ss-mc-ma systems with pilot symbol aided channel estimation in the asynchronous uplink

SS-MC-MA Systems with Pilot Symbol Aided Channel Estimation in the Asynchronous Uplink*

STEFAN KAISER German Aerospace Center (DLR), Institute for Communications and Navigation, 82234 Oberpfaffenhofen, Germany

Stefan. Kaiser @)dlr.de

TRLabslUniversity of Alberta, 800 Park Plaza, 1061 1 - 98 Avenue, Edmonton, Alberta, Canada wak @ edm. trlabs. ca

Marconi Communications GmbH, 71522 Backnang, Germany Khaled. [email protected]

WITOLD A. KRZYMIEN

KHALED FAZEL

Abstract. Spread spectrum multi-carrier multiple access (SS-MC-MA) systems exploit in a similar manner as MC-CDMA systems the advantages of the diversity gain due to the spread spectrum technique and the high bandwidth efficiency due to the orthogonal multi-carrier modulation. However, the main difference is that SS-MC-MA systems use FDMA for the user separation and therefore are less vulnerable to multiple access interference, and thus are especially of interest for the uplink of mobile radio systems. They allow asynchronism between the mobile users. In this paper we investigate the effects of IS1 and ICI due to asynchronism in the uplink of an SS-MC-MA system on the performance of pilot symbol aided channel estimation. Moreover, we consider the suitability of power boosting of the pilot symbols to improve the link performance.

1 INTRODUCTION

Spread spectrum multi-carrier multiple access (SS- MC-MA) is an FDMA scheme at subcarrier level in which each user applies code division multiplexing (CDM) of symbols on its own subcarriers [ 1 I. This multiple access scheme combines the advantages of multi-carrier (MC) modulation implemented as orthogonal frequency division multiplexing (OFDM) [2], and the spread spectrum technique. The advantage of SS-MC-MA becomes evident in the uplink of a mobile radio system where it is preferred to tolerate asynchronisrn between the users [3 ] . A simple SS-MC-MA uplink scheme can be implemented which synchronizes on the frame structure received on the synchronous downlink. The approach allows a certain amount of intersymbol interference (ISI) and intersubchannel interference (ICI), but in return reduces the loss in bandwidth efficiency due to the guard intervals, and the complexity of a dedicated uplink synchronization scheme is avoided,

*Parts of this work were performed during S. Kaiser's research visit at the Telecommunications Research Laboratories (TRLabs). The work was funded by T R h b s , the German Aerospace Center (DLR). and the Natural Sciences Br Engineering Research Council (NSERC) of Canada.

Moreover, an uplink FDMA scheme requires a lower peak power compared to other multiple access techniques, re- ducing the costs of the mobile transmitter.

In this paper we investigate the effects of IS1 and ICI on pilot symbol aided channel estimation in an SS-MC- MA receiver. One-dimensional filtering with FIR filters designed as Wiener filters [4] in time dimension is applied. The performance with the proposed channel estimation in an asynchronous uplink is compared to results with perfect channel knowledge, and also with the synchronous uplink. Power boosting of the pilot symbols is a means to improve channel estimation and is e.g. applied in the European terrestrial digital video broadcasting (DVB-T) standard [5 ] . We investigate the suitability of power boosting of the pilot symbols in SS-MC-MA systems.

The paper is organized as follows. In Section 2 the prin- ciples of operation of SS-MC-MA are described. The uplink channel estimation concept is introduced in Section 3. In Section 4 we present a method to mitigate interference in the uplink, intentionally allowed to reduce the loss of bandwidth efficiency. Performance of the proposed scheme with pilot symbol aided channel estimation is discussed in Section 5. Finally, Section 6 summarizes the results.

Vol. I I , No. 6. November-December 2000 605

S. Kaiser, W.A. Krzymien, K. Faze1

2 SYSTEM DESCRIPTION

The block diagram of the mobile SS-MC-MA transmitter for user k is shown in Figure 1. Each user k,

data source of user k

symbol

1-

user specifi frequency mapper ]

Figure I : SS-MC-MA transmitter.

k = 1, . . . , Ii', transmits exclusively on a subset of L subcarriers of N , available subcarriers. K is the number of users. The total number of subcarriers is given as N , = K L . After channel coding, code bit interleaving, and symbol mapping, L complex-valued data symbols d!", 1 = 0, . . . , L - 1, of user k are spread by multiplication with orthogonal Hadamard codes of size L, and superimposed on the same subset of L subcarriers. The L orthogonal spread- ingcodesareq = ( C ~ , O , C I , ~ , . . . , c 1 , ~ - 1 ) , 1 = 0 , . . . , L-1.

where The resulting sequence is s(~) = ($1, $1,. . . , SL-l), (k)

L - 1

,$!"I = C dfk) cl,j, j = 0,. . ., L - 1. (1)

The L elements Sj[") modulate in parallel the subcarriers assigned to user k. In order to optimally exploit the frequency diversity offered by the mobile radio channel, the subcarriers assigned to different users are interleaved so that the subcarriers used by a given user are spaced by KIT,. This subcarrier assignment is referred to as user specific frequency mapping [I]. To achieve MC modu- latioddemodulation, the OFDM operation is applied. It is efficiently implemented with I F F T m algorithms [2]. A block of N, subcaniers modulated by one set of ~ ( ~ 1 , k = 1, . . . Ii, is referred to as an OFDM symbol of duration T, . Possible IS1 and ICI can be mitigated by inserting a guard interval of duration T, between successive OFDM symbols [ 6 ] . The guard interval is occupied by a cyclic prefix, resulting in the extended OFDM symbol of duration

l = O

T: = Tg + T, .

To enable channel estimation for coherent detection in the receiver, pilot symbols are multiplexed into the transmitted data.

The block diagram of the SS-MC-MA receiver, located at the base station, is shown in Figure 2. After MC demodulation with the inverse OFDM operation and deinterleaving (i.e., user specific frequency demapping), the demodulated

channel

data sink of user k

frequency demappcr

I I -u ..

Figure 2: SS-MC-MA receiver.

sequence dk) = (Rf), R?), . . . , is obtained. In this work a maximum likelihood detector is applied for the joint detection of L data symbols'of user k. After symbol demapping, code bit deinterleaving, and channel decoding, the detected source bits of user k are obtained.

3 UPLINK CHANNEL ESTIMATION

The proposed SS-MC-MA system requires pilot- symbol-aided channel estimation for coherent detection. For the uplink, one-dimensional filtering in time dimension is used. It should be noted that in the downlink, since it constitutes a simple broadcasting configuration, a more ef- ficient two-dimensional filtering is preferable, in which the overhead due to pilot symbols can be reduced [7]. In this paper, we focus on the uplink and apply one-dimensional low-pass FIR filters for subchannel estimation. The FIR filters are designed to become optimal Wiener filters in the worst case condition when the channel's Doppler power spectrum is rectangular with the maximum Doppler shift fD,,,., = 333.3 Hz. The Wiener filter is optimal in the sense of minimizing the mean square error

J = E{IHn - gnI2} (3)

between the channel transfer function Hn and its estimate Hn. Let us denote the spacing between the pilot symbols in time direction by Nt and the normalized channel bandwidth by T:~D,,,,, . Then, the sampling theorem requires that

In the proposed channel estimation, the pilot symbol spacing is chosen such that the channel transfer function is times two oversampled in order to achieve reasonable complexity and performance. Thus, it is sufficient to use 5-tap filters [7]. With times two oversampling, the pilot symbol spacing is given by

NtT,'fD,,,,, I 1/2. (4)

The pilot symbol grid with the system parameters given in Section 5 is illustrated in Figure 3, where according to (3, the pilot symbol spacing is Nt = 5 .

606

SS-MC-MA Systems with Pilot Symbol Aided Channel Estimation in the Asynchronous Uplink

- ?;,= 4.4 ms

0 data bearing symbol of user k

pilot symbol of user k

[7 data bearing or pilot symbol of user g * k

Figure3: Pilot symbol grid within an OFDM frame with 31 OFDM symbols.

4 INTERFERENCE MITIGATION

In the uplink, significant time offsets between the signals arriving at the base station occur due to different propagation distances between the mobile stations and the base station. The maximum time offset between signals from different users within a cell arriving at the base station is amax = 2R/c, where R is the radius of the cell and c is the speed of light. The factor 2 results from the summation of the propagation delays in the downlink and the uplink. The delays of the signals of the li users due to the propagation distance to the base station are b - ( k ) € [O,drnax], k. = 1, . . . , K . With the assumption of uniform surface dis- tribution of users within the cell, the resulting linear prob- ability density function of the signal delays at the base station is

Without compensation of different propagation delays of the signals in the uplink and an insufficient guard interval, the user synchronism is lost and interference results, which can significantly deteriorate the performance of an OFDM system.

In the following the resulting IS1 and ICI in an asynchronous multi-carrier link is described. We assign each subchannel n its own delay Sn, n = 0 , . . ., N , - 1. The delays on subcarriers assigned to the same user are deter- mined by the location of the user, and are the same. One arbitrary symbol Sn,i is transmitted per subcarrier in one OFDM symbol. The index n is the subchannel index and

i is the OFDM symbol index (discrete time). The symbols Sn,i are equivalent to Sj"' with their dependence on discrete time explicitly shown. In the sequel, for the sake of clarity, the notation Sn,i is preferred to describe the IS1 and ICI. There are N, orthogonal subcarrier frequencies fn , separated from each other by l/Ts. The nth orthogonal basis function is defined as

4n(t) = { e iarrJnt for otherwise

- Tg 5 t < T, (7) 0

and the signal transmitted on subcarrier n is given in time domain by

m

~ ~ ( 2 ) = C Sn,i &(t - iT: - b-,,). (8) iZ-00

Considering multipath propagation with Np paths, the impulse response of the wideband mobile radio channel as- sociated with the nth subcarrier is given by h,(t), with pn,F as complex-valued attenuation and Tn,p as excess delay of the pth path of subchannel n. For simplicity it is assumed that all subchannel impulse responses hn ( 2 ) . n = 0, . . . , N , - 1, have the same number of paths NF. The total delay cnrF per arriving path p on subchannel n at the base station is the sum of the propagation delay 5, and path delay rn,p, i.e., En,p = b-n -k rn ,p , where En,p E [o, Emax], cmax = 6max + ?inax, and rmax is the maximum excess delay of any subchannel.

The received signal y(t) is the sum of the convolutions of zn( t ) with h,(t) over all subcarriers and an additive noise term n(t) . The demodulation of subcarrier rn in the receiver involves a correlation with 4R(t - iT') over the effective OFDM symbol duration T,, i.e.,

(9)

The asterisk denotes complex conjugation. The term Nm,, corresponds to the additive noise component n(2) after demodulation. The output Ym,i of the demodulator is the noise-free decision variable that includes IS1 and ICI. Ym,i can be written as

Vol. 1 1, No. 6, November-December 2000 601

S . Kaiser, W.A. Krzymien, K. Faze1

-

and

Pm,n(En,p) = (12)

if 0 5 En,p < Tg

7r(n - m) Tg if ~ n , p sin ( ~ ( n - m)('C - c n , p ) / T ~ ~

e~n(2ncm,p-(n--m)(T: - e n , p ) ) /TJ

The desired component in (10) on subcarrier m is fm,q S m , i pm,,(Em,p), and the IS1 on the same subcar- ner IS pm,p &,,,-I Am,,(~,,p). ICI from the subcarriers n # m is given by Pn,p (Sn , ; pm,n(€n,p) + Sn,i-i Am,n(fn ,p) ) - The average signal-to-interference power ratio (SIR) on subcarrier m is

guard interval OFDM symbol ,\ I \ / \

I \ \

, \ , \ , * E Emax

A +To T, + c I I

I user k-1

P, Prcr + PISI' (SIR) , =

where PJ is the power of the desired component and PICI + P I ~ I is the interference power.

In order to keep the loss in bandwidth efficiency due to the insertion of guard intervals at a tolerable level, we consider an uplink scheme which allows IS1 and ICI by choosing Tg < i.e., we use a short guard interval. At the same time the detection interval in the receiver is chosen such that the residual interference is minimized [3]. Principle features of this approach are illustrated in Fig- ure 4. A + Tg is the beginning of the integration interval

detection interval

A -

I I userk

c I I userk+l

Figure 4: Principle of inrerjference mitigation.

for demodulation (of duration T,), which is the same for all users. The time shift A can take on values in the interval [O, E,,, - Tg). It is explained in [3] how to find the op- timum A. It is also explained there why the choice of the same correlation interval for all users results in minimum combined performance loss due to ICI and ISI.

As explained in [3], we can define the worst case user as the user which is next to the base station and has a delay of S ( k ) z 0 ps. Due to the fixed detection interval starting at A + T,, the worst case user in addition to the ICI from other users suffers from the maximum possible ISI.

5 SIMULATION RESULTS

The investigated SS-MC-MA system is designed for medium size cells with a radius of about 2 km, typical for future outdoor cellular mobile radio systems. This results in maximum time offsets between the users of smsx = 1 3 . 3 ~ ~ . The transmission bandwidth is 2 MHz, the carrier frequency fc = 2 GHz and the number of subcarriers N , = 256. The guard interval'duration Tg is 15 ps. The Hadamard codes for spreading are of length L = 8. QPSK symbol mapping is used and convolutional codes with rate 1/2 and memory 6 are applied. The channel estimation involves 5-tap FIR filtering in time dimension on each subcarrier. The separation of pilot symbols is 5 symbols. The system uses a TDMA frame structure where N f p is the number of time slots per TDMA frame, as explained in detail in [ 1 ,- 81. The duration of one TDMA frame is 17.7 ms. One time slot contains 31 OFDM symbols, and NfP = 4 time slots form a TDMA frame. The user capac- ity of the system is KJYs = N f p I< = 128. The net bit rate per user is 10.8 kbitfs. It is possible to assign to one user several or all transmission links, such that a net bit rate up to 1.4 Mbit/s is obtained.

The mobile radio channel model is taken from [9]. The 'Outdoor Residential -High Antenna' (Channel B) channel model with maximum excess delay T,~, = 15 ps is chosen. Velocity of the mobile user is 30 k d h , resulting in the maximum Doppler frequency of 55.6 Hz, and the classical Doppler spectrum is assumed [9, 101. Thus, the maximum delay is emax = 28.3 ps. All Monte Carlo simulation results shown in the following are for the worst case user and the most critical case of a fully loaded system. More- over, the signal-to-noise ratio (SNR) degradation due to the guard interval and pilot symbols is taken into account in the results. In Figures 5 and 6 the data symbols and pilot symbols have the same energy, i.e. E, = E p , where E, is the energy of the data symbols and Ep the energy of the pilot symbols. The Energy per information bit is Eb.

In Figure 5 the bit error rate (BER) versus the SNR Et,/No is shown for different cell sizes. N 0 / 2 is the two- sided noise spectral density. Results with pilot symbol aided channel estimation (real CE) and with perfect channel knowledge (perf. CE) are shown. Moreover, as a lower bound, the BER performance of SS-MC-MA without IS1 and ICI is plotted. Pilot symbol aided channel estimation produces tolerable BER performance in cells with up to 2 km radius. When R = 2 km, the performance degradation due to the presence of IS1 and ICI is the same whether the perfect channel knowledge is available, or the pilot symbol aided channel estimation is used. However, if the interference exceeds a certain amount (e.g. for R = 3 km) the SNR degradation with pilot symbol aided CE becomes more severe. For cells with R > 2 km a combination of coarse timing alignment followed by the proposed interference mitigation algorithm would be reasonable in order to reduce the complexity of the time synchronization.

608 E'IT

SS-MC-MA Systems with Pilot Symbol Aided Channel Estimation in the Asynchronous Uolink

lo-'

D-Z perf. CE, R 9 k m

lo-2

LI: W m

1 o-3

0 10 12 14 16 18 20 EJNo in dB

1 oA

Figure 5: Performance of asynchronous SS-MC-MA with pilot symbol aided channel estimation.

i

-0 R=lkm n R=2km +m R=Skm

10 12 14 16 10 20 EJN, in dB

1 o - ~

Figure 6: Mean square error of the channel estimation.

Figure 6 shows the mean square error (MSE) J between the channel estimate and the exact channel state versus the SNR of the pilot symbols EP/No for different cell sizes. We can observe the effects of IS1 and ICI on the accuracy of the channel estimation.

The influence of power boosting of the pilot symbols on the performance of SS-MC-MA systems is presented in Figure 7. The required Eb/No versus the pilot to data symbol energy ratio E p / E s for the BER of 4 . is shown. It follows that the best performance is obtained with a pilot symbol power boost of E p / E s = 3 dB. How- ever, the improvement compared to E p / E J = 0 dB is very small. Thus, for the SS-MC-MA system under investiga- tion pilot symbol power boosting is of interest only when the pilot symbols are used additionally, e.g. for carrier recovery, which is then more accurate.

no ISI. no ICI 18

j 5 t 1 4 ' ' ' ' ' ' ' " ' ' " ' " '

-6 . -3 0 - 3 6 9 EJE, in dB

2

Figure 7 Influence of power boosting of the pilot symbols at a BER of 4 ' lod4.

6 CONCLUSIONS

The effects of IS1 and ICI on the accuracy of a pilot symbol aided channel estimation and detection error rate in an asynchronous uplink of an SS-MC-MA mobile radio system have been investigated. The presented system allows IS1 and ICI in the uplink but in return reduces the loss in bandwidth efficiency due to shortened guard interval and avoids the complexity of a dedicated uplink synchronization scheme. It has been shown that the proposed channel estimation can tolerate a certain amount of IS1 and ICI without significant degradation in estimation accuracy. For the system assumed in simulations the asynchronous uplink of the SS-MC-MA scheme performs satisfactorily when mobile radio cell radii do not exceed 2 km. The power boosting of the pilot symbols is only of interest when the boosted pilot symbols are additionally used for other purposes, such as carrier recovery.

Manuscript received on April 18, 2000.

REFERENCES

[ l ] S . Kaiser and K. Fazel. A flexible spread-spectrum multi- camer multiple-access system for multimedia applications. In Proc. IEEE Int. Symp. on Personal, Indoor and Mobile Radio Commun. {PIMRC'97), pages 1OQ-104, Sept. 1997.

[2] S . Weinstein and P. M. Ebert. Data transmission by frequency-division multiplexing using the discrete Fourier trans- form. IEEE Trans. Commun. Tech., Vol. 19, pages 628-634, Oct. 1971.

[3] S . Kaiser and W. A. Knymien. Performance effects of the uplink asynchronism in a spread spectrum multi-canier

Vol. 1 1, No. 6, November-December 2000 609

S. Kaiser, W.A. Krzymien, K. Fazel

multiple access system. European Transactions on Telecom- munications (Em)), Vol. 10, pages 399406, July-August 1999.

[4] S. Haykin. Adaptive Filter Theory. Upper Saddle River, NJ. Prentice Hall, third ed., 1996.

[ 5 ] ETSI ETS 300 744. Digital video broadcasting (DVB); frame structure, channel coding and modulation for digital terrestrial television (DVB-T). Mar. 1997.

[6] E. Viterbo and K. Fazel. How to combat long echoes in OFDM transmission schemes: Sub-channel equalization or more powerful channel coding. In Proc. IEEE Global Telecommun. Con& (GLOBECOM’95), pages 2069-2074, Nov. 1995.

[7] P. Hoeher, S. Kaiser, and P. Robertson. Pilot-symbol-aided channel estimation in time and frequency. In Proc. ZEEE Global Telecommun. ConJ (GLOBECOM’97), Commun. Theory Mini Confi, pages 90-96, Nov. 1997.

[XI S . Kaiser. Multi-Carrier CDMA Mobile Radio Systems - Analysis and Optimization of Detection, Decoding, and Channel Estimation. Dusseldorf: VDI-Verlag, Fortschrit- tberichte VDI, Series 10, No. 531, 1998, Ph.D. thesis.

[9] K. Pahlavan and A. H. Levesque. Wireless Information Net- works. New York. John Wiley & Sons, 1995.

[lo] W. C. Jakes. Microwave Mobile Communications. New York. John Wiley & Sons, 1974.

610 ETT

ss-mc-ma systems with pilot symbol aided channel estimation in the asynchronous uplink

Documents