B. P. Lathi, Zhi Ding - Modern Digital and Analog Communication Systems-Oxford University Press (2009)

702 DIGITAL COMMUNICATIONS UNDER LINEARLY DISTORTIVE CHANNELS
Figure 12. 1 2
Using a bank of
receiver gain
adjusters for N
independent
AWGN channels
in OFDM to
achieve gain
equalization.
s,
• I\ • • •
. . . .
w. t . . .
channel equalization is not necessary. However, because each subchannel has a different gain,
the optimum detection of {sn} from
z[n] = H[n] s n
n = 1, ... , N
would require knowledge of the channel gain H[n]
in = dee (H[nr 1 z[n])
n = 1, ... , N
This resulting OFDM receiver is shown in Fig. 12.12. For each subchannel, a one-tap
gain adjustment can be applied to compensate the subchannel scaling. In fact, this means that
we need to implement a bank of N gain adjustment taps. The objective is to compensate the
N subchannels such that the total gain of each data symbol equals unity before the QAM
decision device. In fact, the gain equalizers scale both the subchannel signal and the noise
equally. They do not change the subchannel SNR and do not change the detection accuracy.
Indeed, equalizers are used only to facilitate the use of the same modular decision device on all
subchannels. Oddly enough, this bank of gain elements at the receiver is exactly the same as
the equalizer in a high-fidelity audio amplifier. This structure is known henceforth as a one-tap
equalizer for OFDM receivers.
12.8 DISCRETE MULTITONE (DMT) MODULATIONS
A slightly different form of OFDM is called discrete multitone (DMT) modulation. In DMT,
the basic signal processing operations are essentially identical to OFDM. The only diffe rence
between DMT and a standard OFDM is that DMT transmitters are given knowledge of the
subchannel information. As a result, DMT transmits signals of differing constellations on
different subchannels (known as subcarriers). As shown in Fig. 12.13, the single RF channel is
split into N subchannels or subcarriers by OFDM or DMT. Each subcarrier conveys a distinct
data sequence:
{ · ·· Sj [k + l] s;[k] s;[k - 1] · · ·}
The QAM constellations of the N sequences can often be different.
Because the original channel distortion is frequency selective, subchannel gains are generally
different across the bandwidth. Thus, even though DMT or OFDM converts the channel
with ISI distortion into N parallel independent channels without ISi, symbols transmitted over