Essay: CDMA systems

In CDMA systems, the narrowband message signal is multiplied by a very large bandwidth signal is a Spread sequence as space time block code Space’time block coding is a technique used in wireless communications to transmit multiple copies of a data stream across a number of antennas and to exploit the various received versions of the data to improve the reliability of data-transfer. The fact that the transmitted signal must traverse a potentially difficult environment with scattering, reflection, refraction and so on and may then be further corrupted by thermal noise in the receiver means that some of the received copies of the data will be ‘better’ than others. This redundancy results in a higher chance of being able to use one or more of the received copies to correctly decode the received signal. In fact, space’time coding combines all the copies of the received signal in an optimal way to extract as much information from each of them as possible [xxxx]. An STBC is usually represented by a matrix. Each row represents a time slot and each column represents one antenna’s transmissions over time.

Here, is the modulated symbol to be transmitted in time slot from antenna . There are to be time slots and transmit antennas as well as receive antennas. This block is usually considered to be of ‘length’ . The code rate of an STBC measures how many symbols per time slot it transmits on average over the course of one block [xxxxx] If a block encodes symbols, the code-rate is

The design of STBCs is based on the so-called diversity criterion derived by Tarokh et al [1]. in their earlier paper on space’time trellis codes. Orthogonal STBCs can be shown to achieve the maximum diversity allowed by this criterion.
Call a codeword

And call an erroneously decoded received codeword

Then the matrix

Complex orthogonal space-time block code for any number of transmits antennas n , is described by a p ?? n transmission matrix ??

All users in a CDMA system use the same carrier frequency and may transmit simultaneously. Each user has its own spreading codeword which is approximately orthogonal to all other code words. The receiver performs a time correlation operation to detect only the specific desired codeword. All other code words appear as noise due to decorrelation. For detection of the message signal, the receiver needs to know the codeword used by the transmitter. Each user operates independently with no knowledge of the other users. CDMA is achieved by modulating the data signal by Space Time Block Code..

Improving the capacity of code-division-multiple access (CDMA) systems through advanced signal processing has been an area of intensive research for many years, with limited success. Multiantenna technologies called multiple-input multiple outputs (MIMO) are an obvious candidate to increase, particularly, downlink capacity. Nearly all research on MIMO’CDMA, however, has focused on increasing the throughput achieved per user, rather than increasing the number of supportable users, which is still the most important design goal in QoS-constrained voice systems. The objective is to evaluate the MIMO OFDM performance and to reach its optimal data transmission. This was accomplished by selecting an OFDM standard and evaluating its performance under several user-channel profiles

A novel transmission scheme is developed to effectively combine permutation spreading technique with MIMO-OFDM to obtain improved bit error rate performance in the presence of frequency selective fading channels with low system complexity. Unlike conventional MIMO-OFDMA, where users are separated in different frequency bands (sub channels), and each user is coded separately using STBC or SFBC, the proposed new scheme enables multi access by joint code design across multiple antennas, subcarriers, and users. Such system will benefit from the combined space and frequency domain freedom as well as multiuser diversity. Hence, better spectrum efficiency is achieved while improving bit error rate performance with respect to signal-to-interference ratio.

Orthogonal space-time block codes (O-STBC) achieve high transmit diversity and have a low complexity decoding algorithm at the receiver using any number of transmit and receive antennas.

Source: Essay UK - http://doghouse.net/essays/information-technology/essay-cdma-systems/


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