![](https://csdnimg.cn/release/download_crawler_static/2065181/bg13.jpg)
Wireless Communication and CDMA
1-5
signals intended for different users. The channel assignment is essentially
determined by the set of codes that are used for that particular link. Thus, the
signal transmitted at any time in a logical channel is determined by:
•The frequency of operation for the base station
•The current symbol
•The specific orthogonal spreading code assigned for the logical channel
•The PN spreading code
CDMA Signal Processing
In the demodulation of CDMA signals, the different paths may be
independently received, which greatly reduces the severity of the multipath
fading. However, multipath fading is not completely eliminated because
occasionally there may be multiple paths that cannot be independently
processed by the demodulator.
Different users in CDMA employ signals that have very small
cross-correlation. Thus, correlators can extract individual signals from a
mixture of signals even though they are transmitted simultaneously in the
same frequency band. CDMA systems employ wideband signals with good
cross-correlation properties, which means the output of a filter matched to one
user’s signal is small when it receives a different user’s signal as input.
In direct-sequence spread-spectrum systems, a high-rate antipodal
pseudorandom spreading sequence modulates the transmitted signal so that
the bandwidth of the resulting signal is roughly equal to the rate of the
spreading sequence. The cross-correlation of the signals is then largely
determined by the cross-correlation properties of the spreading signals.
Although CDMA signals overlap in both time and frequency domains, they can
be separated, based on their spreading waveforms.
Spreading rates can be chosen to exceed the coherence bandwidth so that the
channel becomes frequency selective. For instance, different spectral
components are affected unequally by the channel, and only parts of the signal
are affected by fades. Expressing the same observation in time domain terms,
multipath components are resolvable at a resolution equal to the chip period
and can be combined coherently, for example, by means of a rake receiver.
Coherent combination of multipath components requires an estimate of the
channel impulse response. Such an estimate can be calculated from a training
sequence or by means of a pilot signal.