Impact of Multipath Fading and Partial-Band Interference on the Performance of a COFDM/CDMA Modulation Scheme for Robust Wireless Communications

The performance of a COFDM’CDM4 radio communication system in the presence of partial-band jamming is presented where the eflects of multipath and additive noise are included The importance of interleaving and coding is emphasised and their impact on the bit error-rate performance is evaluated. In the first part of the paper, the effect of jamming on a COFDA4 system is obtained under worst-case conditions. For single tone jamming, the results show that the effect of the jamming signal on the bit error rate is acceptable, even at signal to jamming power ratios as low as –15dB, when interleaving and convolutional coding is used. For the case where 20% of the transmission spectrum is corrupted by partial bandjamming, it is necessa~ for the signal to jamming power ratio to be 5dB or more to achieve an acceptable bit error rate. To provide additional resilience, the case of an 0FDM7CDM4 scheme is examined where the spread-spectrum process gain is used to provide additional protection against jamming. INTRODUCTION Due to its inherent frequency diversity and flexibility, MFSK modulation has long been considered as a highly robust modulation scheme in the presence of tone jamming, particularly when used in conjunction with spread-spectrum modulation [ 14]. In [1], for example, the anti-jamming performance of A4-ary non-coherent FSK with fast frequency hopping was evaluated and a method for analysing the performance of M-ary operation with any combination of signal tone distribution, jamming distribution, jamming form and receiver system noise was presented. In [2], the performance degradation resulting from the multi-tone interference of orthogonal non-coherent FH/MFSK, where the effect of thermal noise is not negligible was investigated for both Rayleigh and Rician frequency selective fading channels. In [4], the effect of partial-band noise jamming on fast frequency-hopping spread-spectrum (FFHSS) using binary frequency shift keyed (BFSK) non-coherent receivers was examined for Rician channels. However, non-orthogonal modulation schemes require ffequency guard bands and this reduces their spectral efficiency. Orthogonal frequency division multiplexing (OFDM) is a variant of MFSK that has attracted considerable attention recently for mobile applications because of its very high spectral efficiency [5-6]. In particular, the modulation scheme is capable of operating at extremely high data transmission rates, relative to the delay-spread of the multipath channel, using a combination of cyclic extension time guard-band to combat 1S1 and flequencydomain equalisation (when coherent demodulation is used) to minimise the effects of fading. Frequency-domain equalisation can be much easier to implement than time-domain equalisation. The basic concept of OFDM is to transmit high-speed serial data at a much lower rate in parallel on IV sub-carriers that are orthogonally spaced. Consequently, a single broadband datastream is represented by many narrowband data-streams. Orthogonality of the sub-carriers allows the frequency guardband, required for MFSK, to be removed. In the presence of highly frequency-selective fading, the channel has a narrow coherence bandwidth due to multipath echoes and this modulation scheme provides frequency diversity in these circumstances. In effect, the frequency selective fade for the single carrier system has been replaced by uncorrelated flat fades for the OFDM system. This, in itself, is not an advantage but by using forward error-correction (FEC) coding and interleaving in the timeand fi’equency-domains, advantage can be taken of the uncorrelated fades to improve the bit error rate (BER). In some respects, the generation of errors due to tone jamming and highly frequency selective fading are similar. Consequently, this paper examines the performance of a COFDM system in both single-tone and partial-band (multi-tone) jamming using a Monte-Carlo simulation method. For illustration, a simple non-coherent receiver, with no frequency domain equalisation but with coding and interleaving, was used, The jamming signal bandwidth has been chosen to contain spectral components of equal power that fall within the bandwidth of the COFDM signal. The wanted signal is subjected to time-varying multipath Rayleigh fading, which is much more extreme than Rician fading, and the effect of the thermal noise is also included. In all the simulations, it is assumed that perfect synchronisation between the transmitter and the receiver has been achieved. The paper shows that this type of coded OFDM system (COFDM) is extremely robust in mobile radio channels containing both multipath fading and partial-band jamming signals. Recently, Kondo and Milstein [7] have proposed a multicarrier CDMA spread-spectrum system and Rowitch and Milstein [8] have examined the partial-band jamming performance of this system. In this paper, we consider the partialband jamming performance of a different multi-carrier CDMA configuration in which the data is spread by the CDMA spreading sequence prior to OFDM modulation. This configuration is called OFDM/CDMA. 0-7803-4902-4/98/$10.00 (c) 1998 IEEE OFDM SYSTEM AND CHANNEL MODEL In this parallel data transmission scheme, the data (represented by complex-valued symbol mappings appropriate to the modulation scheme being adopted, eg. m’PSK or QAM) is transmitted simultaneously over N orthogonal sub-carriers, equally spaced within the desired transmission bandwidth, W. The base-band continuous-time signal, x(9, of the OFDM system may be generated using a set of orthogonal basis functions CII~,k (t) which are defined as: @j,k (t) = recf(t – jT). e2izkt ‘T (1)