Digital Signal Processing Reference
In-Depth Information
the fact that the average signal power that is available at the output of the chan-
nel should be at least 20 dB (100 times!) higher than the sensitivity level of
the receiver. For five resolved multipath components, only 4
.
9 dB of the link
budget must be spent to achieve the same 90% reliability requirement.
As a final remark, it is stressed that the fading problem discussed here is inde-
pendent of the problems related to frequency-selective fading originating from
isi: in the discussion above it was supposed that the transceiver system did
perfectly separate all resolvable multipath components.
Case study: reliability issues of 802.11a/g
One of the main advantages of using ofdm as the modulation method in a
wideband communication system is that the problem of isi is easily circum-
vented by splitting the frequency band in multiple subchannels. Each of the
subchannels is then modulated at a symbol rate which is much lower than the
rms delay spread of the channel. In the 802
.
11a/g system, for example, the
duration of a single ofdm symbol block (4
s) is guaranteed to much longer
than the delay spread in a typical indoor environment (10-70 ns). The rate at
which the data samples
12
are modulated is 50ns. At the antenna terminal of the
receiver, the energy related to one transmitted symbol is thus spread over two
or three adjacent data symbols.
In theory, the 802
.
11a/g system is thus able to combine the signal power of two
or three multipath components. This should not be interpreted as if there were
a bundle of individual symbols streams at the output of the ofdm receiver: it
is the sum of the signal power from all resolved multipaths that is equal to the
total power available from all subbands. Apart from the issue of isi, this implies
that if the number of virtually resolved multipath components becomes higher,
the deviation on the total power in all ofdm subbands becomes lower and the
system becomes more reliable. Since the rms delay spread of 802
.
11a/g is
limited to only three or less symbol periods for the typical indoor channel, the
reliability of the system is doomed to collapse in nlos fading channels that
lack a dominating multipath signal component: it follows from Figure 4.8 that
7 up to 10 dB of the total link budget is irreversibly lost under static nlos
conditions. In practice, this means that the transmitter is forced to reduced the
modulation depth in order to obtain a more reliable value for E
b
/N
0
.From
Table 1.1 (p. 16), it can be verified that a loss of 10 dB is equivalent to a
reduction of 4 in raw signal throughput, or a link failure when the modulation
depth cannot be reduced any more. Note that this problem will arise both in the
adaptive as well as in the bit interleaved (bicm) bit-loading schemes because it
μ
12
An ofdm symbol is composed of 64 complex data samples preceded by a 16-sample cyclic prefix. The
80 data samples with a duration of 50 ns result in the ofdm symbol period of 4
μ
s [Wla07].
