Texto completo

(1)

function of which is given by R(z)=Jo(2rrffl), where Jo(.) is the Bessel fimction of the first kind of order zero andfd represents the maximum Doppler shift introduced by the fading process. The order of the prediction error filter is set to ,u

+

1 = 4 , and 100 training symbols are used for the initial acquisition of the filter. The super- iority of the proposed receiver to Yu and Pasupathy’s I-MLSE receiver is clearly seen in the case of fast fading. Fig. 1 shows the BEP performances of the various I-MLSE receivers when the actual normalised maximum Doppler frequency of the channel is 0.3. The proposed receiver provides better performance than Yu and Pasupathy’s receiver optimised for fdT=O.3. In addition, while two samples per symbol period need to be collected for Yu and Pasupathy’s receiver to avoid the irreducible error floor at high signal-to-noise ratio (SNR) [2], the proposed receiver produces the inverse log-linear BEP plot without increasing the Nyquist sampling rate. In Fig. 2, the BEP performances of the proposed receiver are shown for different values of normalised Doppler shift. The proposed receiver never exhibits the error floor phenomenon even in an extremely fast fading channel, which is in contrast to the observation from Fig. 1 that the BEP of the Yu and Pasupathy’s receiver atfdT= 0.3 saturates to the level that is higher than A s h T i s increased from 0.001 to 0.3, the power penalty paid by the proposed receiver at BEP = lo-* is < 5 dB.

EbINo, dB

Fig. 2 BEP performances of proposed receiver for dvferent values of normalised Doppler bandwidth

-A- fdT=0.001 - -0- - fdT=0.01

fdT=O.l

. . .. . . fdT= 0.3

Conclusion: The adaptive implementation of the I-MLSE receiver using the per-survivor processing concept is addressed. In the presence of uncertainties associated with the channel, the perfor- mance enhancement of Yu and Pasupathy’s model-based I-MLSE receiver over CDD diminishes as the degree of the mismatch between the assumed statistical model and the actual statistics of the channel expands. On severely fast fading channels, BEP performance reaches the error floor at high SNR unless the Nyquist sampling rate is doubled. Conversely, simulation studies illustrate that the proposed receiver is very robust to the uncertainty associated with the channel dynamics. It uniformly provides the inverse log-linear BEP curve over all simulated values of Doppler bandwidth without the need of increased sampling rate.

0 IEE 2002

Electronics Letters Online No: 20020005 DOI: 10.1049/el:20020005

Dongjun Lee (Strategic Technologies Group, Aware, Inc., 3685 Mt.

Diablo Blvd., Suite 395, Lafayette, CA 94549, USA)

10 August 2001

References

1 DAM, W.C., and TAYLOR, D.P.: ‘An adaptive maximum likelihood receiver

, for correlated Rayleigh-fading channels’, ZEEE Trans. Commun., 1994, 42, pp. 26862692

2 YU, X., and PASUPATHY, S.: ‘Innovations-based MLSE for Rayleigh fading channels’, IEEE Truns. Commun., 1995, 43, pp. 15341544 3 VITETTA, G.M., and TAYLOR, D.P: ‘Maximum likelihood sequence

estimation of uncoded and coded PSK signals transmitted over Rayleigh flat-fading channels’. Proc. IEEE ICC’94, New Orleans, Louisiana, May 1994, pp. 1-7

4 ADACHI, F., and SAWAHASHI, M.: ‘Viterbi decoding differential detection of DPSK’, Electron. Lett., 1992, 28, (23), pp. 2196-2198

5 RAHELI, R., POLYDOROS, A,, and TZOU, c.-K.: ‘Per-survivor processing: a general approach to MLSE in uncertain environments’, ZEEE Trans.

Commun., 1995,43, pp. 356364

CDMA receiver with subband preprocessing I. Barbancho, L.J. Tardon and J.T. Entrambasaguas

A code division multiple access (CDMA) receiver that employs subband processing techniques to mitigate multiple access interference (MAI) is presented, The incoming signal is processed using wavelet packets and a new version of the received signal with most MA1 eliminated is reconstructed. A correlation receiver is then employed.

This receiver achieves good BER performance even for low signal-to- interference ratios.

Introduction: Multiple access interference (MAI) limits the capacity of code division multiple access (CDMA) systems. To overcome this problem, different strategies, e.g. power control algorithms or multi- user receivers, can be adopted [l]. Focusing attention on the second strategy, in this Letter we present a receiver that employs wavelet packet decomposition to eliminate MAI. Our technique is based on wavelet packet analysis and involves three basic steps to reduce interference: wavelet packet decomposition, processing of subband coefficients and reconstruction of the signal with the modified coefficients. This technique to reduce interference is similar to some methods used by the image processing community for denoising [2]. After subband processing, any CDMA receiver can be employed.

In our case, a standard correlation receiver has been chosen. This receiver achieves better bit error rate (BER) performance than the adaptive minimum mean squared error (MMSE) receiver and it only needs to know the spreading code and timing of the user to be detected.

Receiver with subband preprocessing: The block diagram of the proposed receiver is shown in Fig. 1. The receiver consists of two main parts, a preprocessor and a correlation (CR) receiver. The aim of the preprocessor is to eliminate MA1 before giving the signal to the CR ieceiver.

s(p)

Fig. 1 Block diagram of proposed receiver

ELECTRONICS LE’ITERS 3rd January2002 Vol. 38 No. I 49

(2)

In the preprocessor, the incoming signal s ( p ) will be decomposed using a wavelet packet basis. As it is known, for a particular orthogonal wavelet function several wavelet packet bases can be generated depend- ing on the selected tree structure. In this case, a balanced decomposition tree of depth n has been chosen [2]. Therefore, the signal s ( p ) is processed with a filter bank to obtain the coefficients of the signal in each of the 2" subbands. In the forthcoming expressions, the subband coefficients will be denoted by Isn,/@), 0 ij 5 2" - 1) and let {h",,, 0 5 j 5 2" - 1 } and {gH,/, 0 i j 5 2" - 1 } denote the analysis and synthesis filters, respectively. Note that, if the filter bank is properly chosen, the original signal can be perfectly reconstructed using the coefficients {sn,,(p), 0 sj 5 2" - I}.

The aim now is to process these subband coefficients to obtain a new set of subband coefficients {sk,,(p), 0 sj 5 2" - I } such that the signal reconstructed from these new coefficients, s'(p), presents better char- acteristics than the original one for CDMA detection. In this CDMA framework, the improved signal will be one in which most MA1 has been eliminated.

With this criterion in mind, the core idea will be to eliminate the subbands, which contain higher interference power than the desired user's power. To attain this objective, the signature waveform assigned to the user to be detected will be decomposed, and the subband coefficients with entropy lower than a certain threshold will be discarded.

Therefore, before starting the reception process itself, the subbands to be eliminated must be selected according to the specific signature waveform assigned to the user to be detected. To achieve this objective, the following steps are performed:

al-Obtain the subband coefficients { s n J , 0 sj 5 2" - I } corresponding to the complete wavelet packet tree decomposition of the user code waveform.

bl-Measure the Shannon entropy [3] of the vector of coefficients {s,,,, 0 5 j 5 2" - 1 ) in each subband:

I 0-5-

10-6-

cl-Determine the subbands the entropy of which is smaller than a particular threshold 6. These subbands will be removed in the second phase, so the modified user code waveform coefficients will fulfil (2):

Once the subbands to be eliminated are identified, the receiver is ready to start the detection process. The process will be as follows:

a2-Decompose the received signal s(p).

b2-Eliminate the subband coefficients vectors determined in step c l . c2-Reconstruct the signal with the selected subbands, according to (3):

d2-Perfom the CDMA detection using the reconstructed signal s'(p).

Note that any CDMA detector could be employed.

Further considerations must now be taken into account concerning the preprocessing subsystem with regard to the choice of the wavelet function for the wavelet packet basis { h n , j ( p ) , 0 sj 5 2" - l}, the depth of the decomposition tree ( n ) and the threshold (6). These are key design decisions since the performance of the receiver will heavily depend on an appropriate choice of these three factors. Some of the criteria to be taken into account to make the proper choice are the following:

(i) The wavelet function selected for the wavelet packet decomposition must be such that the entropy of the user code waveform will be concentrated on a reduced number of subbands for a certain decom- position depth.

(ii) The decomposition must be deep enough to distinguish the subbands that carry most of the user code waveform energy and those that will carry most interference.

(iii) Subband preprocessing must remove more interference power than the desired user's power from the received signal. This condition can be

checked out using (4):

(4)

where N is the number of active users, i denotes the user to be detected, and

d,,

p L L , p:J and pu are the autocorrelations and crosscorrelations, respectively, after and before the reconstruction.

Implementation and results: The proposed receiver has been tested on a CDMA system with Gold codes of length 3 I . The channel is an additive white Gaussian noise (AWGN) channel and the users are asynchronous. The wavelet used for the wavelet packet decomposition has been the biorthogonal wavelet of order N,=3 and Nd=7 described in [4]. This wavelet family exhibits the property of linear phase, which is needed for signal reconstruction. The depth of the dyadic binary wavelet packet tree is n = 4 . After wavelet packet preprocessing, a correlation receiver is employed.

'O01

\

10-74

0 2 4 6 8 10 12 14 16

SNR

Fig. 2 BER with six active users and perfect power control (Pi = Pd, _ _ subband receiver

_ _ _ _ _

correlation receiver , . . . MMSE single user bound receiver

l 0 O l

0 5 10 15 20 25 30

SNA

1 0 4

Fig. 3 BER with six active users and P, = 5Pd __ subband receiver

_ _ _ . - correlation receiver , . . . single user bound

MMSE receiver

Figs. 2 and 3 show comparison of the BER performance of the proposed subband receiver with the simple correlation receiver, the adaptive MMSE receiver and the single user bound. Comparison is made for different signal-to-interference ratios (SIR) and signal-to-noise ratios (SNR), considering six active users ( N = 6). In Figs. 2 and 3, the BER is depicted as a function of the SNR. In Fig. 2, perfect power control is supposed (all users are received with exactly the same power).

50 ELECTRONICS LEl7ERS 3rd January 2002 Vol. 38 No. I

(3)

In Fig. 3, the power of the desired user (Pd) is supposed to be five times smaller than the power of each of the other interferer users (Pi).

It can be seen that the new receiver with subband preprocessing achieves good BER, even for scenarios with low signal-to-interference ratios where detection with the correlation receiver or the adaptive MMSE receiver is unreliable.

Conclusions: A CDMA receiver employing wavelet packet analysis for interference suppression has been presented. This receiver, which only needs to know the code and timing of the user to be detected, decomposes the incoming signal using a particular wavelet packet basis. The subband coefficients are then analysed and those subbands with higher interference power than desired user’s power are elimi- nated. Following preprocessing, which will have reduced the MA1 in the received signal, any CDMA receiver can be employed. A simple correlation receiver has been considered. The proposed receiver has been tested for different signal-to-noise ratios and signal-to- interference ratios in an asynchronous scenario and it is confirmed that the new receiver achieves very good BER performance even with very low signal-to-interference ratios.

0 IEE 2002

Electronics Letters Online No: 20020006 D o l : IO. 1049/e1:20020006

I. Barbancho, L.J. Tardon and J.T. Entrambasaguas (Departamento Ingenieria de Comtmicaciones, ETSI Telecomunicacibn, Universidad de Mcilaga, Campus Universitario de Teatinos s / n , Malaga E-29071, Spain)

E-mail: ibp@ic.uma.es

7 October 2001

References

KOULAKIOTIS, D., and AGHVAMI, H.: ‘Data detection techniques for DS/CDMA mobile systems: a review’, IEEE Pers. Commun., June

RAO, R M., and BOPARDIKAR, A.S.: ‘Wavelet transforms. Introduction to theory and applications’ (Addison Wesley, 1998)

best basis selection’, IEEE Trans. In$ Theory, 1992,38, (2), pp. 713-718

DAUBECHIES, 1 : ‘Ten lectures on wavelets’, CBMS-NSFSeries on Applied Mathematics (SIAM), 1992

2000, pp. 24-34

COIFMANN, R.R., and WICKERHAUSER, M.V: ‘Entropy-based algorithms for

Code reservation CDMA for integrated voice and data service in cellular environment

Qian Liang and Wentao Song

A new media access control (MAC) protocol for packet code division multiple access (CDMA) in a cellular environment is proposed. When the overall system load remains low, thc proposed protocol is shown to yield improvement over conventional ones for both voice and data traffic at the expense of introducing code reservation and modification of contention mechanism.

Introduction: As an extension of packet reservation multiple access (PRMA) operating in a packet environment, a joint code division multiple access (CDMA)/PRMA was proposed [ l ] to support inte- grated traffic for the third generation mobile communication. Never- theless, since CDMA is interference-limited, inherited parameters which control the channel access manner are not always efficient [2, 4, 51. While the work in [2] focused on optimising access probability (AP) for different traffic, other studies [3-51 suggested different approachcis. However, when they tried to control the access behaviour by optimising AP, there were always malign deviations between the transmission controlled by broadcast AP and the expected optimised one. In this Letter, we propose a new methodology. Rather than controlling the access manner via AP, the base station first gathers as ELECTRONICS LETTERS 3rd January 2002 Vol.

much contention information as possible, and then assigns resources accordingly. The approach is proved by simulation to largely improve resource assignment inefficiency.

Description: The frame structure of code reservation CDMA (CR/CDMA) is similar to that of CDMA/PRMA. When a mobile terminal, no matter what its traffic type, tries to transmit information packets but has no reservation, it should wait until the first time slot of a frame, which is completely dedicated as the contention slot. The mobile can then choose one of the codes in the contention slot and transmit its request freely, without having to check the result of the Bernoulli experiment. If the request is successfully demodulated by the base station, the corresponding terminal will be assigned with reserved resource in the rest information slots. Upon contention code collision, the mobile terminal should continue to compete in the contention slot of the successive frame. The base station has complete control over the assignment in an explicit manner, which has been proved to effectively alleviate multiple access interference [3, 41.

It is reasonable to,hypothesise that the useful bits in the contention packet can be much less than those of a normal packet, which indicates a higher processing gain, and more simultaneous successfully demo- dulated codes. The principle is similar to ‘mini-slot’, but the resource is divided over the code axis, which is more natural in a real CDMA system. Here, 7 x 8 = 56 contention codes are supposed to be available, which is proved to be enough for high-throughput and stable system operation with the following modification.

For integrated traffic, the contention procedure is inclined to be unstable if every packet in data transmission (DT) contends for resources. Considering the reservation merit of PRMA, we set a threshold in the DT buffer. Because data packets are insensitive to time delay, the buffer will first accumulate packets in a first-in-first-out manner until the threshold is reached. Then DT enters the contention mode in the same manner as voice transmission (VT), and on receiving positive acknowledgment, can enjoy reserved resource until the buffer reaches zero again, or its reservation is replaced by VT. With less chance to contend the collision caused by DT decreases dramatically.

Because privilege should be given to VT to guarantee real-time delivery, the following resource assignment algorithm is adopted when the base station correctly demodulates one contention packet:

slot t find-slot (min-traflc);

num c find-reserved-code-num (slot);

case packet-t);pe of

voice: if NOT ((MAI(slot,num

+

I ) = = 0)

OR(data-termin-num-in-frame( ) = = 0)) begin termin t find-data-termin (min-buffer-size);

slot t find-slot (termin);

suspend (termin);

end

slot-assign (slot);

end

data: if saturation-ctrl (slot,num) begin slot-assign (slot);

Simulation results: To assess the effectiveness of the proposed protocol, we simulated and compared its performance with explicit and conventional CDMA/PRMA. Most of the simulation parameters are in accordance with those of [l].

Fig. 1 gives the simulation result of voice-only traffic. If voice traffic in a cellular environment is low, the proposed protocol has the best performance among the three. Because of the explicit method of resource assignment, ploSs of the proposed and explicit protocol remains almost unchanged whereas ploss of the conventional one increases gradually. Owing to the dedicated contention slot and high processing gain codes in it, CR/CDMA even outperforms the explicit one. Theplo,, of CR/CDMA has a saturation point, similar to that of explicit CDMA/PRMA. Because certain channel resources should be reserved for contention, it is easy to speculate that CR/CDMA has a tuming point smaller than that of the explicit one. Furthermore, ploss of CR/CDMA should increase more rapidly after the saturation point, for more information slots means more stable traffic accommodation. Because of the existence of the contention slot, the conventional protocol outperforms CR/CDMA when the system load exceeds 330. But the advantage is small: (350 - 341)/350 -2.6% ifp,,,, of 1% is adopted.

38 No. 1 51

Figure

Actualización...

Referencias

Actualización...

Related subjects :