Modulation-Aware Decoding: Signal Reconstruction

The wideband nature of analog signals involved in high-speed digital communication systems requires a likewise high bandwidth of the transmission channel. Most wideband transmission mediums suffer from frequency-selectivity caused by reflections or multipath propagation. In addition, wideband wireless channels are increasingly vulnerable to imperfections such as time-varying channel-selective fading and in-band interference. In the previous sections, it was discussed that ofdm has been widely used to reduce intersymbol interference and that adaptive bit loading can be employed to avoid those parts of the spectrum that are contaminated by narrowband interference. All modulation techniques analyzed before try to preshape the transmitted signal is such a way that the receiver is able to extract the original information, even when the received symbol stream gets corrupted by the non-ideal transfer characteristics of the channel. For this reason, this type of channel coding techniques have been classified under the name modulation-aware coding systems. All these techniques have a common flaw, though: the characteristics of the channel are known to the receiver, but not at transmission side. A lot of the effort that goes into encoding, modulation and decoding gets lost due to inaccurate assumptions about the wireless channel. As a consequence, a significant portion of signal power must be allocated to the redundancy of the system, which is at the expense of the effective throughput of information. Also, the responsibility of adaptive channel coding is entirely allotted to the transmitter, so this type of approach is not very useful for a point-to-multipoint broadcast system.

This section describes an interferer suppression and signal reconstruction (issr) method for wideband communication systems. The issr technique shifts a big portion of the responsibility of dealing with non-idealities of the channel to the receiver. The signal reconstruction strategy used in issr should not be confused with channel estimation and equalization. Equalization is focussed on the compensation of the frequency response of the channel. Instead of this, issr can decide to consider seriously corrupted frequency bands as irreversibly lost and subsequently tries to reconstruct the original data using the remaining, unaffected parts in the spectrum. On the other hand, issr is not a holy grail system and cannot serve as a replacement for channel equalization. issr is meant to be put in action when channel equalization fails due to insufficient signal quality in some of the subbands. The ability to deliberately ignore a specific part of the spectrum is also the biggest strength of issr. When a certain subband suffers from destructive fading, equalization will boost the gain for that particular band in an attempt to reduce isi. However, the useful information within this band is often limited due to the background noise of the wireless channel. A lot of noise is thus inevitably introduced by the channel equalization filter.