DIVERSITY TECHNIQUES FOR OFDM BASED WLAN SYSTEMS J.D. Moreira1, V. Almenar1, J.L. Corral1, S. Flores1, A. Girona1, P. Corral2 1
Departamento de Comunicaciones, Universidad Politécnica de Valencia, EPS Gandia, 46730 Grao de Gandia, SPAIN {valmenar, jlcorral, sflores, agirona}@dcom.upv.es 2 Departamento de Física y Arquitectura de Computadores, Universidad Miguel Hernández, Elche, SPAIN,
[email protected]
Abstract - This paper presents different spatial diversity techniques that can be employed in an OFDM based WLAN system to improve the system performance. We present some results obtained by simulation when a HIPERLAN/2 transceiver is employed. Keywords – Hiperlan/2, OFDM, diversity, multiple antennas. I. INTRODUCTION HIPERLAN/2 (HL/2) is a Wireless LAN (WLAN) standard defined by the ETSI BRAN [1], [2]. This standard will operate in the unlicensed 5 GHz band and will provide data rates up to 54 Mbps in the physical layer in order to support broadband multimedia communications between portable devices and different core networks. The centralized mode of HIPERLAN/2 standard controls the communication between different mobile terminals (MTs) by means of fixed access points (APs) which will give service in a specific coverage area in a cellular way. This paper is focused on the improvements obtained in the quality of data transmission when two or more antennas are used for reception in a typical indoor multipath channel [3] and [4]. Different spatial diversity techniques at the mobile receiver have been analysed and assessed by means of simulations of the HIPERLAN/2 physical layer carried out. Results of the simulations are exposed, which show the benefits of using multiple antennas at the MT. This paper is organised as follows: Section II describes an overview the physical layer of the HIPERLAN/2 standard. Section III shows different diversity algorithms employed at the receiver stage, and a comparison between the obtained Bit Error Rate (BER) results with and without multiple antennas. II. OVERVIEW OF HIPERLAN/2 PHYSICAL LAYER The physical layer (PHY) of HIPERLAN/2 offers information transfer services to the data link control layer (DLC) of HIPERLAN/2. For this purpose, it provides functions to map different DLC Protocol Data Unit (PDU) trains into framing formats called PHY bursts. These are appropriate for transmitting and receiving management and
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user information between an AP and an MT in the centralized mode or between two MTs in the direct mode. The air interface of HL/2 is based on time-division duplex (TDD) and dynamic time-division multiple access (TDMA). There is a basic frame with a fixed length of 2 ms, which comprises five phases with variable duration for broadcast and frame control, downlink, direct link (optional), uplink and random access. In all cases, the transmission format on the physical layer is a burst, which consists of a preamble and a data field. As previously stated, the AP distributes the length of the frame among all phases according to the needs of the system. Table 1 Hiperlan/2 Modes Mode
Modulation
1 2 3 4 5 6 7
BPSK BPSK QPSK QPSK 16 QAM 16 QAM 64 QAM
Coding rate 1/2 3/4 1/2 3/4 9/16 3/4 3/4
Bit rate (Mbit/s) 6 9 12 18 27 36 54
Orthogonal Frequency Division Multiplexing (OFDM) has been selected as the modulation scheme for HL/2 due to its good performance on highly dispersive channels. The baseband signal is built using a 64-FFT, then a cyclic prefix of 16 samples is added to make the system robust to multipath. Since the frequency sampling is 20 MHz, each symbol is 4 µs (80 samples) length, and the guard interval is 800 ns length. In order to facilitate implementation of filters and to achieve sufficient adjacent channel suppression, only 52 subcarriers are used: 48 are data carriers and 4 are pilots for phase tracking. This allows uncoded data rates from 12 to 72 Mbps using variable modulation types from BPSK to 64-QAM. Convolutional coding is used with a 1/2 rate. This rate may be increased to 3/4 or 9/16 by puncturing the coded output bits. As a result, seven different modes with diverse data bit rates are specified for the PHY layer, according to the combination of QAM modulation scheme and coding rate employed by the system. In [5], ETSI BRAN defines a set of five indoor channel models (models A, B, C, D and E), to be used for HL/2
PIMRC 2002
simulations. It has been chosen a tapped delay line model where the average power declines exponentially with time. Except for the first tap of channel D, which has a Ricean K factor of 10, all taps have Rayleigh fading statistics. A classical Doppler spectrum corresponding to a terminal speed of 3 m/s is assumed for all taps. As an example of the results obtained employing the developed simulator, Fig. 1 shows the packet error rate (PER) performance versus C/N for modes 3 and 5 when channel A is used. This channel has 50 ns average rms delay spread, and represents a typical office environment. As one can expect, mode 3 behaviour is better than mode 5 for the same C/N ratio due to the different QAM mapping and code rate employed. It must be noted that BER measurements are done after hard Viterbi decoding, with a perfect knowledge of the channel, and with no frequency nor time offsets.
compensate the channel response of the selected antenna for each OFDM symbol. It must be noted that in a real implementation a power detector is enough to select the proper antenna, this avoids the need of estimating the channel response from both antennas. B. Subcarrier Selection Combining The subcarrier with the highest magnitude response is selected, that is, the output Rk is either RA,k or RB,k for each k, depending on which is greater |HA,k| or |HB,k|. In this case, for each subcarrier the equalizer must compensate the values of the channel response at the subcarrier frequency of the selected entry. C. Equal Gain Combining (EGC) The subcarriers in both antennas are added, this can be done coherently (with phase aligning) or incoherently (without phase aligning). The output of the combiner is given in the first case by Rk = R A,k ( e j arg( H A, k ) ) * + RB ,k (e j arg( H B , k ) ) * , and in the second case by Rk = RA,k + RB,k . So, the values to be compensated by the equalizer are given in the first case by the equation |HA,k| + |HB,k|, and in the second case by the equation HA,k + HB,k. D. Maximal Ratio Combining (MRC) The subcarriers in both antennas are phase aligned and weighted by their power. The output of the combiner is given by R k = R A, k ( H A, k ) * + R B , k ( H B , k ) * . So, the values to be compensated by the equalizer are given by the equation |HA,k|2 + |HB,k|2, for all k.
Fig. 1. Performance of HL/2 in modes 3 and 5 III. RECEIVER DIVERSITY As a way to improve radio link quality, a model of HL/2 receiver with N antenna diversity has been developed. Fig. 2 shows an example of a receiver with dual antenna diversity [4]. Signals from both antennas, labelled as A and B, are demodulated, and the values of the k data subcarriers in an OFDM symbol RA,k and RB,k are introduced into the diversity combiner block. The combiner, according to a diversity algorithm, will merge the subcarrier values and the channel state information in order to form the signal Rk, which will pass through the channel equalizer and send to the inner receiver. We have been working with four different diversity algorithms: antenna selection, subcarrier selection, equal gain combining, and maximal ratio combining. A. Antenna Selection Combining For each symbol, the signal from the antenna with the highest average power is selected, that is, Rk is either RA,k or RB,k for all k depending on which signal is greater. To make a decision the sum of |HA,k|2 or the sum |HB,k|2 for all subcarriers is computed. After selection, the equalizer must
A OFDM demodulator
RA ,k Rk
Channel equalizer
Diversity combiner
B OFDM demodulator
RB,k HA,k HB,k
Hk Channel information
Fig. 2. Receiver with dual antenna diversity IV. RESULTS A. Two antennas diversity Fig. 3 shows the packet error performance (PER) of all these techniques when a channel type A is used in a transmission mode number 3. The thick line shows the PER when no diversity is employed. Antenna selection results are represented by the line with circles, subcarrier selection results by the line with triangles, and maximal ratio combining by the line with crosses. Dashed lines with squares show equal gain combining results. The higher one comes from incoherent method, and the lower one comes from the coherent method.
In view of this results it is clear that incoherent EGC does not contribute any improvement. On the other hand, coherent EGC and MRC have similar complexity but EGC provides worst performance. For these reasons both EGC methods are discarded from now on.
NLOS conditions and 250 ns average rms delay spread [5]. Here we see that if we do not use diversity techniques, the dispersive channel worsens the performance compared to a channel with lower delay spread. Moreover, antenna selection technique does not improve the performance significantly, whereas both subcarrier selection and maximal ratio combining give a performance similar to that obtained in better channel conditions (like in Fig. 4). In this case, the gain in performance is much higher than those obtained before under channel model A.
Fig. 3. PER results for mode 3 in Channel model A
Fig. 5. PER results for mode 5 in Channel model E B. Three antennas diversity In this section we investigate the improvement of using a receiver with three antennas. Fig.6 and Fig.7 give the PER results for mode 3 and mode 5 (channel model A). These results must be compared with Fig 3 and Fig 4. In both cases we can see that antenna selection method gives a low improvement, while maximal ratio combining and subcarrier selection have a gain of 3 dB in performance. C. Effects of correlation between antennas Fig. 4. PER results for mode 5 in Channel model A Fig. 4 shows the PER performance and for transmission mode 5 when (a) no diversity technique, (b) antenna selection, (c) subcarrier selection, and (d) maximal ratio combining are used. Channel conditions are the same as in Fig. 3. Both figures show that antenna selection only improves the performance of the receiver in 1 dB. Maximal ratio combining introduces a gain of 8 dB, meanwhile subcarrier selection improves the results in 6 dB. Although MRC has a better performance than subcarrier selection, this one has a lower complexity. The method with lowest complexity is antenna selection since it does not need to demodulate all the signals as in MRC and subcarrier selection. Finally, Fig. 5 shows the use of these diversity techniques for transmission mode 5 in different channel conditions. Here we have employed a channel model E. This model corresponds to a typical large open space environment for
In this section we investigate the performance of diversity algorithms when there is correlation between the signals received by the antennas. So, we have generated correlated channels, one with a low correlation factor of 0.2, and another one with a high correlation factor of 0.7. Fig. 8 and Fig. 9 show the results for low and high correlation cases, respectively. It can be seen how antenna selection method becomes useless for a high correlation channel and give a low gain in a low correlation channel. MRC and subcarrier selection methods have a similar behavior, they both reduce their performance in the same amount: no more than 3 dB in Fig. 9 (high correlation). V. CONCLUSIONS This work has presented the use of multiple antennas in a Hiperlan/2 receiver. By means of simulation we have evaluated different diversity techniques: antenna selection, subcarrier selection, equal gain (coherent and incoherent)
and maximal ratio combining. We have discarded equal gain combining methods: incoherent method has a poor performance and coherent method has a similar implementation cost than MRC and lower performance. In low dispersive channels (like channel model A), while antenna selection only improves in 1 dB the system performance, MRC and subcarrier selection give a gain of 8 and 6 dB, respectively. However, in a worst environment (like channel E) antenna selection does not give a significant improvement, meanwhile MRC and subcarrier selection show a high reliability (more than 10 dB in gain). Another way of improving the performance of the system is to increment the number of antennas. We have shown that a three-antenna receiver gives a gain of 3 dB in MRC and subcarrier selection, and only 1 dB in antenna selection.
Fig. 8. Results with low correlation between antennas
Finally, we have checked the reliability of these methods against channel correlation. It has been shown that antenna selection reduces drastically its results even under low correlation. MRC and subcarrier selection have a similar behavior and show a good performance under highly correlated channels.
Fig. 9. Results with high correlation between antennas AKNOWLEDGEMENTS
Fig. 6. Results of using 3 antennas for mode 3
This work was supported by the Conselleria de Cultura, Educación y Ciencia, Generalitat Valenciana under Research Project GV00-113-14 and in part by the Universitat Politècnica de València. REFERENCES [1] [2] [3]
[4]
[5]
Fig. 7. Results of using 3 antennas for mode 5
ETSI TS 101 475 v1.2.2 BRAN; HIPERLAN Type 2; Physical (PHY) layer. R. Van Nee and R. Prasad. OFDM for Wireless Multimedia Communications. Archech House, 2000. J. H. Winters, J. Salz, and R. D Gitlin, The impact of antenna diversity on the capacity of wireless communication systems, IEEE Trans. Comm., vol. 42, pp. 1740-1751, 1994. M.R.G. Butler, et al. “The performance of HIPERLAN/2 Systems with multiple antennas”. Proceedings of IEEE Vehicular Technology Conference, Rhodes, 2001. BRAN WG3 PHY Subgroup. Criteria for Comparison. ETSI/BRAN document no. 30701F, 1998.
