Digital Signal Processing Reference
In-Depth Information
problematic when a lot of short data packets must be exchanged, such as the
tcp/ip packets in a wlan network. Under these circumstances, only a very
restricted spreading sequence length can be used.
A perfect example of this is provided by 802
.
11b wlan devices. The dsss
modulation scheme employed in the 802
.
11b standard is
complementary code
keying
(cck). More details on cck can be found in [And00], but the most
important feature of cck is that an 8-chip quadrature spreading code is used.
From the total set of 4
∧
8 (65k) possible spreading codes, a subset of only 64
spreading codes is retained. This allows the receiver to wrap 6 data bits in the
8-chip spreading sequence. The complete sequence can also be rotated by 0
◦
,
90
◦
, 180
◦
or 270
◦
. Using a differential encoding scheme, two extra bits are
thus represented by the rotation angle of the sequence, which results in a to-
tal of 8 bits for each eight-chip codeword. Finally, the encoded data is sent
at 11 Mchip/s through the channel. In the receiver, the channel is monitored
by a bank of 64 parallel correlators and only the codeword with the largest
correlation result is selected. It is indeed thanks to the large amount of paral-
lel processing power that synchronization is achieved within the duration of
a single symbol interval. If the subset of 64 codewords is carefully selected
with respect to the auto- and cross-correlation properties, limited protection
against multipath interference can be achieved. However, the practical coding
gain of cck is limited to only 2 dB above the performance of uncoded qpsk.
This is due to the small minimum distance between neighbouring codewords
[Hee01]. The resistance of cck against multipath interference is very limited
if compared to the results that can be achieved in the (low-rate) gps system.
A severe disadvantage of the cck system used by 802
.
11b compatible devices
is the limited spreading factor. The consequence is that the energy of narrow-
band interferers is indeed averaged over the spectrum, but cannot be removed:
after despreading (correlation), the complete frequency band is still used by
the signal-of-interest. This is in sharp contrast to the spectral redundancy ex-
ploited in the gps system. Chances that a 802
.
11b transmission survives the
interference of a moderate powered in-band spur are thus very slim. But as bad
luck would have it, 802
.
11b devices are only licensed to operate in the unli-
censed 2
.
4 GHz ism
16
band. This band was opened up by the fcc in 1985 for
low-power unlicensed communication devices, only because that part of the
spectrum was already polluted anyway by rf-leakage from applications such
as industrial heaters and microwave ovens.
The rf-energy leaking from a microwave oven, for example, makes repeated
sweeps from 2
.
4to2
.
45 GHz [Kam97], temporarily blanking out parts of
the wireless channel for wireless devices using the same frequency band. To
16
ism band: internationally reserved radio band for industrial, scientific and medical use.
