Code Division Multiple Access (CDMA) continued [Under Multiple Access]
There are two types of CDMA.
1) Frequency Hopping CDMA (or frequency hopping spread spectrum)
2) Direct Sequence CDMA (or direct sequence spread spectrum)
The examples shown in earlier article is for DS-CDMA.
In FH-CDMA, the information bits are carried by hopping the frequencies. The sequence of frequencies forms the code. Refer Wiki page on FHSS for more details. FH-CDMA is used in military systems.
Commercial systems (like IS-95/CdmaOne) use DS-CDMA.
DS-CDMA
Let us have a look at example of DS-CDMA given in earlier article in terms of bit wise operations..
User's data is 0 1
Spreading code is 0 1 0 1
Now these are "Exclusive ORd" - 0 1 0 1 1 0 1 0
On receiving side, we Ex-OR 0 1 0 1 1 0 1 0 with (repeated) C i.e. 0 1 0 1 0 1 0 1
That gives us 0 0 0 0 1 1 1 1 i.e 0 1
Note that in practice, coding is applied more than once, like in IS-95/cdmaOne and cdma2000, it is done thrice (once with Walsh code, then with Short code and Long code) !
In addition to spreading codes, other parameters of DS-CDMA are Chip rate, Spreading factor, and Processing gain.
Chip rate relate to coding rate, chip is individual bit of which code is made up of. IS-95/cdmaOne and cdma2000 use chip rate of 1.2288 Mcps whereas W-CDMA (3G/UMTS) use 3.84 Mcps.
Spreading factor is ratio of code rate to information rate. For IS-95, spreading factor is 64.
Processing gain is same as Spreading factor but related to Power density and so expressed in dB. In effect DS-CDMA converts a narrow band signal (low information rate, low bandwidth, narrow frequency band) to wide band signal (higher chip/code rate, higher bandwidth, wider frequency band). So Power is distributed. At receiving side, spreading code is applied again to get back original information signal. Due to nature of spreading code, we not only get back original narrow band signal, but also noise/inteference signal is spreaded (reducing their power density). This is a difference between CDMA and other multiple access methods. Even though spreaded signal is below the noise power, it can be recovered. Refer the (rough) diagram below:
The ratio or difference between received wide band signal and retrieved narrow band signal is Processing Gain. It is apparent from above discussion that more spread, will give us from better PG. It can be shown that PG = 10 log10 ( Chip rate / Information rate ). IS-95/cdmaOne PG for 9.6 kpbs voice information signal would be 21 dB.
It is apparent that as more number of users are added (and so the power of spreaded signal received), processing gain would reduce. Once PG reach to the level of "despreaded" noise power, we reach the limit of number of simultaneous users that can be supported by DS-CDMA systems. In ideal case, number of unique spreading codes determine number of users that can be supported simultaneously. Refer PG Article by Fakatselis for more on "Processing Gain". Also check out Intro to CDMA from Scott Baxter for discussion of processing gain in view of Shanon's transmission channel capacity equation.
References: Intro to CDMA from Scott Baxter, CDMA tutorial from complex2real site, Wiki page on FHSS, and PG Article by Fakatselis.
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