Contents
What is autocorrelation in CDMA?
Auto-correlation is simply a comparison of a signal against itself. For a digital sequence, such as the short codes used in EIA/TIA-95-B, this comparison is a measure of the number of bits that match relative to the number of bits that do not match.
Why CDMA called spread spectrum communication?
In Code Division Multiple Access (CDMA) systems all users transmit in the same bandwidth simultaneously. Communication systems following this concept are “spread spectrum systems”. In this transmission technique, the frequency spectrum of a data-signal is spread using a code uncorrelated with that signal.
What is spreading in CDMA?
A spreading sequence is a binary sequence that appears random but can be reproduced in a deterministic manner by intended receivers. These spreading sequences are used to encode and decode a user’s signal in asynchronous CDMA in the same manner as the orthogonal codes in synchronous CDMA (shown in the example above).
How the PN sequence code is selected in CDMA?
PN Sequence The DS-CDMA system uses two types of spreading sequences, i.e., PN sequences and orthogonal codes. As mentioned above, the PN sequenc is generated by the pseudo-random noise generator.
Why do we need to choose chip sequence in CDMA?
The key to CDMA is to be able to extract the desired signal while rejecting everything else as random noise. In CDMA, each bit time is subdivided into m short intervals called chips. Typically there are 64 or 128 chips per bit. In order to protect the signal, the chip sequence code used is pseudo-random.
Is DSSS and CDMA same?
Direct sequence spread spectrum (DSSS) transmission employs a chipping code to “spread” the transmission over a wider frequency band than it would normally occupy. DSSS is a form of CDMA (see Chapter 4). But instead of a pseudorandom binary sequence (PRBS), an 11-bit Barker code is used as the spreading sequence.
Where is CDMA used?
In the US, Verizon, US Cellular, and the old Sprint network (now owned by T-Mobile) use CDMA. AT and T-Mobile use GSM. Most of the rest of the world uses GSM. The global spread of GSM came about because in 1987, Europe mandated the technology by law, and because GSM comes from an industry consortium.
Why is code selection important in a CDMA system?
Code selection. The spreading of the data signal in a cdma system is done by applying a code, independent of the data-signal. Code-selection has a large impact on the performance of the system. On the one hand the longer the code, the higher the processing gain which enables us to allow more users in the system.
How is modulation done in a CDMA signal?
In CDMA we do modulation twice. First with a binary sequence g(t), the properties of which we will discuss below and then by a carrier. The binary sequence modulation ahead of the carrier modulation accomplishes two functions, 1. It spread the signal and 2.
How are Gold codes spread in a cdmasystem?
In other words, some Gold codes and Kasami codes have large cross-correlation; and/or they have large autocorrelation for au eq 0. The spreading of the data signal in a cdmasystem is done by applying a code, independent of the data-signal.
What is the autocorrelation function of an M-sequence?
The autocorrelation function of m-sequence is close to \\delta (\au) and is robust to time offset. However, the cross-correlation of two m -sequences tends to be large. Gold codes and Kasami codes, if carefully selected, have autocorrelation functions and cross-correlation functions close to the ideal codes.