Digital Signal Processing Reference
In-Depth Information
Unfortunately, equalization will never compensate for the low signal-to-noise
ratios in subbands affected by destructive fading. Manufacturers of 802
.
11a/g-
compatible devices solve this issue in several ways. The most drastic (but also
effective) measure is to switch to a lower coding rate
R
or decrease the modu-
lation depth over all subchannels at once. More advanced receiver implemen-
tations also exploit the space-diversity offered by the use of multiple antennas
to diminish the possibility of destructive interference [Mat04].
Furthermore, in order to give the error coding algorithm better chances to re-
cover the original message, a two-level bit interleaver [Cai98, Wla07] is placed
between the convolutional encoder and the symbol mapper in the transmitter.
Doing so, it is ensured that (1) consecutive data bits provided by the encoder
are mapped onto non-adjacent subbands and that (2) data bits are placed at
alternating locations in the bit word of the Gray-coded constellation diagram.
Bit-interleaving must be considered as the stopgap solution for when all other
techniques fail. It exploits the frequency diversity across the 52 subbands used
in 802
.
11a/g and prevents a high concentration of errors in the stream of data
bits that is applied to the decoder. It is obvious that bit-interleaving does not re-
ally solve the problems that are caused by the absence of the adaptive loading
technique. It merely avoids a catastrophic failure of the error coding mecha-
nism by equalizing the noise power over a longer sequence of bits. Obviously,
bit-interleaving can and will never be a match for a decent adaptive loading
subsystem. The conclusion here is that there is always a trade-off between
complexity and power consumption at one side and implementation losses (il)
in terms of throughput at the other side. The quantity and nature of channel-
related issues that are faced in the fast-varying wideband wireless environment
have forced the IEEE 802
.
11a/g standardization committee to trade a signifi-
cant amount of throughput performance for a more manageable hard- and soft-
ware complexity.
2.6
Wideband single-carrier modulation
It has become abundantly clear that high-speed communication systems are not
just an upscaled version of their narrowband counterparts. Multipath disper-
sion in the wideband channel opens a whole spectrum of new problems related
to intersymbol interference (isi) and frequency-selective fading. As if that's
not enough, most broadband wireless applications with a high market poten-
tial are only licensed to operate in unlicensed frequency bands, crowded with
a myriad of intended and not so intended interferers. At first sight, it would
seem that every broadband system with a single-carrier modulation scheme
is an easy victim on every front in such an hostile environment. Although it
is perfectly possible to combat the problem of isi with a time- or frequency
