Pilot-assisted channel estimation in MC-CDMA for future mobile cellular systems

The effect of two different pilot-assisted channel estimation techniques on BER in Multi-Carrier CDMA (MCCDMA) is considered. The first technique involves sending both data and pilot symbols in the same OFDM symbol. The second technique involves sending data and pilot symbols in separate OFDM symbols. In both cases multi-access interference is present. Simulations of both systems in a UMTS multipath channel are presented. The results of this investigation conclude that the second technique outperforms the first technique, however both systems suffer a reduction in throughput. Introduction The deployment of 3rd Generation (3G) mobile cellular systems in Japan in late 2001 and the anticipated deployment of such systems in Europe in late 2002 has focussed attention in the research community on the requirements of future mobile cellular systems. These future systems have been termed ‘beyond 3G’ or ‘4G’ systems [1, 2]. The main features of such a system will be greater capacity and access speed in the downlink. Multi-carrier CDMA (MC-CDMA) [3, 4] is a promising candidate for the air interface of such systems. This paper will focus on channel estimation techniques in MC-CDMA using pilot symbols. 1 MC-CDMA basics MC-CDMA is a modulation / multiple access scheme which is a combination of Orthogonal Frequency Division Multiplex (OFDM) and Code Division Multiple Access (CDMA). In OFDM systems data is transmitted over multiple subcarriers instead of a single carrier. CDMA is a multiple access scheme where different users in a system are identified by a unique spreading code. In a MC-CDMA transmitter with N subcarriers the data symbol from user u is replicated N times and multiplied by a chip from a user specific spreading code, c(n, u), where n = 0..N-1. The n th spread symbol from each user is combined and then modulated onto one of the N subcarriers. In effect spreading is performed in the frequency domain. In discrete-time MC-CDMA each of the spread symbols in the transmitter is modulated onto N subcarriers using an N-point inverse fast Fourier transform (IFFT). This creates an N–sample OFDM symbol at the output of the IFFT. In the receiver an N-point FFT is used to demodulate the OFDM symbol. The signal at each subcarrier is multiplied by a channel equalising coefficient, a(n), and then despread using the user specific spreading code. The despread symbols are combined to form an estimate of the original data symbol for user u. A single user MC-CDMA transmitter and receiver is illustrated in Figure 1. 2 Channel estimation Channel estimation will permit the channel equalisation coefficients a(n) to be derived. The most popular method of performing channel estimation in wireless channels is to use pilot symbols. Pilot symbols are interspersed in time with data symbols in the downlink of the 3G UMTS standard [5]. In terrestrial Digital Video Figure 1: Digital MC-CDMA transmitter (a) and receiver (b). a(N-1) c(N-1,u) (b) (a) x x