Chapter: Analog and Digital
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 DVB-DTTB system promises to become a high-speed 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.
Multi-Carrier CDMA also applies OFDM-type of transmission to a multi-user synchronous DS-CDMA signal. In conventional DS-CDMA, 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, MC-CDMA chips are long in time duration, but narrow in bandwidth. Multiple chips are not-sequential, 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 MC-CDMA link, as well as to case of symbols from multiple users which are multiplexed onto a common multi-carrier signal. We will use the term multi-user 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 MC-CDMA uses OFDM, it is also vulnerable to rapid time variations of the channel.
Typical MMSE MC-CDMA systems can not achieve the full channel capacity, whereas for C-OFDM with ideal error correction decoding and channel state information presumably we see no fundamental restrictions. The main performance penalty of MC-CDMA 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 local-mean SNR of the received signal, and becomes small for moderate below 10 or 15 dB. For high SNR, the merits of MC-CDMA 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 C-OFDM.
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, Inter-Carrier 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 delay-line 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 time-derivatives 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 DVB-T, 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 MC-CDMA in mobile Rayleigh channels with both Delay and Doppler spreads", IEEE VT, Vol. 50, No. 6, Nov. 2001, pp 1375-1387.
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. 572-583
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. 238-245.