Wireless Communications and Networks

exploit multiple scattering between the scatters [25, 26]. After the time-reversal operation,
the whole multiple-scattering medium behaves, in effect, as a coherent focusing source
whose large angular aperture enhances the resolution at the final focus. Owing to the high
resolution of a short UWB pulse, rich resolvable multipath makes the UWB channel act
like an underwater acoustical channel [27, 28]. The success of using time reversal for
underwater acoustical communications motivates the research UWB radio.
In a time-reversal scheme [29], the channel impulse response (CIR) h(t) is used as a
prefilter at the transmitter. After the encoded information bits pass through the channel,
the response of the channel is the autocorrelation of the CIR as shown in Figure 1.3.
A pilot signal can be used to sound the channel such that the transmitter has the CIR
available for precoding. This scheme is used to shift the design complexity from the
receiver to the transmitter. In the time-reversal scheme, a signal is precoded such that it
focuses both in time and in space at a particular receiver. Owing to temporal focusing,
the received power is concentrated within a few taps and the task of equalizer design
becomes much simpler than without focusing.
Time reversal can be justified in many ways. Let us use our version of justification. The
autocorrelation operation of the matched filter is a process of pulse compression. A UWB
channel can be treated as a quasi-static, reciprocal channel [30, 41]. Each multipath can
be treated as a `chip' as in a pseudonoise (PN) sequence. The longer the train of pulses
in a rich multipath channel, the sharper is the autocorrelation of the CIR. If we precode
the information symbols using the time-reversed version of CIR at the transmitter, the
resultant channel response, equal to response of a filter matched to the CIR will output
very sharp response equivalent to the autocorrelation of the CIR (similar to PN code)
(Figure 1.3(b)). This sharpened matched filter response will reduce ISI greatly if the
length of the multipath pulse train is large. This exploits the rich multipath, rather than
treating it as a nuisance. This simple precoding scheme yields a concentration of power
at only the intended receiver at a particular time. In Figure 1.3(b), the peak accounts for
50 % of the total energy of the CIR. Another important advantage of the time reversal is
spatial focusing. Spatial focusing results in very low cochannel interference in a multicell
system, resulting in a very efficient use of bandwidth in the overall network. Finally, time
reversal reduces the problem of the per-path pulse distortion since the autocorrelation
operation (correlating with its time reversal) yields a much smoother symmetric signal
waveform at the receiver (see Figure 1.3(b)).
1.3.4 MAC