ST2115
Scientech Technologies Pvt. Ltd.
17
Advantages of Asynchronous CDMA over other techniques
Asynchronous CDMA's main advantage over CDM (
Synchronous
CDMA), TDMA
and FDMA is that it can use the spectrum more efficiently in mobile telephony
applications. TDMA systems must carefully synchronize the transmission times of all
the users to ensure that they are received in the correct timeslot and do not cause
interference. Since this cannot be perfectly controlled in a mobile environment, each
timeslot must have a guard-time, which reduces the probability that users will
interfere, but decreases the spectral efficiency. Similarly, FDMA systems must use a
guard-band between adjacent channels, due to the random doppler shift of the signal
spectrum which occurs due to the user's mobility. The guard-bands will reduce the
probability that adjacent channels will interfere, but decrease the utilization of the
spectrum.
Most importantly, Asynchronous CDMA offers a key advantage in the flexible
allocation of resources. There are a fixed number of orthogonal codes, timeslots or
frequency bands that can be allocated for CDM, TDMA and FDMA systems, which
remain underutilized due to the bursty nature of telephony and packetized data
transmissions. There is no strict limit to the number of users that can be supported in
an
Asynchronous
CDMA system, only a practical limit governed by the desired bit
error probability, since the SIR (Signal to Interference Ratio) varies inversely with the
number of users. In a bursty traffic environment like mobile telephony, the advantage
afforded by Asynchronous CDMA is that the performance (bit error rate) is allowed
to fluctuate randomly, with an average value determined by the number of users times
the percentage of utilization. Suppose there are 2N users that only talk half of the
time, then 2N users can be accommodated with the same
average
bit error probability
as N users that talk all of the time. The key difference here is that the bit error
probability for N users talking all of the time is constant, whereas it is a
random
quantity (with the same mean) for 2N users talking half of the time.
In other words, Asynchronous CDMA is ideally suited to a mobile network where
large numbers of transmitters each generate a relatively small amount of traffic at
irregular intervals. CDM (
Synchronous
CDMA), TDMA and FDMA systems cannot
recover the underutilized resources inherent to bursty traffic due to the fixed number
of orthogonal codes, time slots or frequency channels that can be assigned to
individual transmitters. For instance, if there are N time slots in a TDMA system and
2N users that talk half of the time, then half of the time there will be more than N
users needing to use more than N timeslots. Furthermore, it would require significant
overhead to continually allocate and deallocate the orthogonal code, time-slot or
frequency channel resources. By comparison, Asynchronous CDMA transmitters
simply send when they have something to say, and go off the air when they don't,
keeping the same PN signature sequence as long as they are connected to the system.
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