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

Figure 12. 13
DMT transmission
of N
different symbol
streams over a
single FIR
channel.
:&•
SN [kl
Add
s, [k 1--.:_. IDFT cylic
:i' i [k ]
prefix
12.8 Discrete Multitone (DMT) Modulations 703
>-----, Parallel ,____,_ L h[k]z- ,
to serial
io::O
FIR channel
L
SN - L +l
ZN ZN-I . . . Z I
r z 1
R _ e _ B·
m - ov - e
Serial to
cyclic f-------- ·: DFT .
parallel
prefix
different subcarriers will encounter different SNRs at the receiver end. In DMT, the receivers
are responsible for conveying to the transmitter all the subchannel information. As a result, the
transmitter can implement compensatory measures to optimize various performance metrics.
We mention two common approaches adopted at DMT transmitters:
Subcarrier power loading to maximize average receiver SNR.
Subcarrier bit loading to equalize the bit error rate (BER) across subcarriers.
Transmitter Power Loading for Maximizing Receiver SNR
To describe the idea of power loading at the transmitter for maximizing total receiver SNR,
let s;[k] be the data stream carried by the ith subchannel and call {s;[k]} an independent data
sequence in time k. Let us further say that all data sequences {s; [k]} are also independent of
one another. Let the average power of s;[k] be
The total channel input power is
whereas the corresponding channel output power at the receiver equals
Hence, the total channel output SNR is
M
L IH [i] l 2 · P;
i=l
"' M IH [i] l 2 . P· 2
L..i=I
NN/2
M
l = - "IH [i] l 2 . P ·
NN
i=I