JPL's Wireless Communication Reference WebsiteChapter: Analog and Digital
Transmission

The OFDM receiver structure allows relatively straightforward signal processing to combat channel delay spreads, which was a prime motivation to use OFDM modulation methods in several standards. In DAB, mobile reception leads to disadvantageous channel conditions, with both (frequency) dispersion and rapid variations of the channel with time. Reception of DTTB broadcast television 'on the move' may not be seen as a major market today. Nonetheless, the DVBDTTB system promises to become a highspeed delivery mechanism for mobile multimedia and internet services. Field test have been conducted in summer 1999 by Nokia, Deutsche Telecom and ZDF to offer mobile computing and web browsing over DTTB broadcast links with a GSM return channel. This involves OFDM reception over channels with a Doppler spread and the corresponding time variations, which are known to corrupt the orthogonality of the OFDM subcarrier waveforms. In such case, Intercarrier Interference (ICI) occurs because signal components from one subcarrier cause interference to neighboring subcarriers.
MultiCarrier CDMA also applies OFDMtype of transmission to a multiuser synchronous DSCDMA signal. In conventional DSCDMA, each user bit is transmitted in the form of many sequential chips, each of which is of short duration, thus having a wide bandwidth. In contract to this, due to the OFDM transform, MCCDMA chips are long in time duration, but narrow in bandwidth. Multiple chips are notsequential, but transmitted in parallel on different subcarriers. Here we address the synchronous downlink. Our models apply to the case of a single broadcaster simultaneously sending data symbols over one MCCDMA link, as well as to case of symbols from multiple users which are multiplexed onto a common multicarrier signal. We will use the term multiuser interference (MUI) for any mutual interference between different symbols due to frequency dispersion of the channel, eventhough in the broadcast scenario all signals belong to the same user. as MCCDMA uses OFDM, it is also vulnerable to rapid time variations of the channel.
Typical MMSE MCCDMA systems can not achieve the full channel capacity, whereas for COFDM with ideal error correction decoding and channel state information presumably we see no fundamental restrictions. The main performance penalty of MCCDMA appears to be due to the absence of a method to exploit correlated noise in the decision variables of the various user symbols. The performance penalty depends on the localmean SNR of the received signal, and becomes small for moderate below 10 or 15 dB. For high SNR, the merits of MCCDMA should be sought particularly in its ease of implementation, as it is not substantially more complicated than for OFDM. Its error correction coding can be simpler than for COFDM.
For more details we refer to the following subsections:
Time variations are known to corrupt the orthogonality of the OFDM subcarrier waveforms. In such case, InterCarrier Interference (ICI) also called "FFT leakage", occurs because signal components from one subcarrier spill into other, mostly to neighboring subcarriers.
The basic idea to repair InterCarrier Interference by adaptively combining subcarrier signals is intuitively appealing and can work if the delay spread is negligibly small. In such case all subcarriers experience the same amplitude and phase shifts, thus the ICI arrives with same crosstalk coefficients. However, for channels with both a delay and a Doppler spread, a practical implementation is more complicated, as it would typically require a large matrix multiplication. This matrix has its dominant contributions along the main diagonal, but is not of a Toeplitz structure that would allow a delayline filter structure. Moreover, extensive channel estimation poses problems in practice. So the prime challenge is to find a realistic channel representation which allows a computationally attractive implementation. One approach is the use of timederivatives of amplitudes of subcarriers. Following this concept we provide:
Novel signal processing methods can allows substantial improvements of the link performance at limited receiver complexity. If used for DVBT, it would not require requiring any modification to the transmit standard. If the solutions are considered in the definition of future mobile multimedia or fourth generation mobile systems (4G), it may allow the use of OFDM over a much wider range of parameters than hitherto believed
PDF J.P.M.G. Linnartz, "Performance Analysis of Synchronous MCCDMA in mobile Rayleigh channels with both Delay and Doppler spreads", IEEE VT, Vol. 50, No. 6, Nov. 2001, pp 13751387.
450k PDF A. Gorokhov, J.P.M.G. Linnartz, "Robust OFDM receivers for dispersive time varying channels: equalization and channel acquisition", IEEE Transactions on Communications, Vol. 52, No. 4, april 2004, pp. 572583
PDF 500k S. Tomasin, A. Gorokhov H. Yang, J.P.M.G. Linnartz, "Iterative Interference cancellation and channel estimation for mobile OFDM", IEEE Transaction in Wireless Communication, Vol. 4, No. 1, Jan. 2005, pp. 238245.