1.
Spread spectrum communication systems are widely used today in a variety of applications for different
purposes such as access of same radio spectrum by multiple users (multiple access), anti-jamming
capability (so that signal transmission can not be interrupted or blocked by spurious transmission from
enemy), interference rejection, secure communications, multi-path protection, etc. However,
irrespective of the application, all spread spectrum communication systems satisfy the following criteria(i) As the name suggests, bandwidth of the transmitted signal is much greater than that of the message
that modulates a carrier. (ii) The transmission bandwidth is determined by a factor independent of the
message bandwidth.
On BPSK modulation, the spread signal becomes, s(t) = d(t).coswt. Fig.7.38.1 (b) shows the
baseband processing operations necessary after carrier demodulation. Note that, at the receiver,
the operation of despreading requires the generation of the same spreading code incorrect phase
with the incoming code. The pseudo noise (PN) code synchronizing module detects the phase of the
incoming code sequence, mixed with the information sequence and aligns the locally generated
code sequence appropriately. After this important operation of code alignment (i.e.
synchronization) the received signal is ‘despread’ with the locally constructed spreading code
sequence. The dispreading operation results in a narrowband signal, modulated by the information
bits only. So, a conventional demodulator may be used to obtain the message signal estimate.
Advantages of Spread Spectrum (SS) Techniques
Reduced interference: In SS systems, interference from undesired sources is considerably
reduced due to the processing gain of the system.
b) Low susceptibility to multi-path fading: Because of its inherent frequency diversity
properties, a spread spectrum system offers resistance to degradation in signal quality due to
multi-path fading. This is particularly beneficial for designing mobile communication systems.
c) Co-existence of multiple systems: With proper design of pseudo-random sequences, multiple
spread spectrum systems can co-exist.
d) Immunity to jamming: An important feature of spread spectrum is its ability to withstand strong
interference, sometimes generated by an enemy to block the communication link. This is one reason
for extensive use of the concepts of spectrum spreading in military communications.
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RAKE receiver, used specially in CDMA cellular systems, can combine multipath
components, which are time-delayed versions of the original signal transmission.
The basic idea of a Rake receiver was first proposed by Price and Green.
Rake receiver is multiple parallel receivers used to combat multi-path
interference and Inter-symbol interference.
Due to reflections from obstacles a radio channel can consist of many copies of
originally transmitted signals having different amplitudes, phases, and delays. If
the signal components arrive more than duration of one chip apart from each
other, a Rake receiver can be used to resolve and combine them. The Rake
receiver uses a multipath diversity principle.
When the signal travels from transmit end to the receive end, it will go through
multiple paths. This results into multiple versions of the transmit signals received
at the receiver. Each of these signals will have different attenuations and path
delays.
In the cellular systems this multipath interference is exploited to recover the
transmitted signal.
Rake receiver reduces effects of fading and provides spectral efficiency
improvement of CDMA.
Rake receiver attempts to collect the time shifted versions of the original signal
by providing a separate correlation receiver for each of the multipath signals.
This can be done due to multipath components are practically uncorrelated from
another when their relative propagation delay exceeds a chip period.
A Rake receiver utilizes multiple correlators to separately detect M strongest
multipath components. The outputs of each correlator are weighted to provide
better estimate of the transmitted signal than is provided by a single component.
Demodulation and bit decisions are then based on the weighted outputs of the M
correlators.
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5.
Assume M correlators are used in a CDMA receiver to capture M strongest
multipath components. A weighting network is used to provide a linear
combination of the correlator output for bit decision. Correlator 1 is synchronized
to the strongest multipath m1. Multipath component m2 arrived t1 later than m1
but has low correlation with m1.
The outputs of the M correlators are denoted
as Z1,Z2,…,andZM.Z1,Z2,…,andZM. They are weighted by α1, α2,…, and αM,
respectively.