Concept:
As per the previous discussion, CDMA stands for Code Division Multiple Access. It is a multiplexing access method in which multiple users are allocated different codes to access the same channel. Where channel is the set of frequencies and code is nothing but the sequence of n symbols (0 or 1). Mobile telephony systems which use CDMA are referred as CDMA systems.
CDMA uses cellular network and is used for long distance communication. The initial evolution of CDMA was as 2.5 G. Nowadays CDMA supports high data rate and is thus considered as 3G. It transmits not only voice but also data and multimedia streams.
CDMA uses cellular network and is used for long distance communication. The initial evolution of CDMA was as 2.5 G. Nowadays CDMA supports high data rate and is thus considered as 3G. It transmits not only voice but also data and multimedia streams.
Features:
The important features of CDMA systems are:
1. It is more robust for multi-path delay.
2. Signals from each mobile station are coded with two or three codes.
3. It gives higher quality voice and data transmission with small bit error rate.
4. It has soft handover.
5. It performs power control by open loop or close loop methods.
The important features of CDMA systems are:
1. It is more robust for multi-path delay.
2. Signals from each mobile station are coded with two or three codes.
3. It gives higher quality voice and data transmission with small bit error rate.
4. It has soft handover.
5. It performs power control by open loop or close loop methods.
CDMA codes allow unique identification of signals from different sources and transmit these signals through the same time, space and frequency slots without interference. Thus it is necessary that CDMA codes should not correlate with each other. The various types of codes are used and are discussed below.
Types of Codes:
1. Autocorrelation Codes:
An autocorrelation code is a multi-bit code which is used for coding the symbols before transmitting, allows the receiver to correlate and extract the symbols automatically. Barker codes and Pseudo-noise (PN) codes are the examples of autocorrelation codes. Barker code gives strong autocorrelation where as PN code gives strong peak with few, low and non-zero values which may results in interference with the users using same spread. Long M-sequence PN code has strong autocorrelation and is used as scrambling code. A short PN code also has strong autocorrelation. for example, in WCDMA short PN code is used.
An autocorrelation code is a multi-bit code which is used for coding the symbols before transmitting, allows the receiver to correlate and extract the symbols automatically. Barker codes and Pseudo-noise (PN) codes are the examples of autocorrelation codes. Barker code gives strong autocorrelation where as PN code gives strong peak with few, low and non-zero values which may results in interference with the users using same spread. Long M-sequence PN code has strong autocorrelation and is used as scrambling code. A short PN code also has strong autocorrelation. for example, in WCDMA short PN code is used.
2. Orthogonal Codes:
Two codes used for coding are referred as orthogonal when there is no effect of interference between two sets of signals on the received output. Orthogonal code do not show the strong autocorrelation property and thus requires the synchronization between transmitter and receiver. It gives zero cross-correlation. Examples of this code are Best code and Walsh code.
Best codes are optimized codes which allows significant correlation and do not create interference in between different channels. Whereas Walsh code are generated from Hadamard matrix with all pairs of rows are orthogonal. Walsh code is used in IS-95/cdmaOne. This code is used for channelization due to orthogonality property. Following table gives information about the different CDMA standards.
Different CDMA standards:
Conclusion:
Here we discussed about concept, features, types of codes used in different CDMA standards and different CDMA standards with their properties and uses also.
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This article is the topic of unit " Communication Technologies-II " from RTMNU MBA 3 rd sem IT syllabus notes.Further topics will be covered in upcoming blogs For more notes you can also refer to other links as given below:
1.
http://rtmnupervasivecomp.blogspot.com/
2.
3.
)
here is a complete information on CDMA....
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ReplyDeleteit is good but,here is no examples on autocorrelation orthogonal codes
ReplyDeleteplease give some examples on autocorrelation orthogonal code
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