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JPL's Wireless Communication Reference WebsiteChapter:
Analog and Digital Transmission
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In contrast to this, several results showed that with a well-designed Coded OFDM system, modest time dispersion can improve, rather than deteriorate, the bit error rate. This interesting, counter-intuitive phenomenon can be explained using arguments of diversity.
If the entire MCM signal is subject to flat fading, i.e., if the channel is non-selective within the transmit bandwidth of the composite signal, bit errors occur on all subcarriers simultaneously. Error correction may then not be able to correct wrong bits, particularly if the channel fades only slowly, as in indoor systems. In a channel with a larger delay spread, the coherence bandwidth can be such that fading only affects a limited number of subcarriers at a time. Forward error correction coding can then repair poor reception at those subcarriers. Experiments within the DAB project revealed that under typical (outdoor) propagation conditions, 1.5 MHz is a minimum bandwidth to exploit such diversity gains.
Interleaving in frequency domain is used to further improve the performance. Signals from different applications or programmes are interleaved to achieve greater independence of fading of subcarriers for individual users or broadcasters.
While most engineers in the field now think of Coded-MCM as a means to combat time dispersion of the channel, in one of the pioneering papers, Cimini proposed its use in rapidly time-varying, i.e., frequency dispersive channels.
If the symbol duration is relatively large, it is unlikely that the symbol energy completely vanishes during a signal fade. However, OFDM subcarriers loose their mutual orthogonality if time-variations of the channel occur, which typically leads to increased bit error rates. Similarly, phase jitter or receiver frequency offset also leads to InterChannel Interference.
This sensitivity to frequency offsets, as well as to nonlinear amplification (see below), is often pointed out to be one of major MCM disadvantages. A (time-varying) frequency error not only erodes the subcarrier orthogonality, but also makes subcarrier synchronization much more difficult to achieve and maintain. There have been surprisingly few contributions on this topic so far, however, it is interesting to see that several researchers are proposing coherent detection schemes based on a frequency-time interpolation, inherent to MCM signalling in this issue.
In a mobile multipath channel, signal waves coming from different paths often exhibit different Doppler shifts (see "scatter function"). The use of Fourier transforms in both the transmitter and receiver, allows MCM communication systems to invoke any measure that was previously used against time dispersion in an attempt to mitigate the effect of frequency dispersion, and vice versa. For instance, it is well known that a maximum-length pseudo noise sequence can be used to find the delay profile of a time dispersive, i.e., frequency selective channel. If such a sequence is transmitted in multi-carrier format, i.e., after Fourier Transformation, it can be used to find the Doppler components of the frequency dispersive channel.
The MCM receiver can detect the individual components by searching shifted versions of the sequence at the output pins of the FFT. The resulting correlation pattern can be used to steer the Local Oscillator to better track the signal.
This idea can be further exploited if a special form of CDMA transmission is used, which is the Fourier Transform of Direct Sequence CDMA (called Multi Carrier CDMA). It maps each bit to all subcarriers, but each subcarrier uses a time-constant phase offset according to a code pattern. Of course, in an ideal frequency non-selective and time-invariant channel, the receiver adds the energy from all subcarriers according to the code pattern. In a frequency-dispersive channel however, multiple shifted versions of the signal can be recognized at the output of the receive FFT. The receiver may "rake" together energy dispersed in the frequency domain. Implementationwise, it is the dual of the conventional rake receiver designed to combat time-dispersion as now commonly used in Direct Sequence CDMA. This "frequency-domain rake" adds multiple frequency-shifted versions of the signal. Hence, the frequency-domain rake does not address time dispersion, but rather it combats frequency dispersion.
