDE, respectively As the FDE weight,

maximal ratio combining (MRC), zero forcing (ZF), equal gain combining (EGC)and minimum mean square error (MMSE) weights are considered They are given by

where Es/N0(=EcSF/N0 is the average received signal energy per data AWGN power spectrum density ratio and * denotes the complex conjugate operation.One-shot observation of the equivalent channel gain ˆHk and the noise ˆk for

symbol-to-MMSE, ZF and MRC weights are illustrated in Figure 9 An L= 16-path fadingchannel is assumed Also plotted in the figure is the original channel gain Hk TheMRC weight enhances the frequency-selectivity of the channel after equalization.Using the ZF weight, the frequency-nonselective channel can be perfectly restoredafter equalization (of course, only if the channel estimation is ideal), but the noiseenhancement is produced at the subcarrier where the channel gain drops However,the MMSE weight can avoid the noise enhancement by giving up the perfectrestoration of the frequency-nonselective channel (the MMSE weight minimizesthe mean square error between Sk and ˆRk Among these FDE weights, theMMSE weight can provide the best compromise between the noise enhancement andsuppression of frequency-selectivity and therefore, gives the best BER performance.After MMSE-FDE, Nc-point IFFT is applied to obtain the time-domain multicodeSC-CDMA chip sequence as

Tc

1

Nc

k=0ˆHk

st+ ˆt + ˆt



where st in the first term represents the transmitted chip sequence, ˆt is theresidual inter-chip interference (ICI) component and ˆt is the noise component.ˆt can be expressed as

(9) ˆt =

2Ec

Nc

⎤

Note that if ˆHk= constant, ˆt = 0 (i.e., this is the case of ZF-FDE and no ICI

is produced) The residual ICI degrades the achievable BER performance (this is

explained later) Multicode despreading is carried out onˆrt to obtain the decisionvariable for the data modulated symbol sequence {dun; n= 0 ∼ Nc/SF− 1},

based on which data demodulation is done

An arbitrary spreading factor SF can be used for the given value of FFT window

size Nc This property allows variable rate transmission even when FDE is used inSC-CDMA systems

MMSE-FDE for SF=16, obtained by computer simulation, as a function of the averagereceived bit energy-to-AWGN noise power spectrum density ratio Eb/N0 QPSKdata modulation and an L= 16-path frequency-selective Rayleigh fading channelhaving a uniform power delay profile (E[hl2 = 1/L are assumed For comparison,

4 8 16

×

MMSE-FDE Rake combining

the BER performance of coherent rake combining and theoretical lower-bound arealso plotted When C= 1, MMSE-FDE and rake combining can achieve almost thesame BER performance However, when C≥ 4, the BER performance using rakecombining significantly degrades due to strong ICI and exhibits large BER floors.MMSE-FDE can always achieve better BER performance than rake combining and

no BER floors are seen However, although MMSE-FDE provides much betterBER performance, the BER performance degrades as the code-multiplexing order

C increases since the orthogonality distortion among codes is produced due to theresidual ICI ˆt As the frequency-selectivity becomes stronger (or L increases),the complexity of the rake receiver increases since more correlators are requiredfor collecting enough signal power for data demodulation However, unlike rakereceiver, the complexity of MMSE-FDE receiver is independent of the channelfrequency-selectivity The use of FDE can alleviate the complexity problem of therake receiver arising from too many paths in a severe frequency-selective channel.These suggest that SC-CDMA with MMSE-FDE is a promising broadband access

as MC-CDMA for 4G wireless networks

Although MMSE-FDE can significantly improve the BER performance oforthogonal multicode SC-CDMA, there is still a big performance gap to the

theoretical lower-bound as shown in Figure 10 This is due to the residual ICI after

MMSE-FDE, given by Eq (9) An ICI cancellation technique can be introduced into

MMSE-FDE to improve the BER performance The ICI in SC-CDMA with SF=1

is equivalent to the inter-symbol interference (ISI) in the non-spread (i.e., SF=1)

SC transmissions; the ISI cancellation techniques can be found in the literature.Similar to ISI cancellation for MC-CDMA, ICI cancellation for SC-CDMA can becarried out either in the time-domain or in the frequency-domain after performingMMSE-FDE

For the frequency-domain ICI cancellation, the replicas of frequency components{Mk; k= 0 ∼ Nc− 1} of the residual ICI ˆt in Eq (9) are subtracted from

 ˆRk  k= 0 ∼ Nc− 1 after MMSE-FDE Mk is given by

Tc

ˆHk − 1

A joint MMSE-FDE and ICI cancellation is repeated in an iterative fashion so as to

improve the accuracy of the ICI replica generation Figure 11 shows the structure

of joint MMSE-FDE and ICI cancellation

Multicode despreading

Data demodulation

Symbol replica generation

Multicode spreading

Figure 11 Joint MMSE-FDE and ICI cancellation

The ith iteration is described below After performing MMSE-FDE with theMMSE weight wik , ICI cancellation is performed in the frequency-domain as(12) ˜Rik= ˆHik− ˜Mik

The MMSE weight wik minimizes the mean square error (MSE) Eek2 forthe given Hk, i.e., Eek2 wik= 0, where ek is the equalization errorbetween ˜Rik after the ICI cancellation and Sk of the transmitted signal stand is defined as

(15) ek= ˜Rik− Ai

The MMSE weight is given as

where¯si−1t is the hard decision replica of transmitted chip block.

The BER performance for the case of SF = 16 is plotted in Figure 12 with

the code-multiplexing order C as a parameter When C= 1, the BER performanceapproaches the theoretical lower-bound by about 0.5 dB As C increases, the BER

w/ICI cancellation (i= 3) w/o ICI cancellation

performance without ICI cancellation degrades This is because a severe nality distortion is produced by the residual ICI The use of ICI cancellation canimprove the BER performance When C= 16, the Eb/N0reduction from the no ICIcancellation case is as much as 6.9 dB for BER= 10−4.

The antenna diversity technique can be used to increase the received signal-to-noisepower ratio (SNR) and hence improve the transmission performance There aretwo types of antenna diversity: receive diversity and transmit diversity (they can

be jointly used) Receive antenna diversity has been successfully used in practicalsystems Recently, transmit antenna diversity has been gaining much attention sincethe use of transmit diversity at a base station can alleviate the complexity problem

of mobile receivers

Space-time block coded transmit diversity (STTD) can achieve the space diversitygain without requiring channel information at the transmitter In MC-CDMA, eachsubcarrier component is STTD encoded and then decoded in conjunction withMMSE equalization This STTD can be applied to SC-CDMA with MMSE-FDE.Here, this is called frequency-domain STTD In frequency-domain STTD, consec-utive chip blocks are encoded in the frequency-domain

STTD encoding for Nt= 2 is shown in Table 1 Two consecutive chip blocks,

set t= 0 ∼ Nc−1 and sot t= 0 ∼ Nc−1, at even and odd time intervals aredecomposed byNc-point FFT into Ncsubcarrier components, {Sek; k= 0 ∼ Nc−1}and {Sok; k= 0 ∼ Nc−1}, respectively, for STTD encoding Then, Nc-point IFFT

is used to obtain the time-domain coded chip blocks This encoding requires FFTand IFFT operations An equivalent time-domain STTD encoding that requires noFFT and IFFT operations is shown in Since



= s∗

eNc− t mod Nc1

Table 2 Equivalent time-domain STTD encoding Nt = 2

Time (in chip block) Antenna #0 Antenna #1

total transmit power)

At a receiver, after the removal of the GI, the even and odd chip blocks received

by the nrth (nr= 0 ∼ Nr−1) receive antenna are decomposed by Nc-point FFT into

Ncsubcarrier components {Ren

rk; k= 0 ∼ Nc−1} and {Ronrk; k= 0 ∼ Nc−1},respectively Renrk and Ronrk can be written as

s o (N c–t)

2 1

s o (t)

2 1

odd

Figure 13 Equivalent time-domain STTD encoding for SC-CDMA

In the above, w0nrk and w1nrk are the MMSE weights, given by

nt =0 H nr nt k 2 +1 C

SF Es N0

nt =0 Hnr ntk 2 +1 C

SF Es N0

where C denotes the code multiplexing order Finally, Nc-point IFFT is applied

to  ˜Sek and  ˜Sok to obtain the time-domain chip blocks for despreading anddata demodulation

When Nt= 3 and 4, four consecutive chip blocks sqt t= 0 ∼ Nc−1, q = 0 ∼ 3,are encoded STTD encoding for Nt= 3 and 4 can be expressed, using the matrixrepresentation, as

tt; t= 0 ∼ Nc− 1} is the coded chip block to be transmitted from the

ntth transmit antenna in the qth time interval

STTD decoding are carried out, in the frequency-domain, jointly with FDE as

where Rqn

rk is the kth frequency component of the chip block received by the

nrth receive antenna in the qth time interval, and wnrntk is the MMSE weightgiven as

a chip-spaced L= 16-path frequency-selective block Rayleigh fading channel with

= 0 dB), and ideal channel estimation The BER

performance using frequency-domain STTD is plotted in Figure 14 for SF=16 Forcomparison, the single transmit antenna case (Nt= 1) is also plotted The transmitdiversity gain similar to that of two-antenna receive diversity (Nr= 2) using MRC

is obtained, but with a 3dB power penalty (this is because the transmit power fromeach antenna is halved to keep the same total transmit power)

High-speed data services of 100M∼1Gbps are demanded in the next ation wireless systems However, the available bandwidth is limited Spacedivision multiplexing (SDM) is a promising technique to achieve highly spectrum-efficient transmission In SDM, different data sequences are transmitted inparallel from different transmit antennas using the same carrier frequency

gener-At a receiver, a superposition of different data sequences transmitted fromdifferent antennas is received A lot of research attention has been paid tothe signal separation/detection schemes, e.g., maximum likelihood detection(MLD), ZF detection, MMSE detection and vertical-Bell Laboratories layeredspace-time architecture (V-BLAST) For high-speed data transmissions, thechannels become severely frequency-selective and the BER performance ofSC-CDMA using SDM degrades due to the inter-chip interference (ICI) and inter-ference from other antennas Therefore, the receiver must have two tasks: signalseparation/detection and channel equalization

Orthogonal multicode SC-CDMA is considered Figure 15 illustrates the

trans-mitter/receiver structure of (Nt, NrSDM, where Nt and Nr denote the number of

data

Frequency-domain signal processing