Introduction
Wireless is a very important aspect of communication which has greatly impacted on the lives of people in the society. It operates without a wire or electric conductor. However, wireless communication has undergone a lot of developments in the past. Several technological inventions have been done to improve it. This paper gives an in-depth and critical analysis of the use of Space–time block coding and the Alamouti code.
For a very long time now, wireless communication has been focused on using antenna. Its use forms an integral part of in this process. However, such antenna arrays were often located only at one end of the wireless link. The most preferred spot was at the receiver. However, this would later change after the presentation by Gerard J. Foschini, Emre Telatar and Michael J. Gans which led to the rise of the revolutionary Space–time block coding. The introduction of this coding system would be a turning point in the use of wireless communication since it enlarged the scope of this type of communication (Emre, T., 2009). Its use indicates that the use of multiple antennas in both the ends of the link can highly improve the capacity of the performance of wireless communication system. This coding can allow the alternative use of multiple transmit and receive antennas.
The use of STBC is crucial because it is a milestone in this communication. It facilitates the transmission of multiple types of data. This is done by transferring the data streams across a variety of antennas. In case it is efficiently executed, it becomes possible for the process of data transfer to tremendously improve. Hence, the transfer of data becomes reliable because of the improved connection between the antennas which are strategically placed in different parts of the link (Jafarkhani, H., 2001).
In its transfer, the transmitted signal has to be scattered, refracted and reflected in a well coordinated manner. As a result, it becomes easier for the transmitted signal to be transferred across a potentially challenging environment (Wang , H. & Xiang-Gen, X., 2003). The data is then corrupted by thermal noise which is found in the receiver. In the long run, a proper execution of the outlined procedure will ensure that the received set of data is better than the others. Therefore, the copies of the data will be of different quality. The antennas will thus the chances of attaining correctly decoded copies of the required set of data. This is a clear indication that Space Time Decoding is a combination of all the received copies of the signal. It increases its capacity to receive the maximum possible amount of signal possible (Tse, D. & Viswanath, P., 2005).
In explaining this coding, these scholars formulated a matrix which combines the time slot and the transmission by the antennas as shown herein:
In the matrix:
Represents modulated symbol which is transmitted
Besides, the code rate can be used to measure the number of symbols transmitted within one block. It can be denoted as:
.
The other aspect of designing STBC is based on the diversity criterion which was propounded by Tarokh et al. It enables the achievement of an Orthogonal STBCs.
Diversity criterion
Codeword
Erroneous codeword
.
The matrix becomes
However, despite this, this type of encoder has been criticized for being inefficient in performing its duties. This has been due to the fact that any code that fulfils its requirements is supposed to sacrifice a good proportion of its data rates. Thus, it has not been the best one to use since there are other better alternatives with an ability to accomplish optimal data rates.
Alamouti's Code
This type of coding was propounded in 1998 by Alamounti Siavash in his paper titled, Transmit Diversity Technique for Wireless Communication. In this presentation, Alamouti introduced the idea of a simple method of applying the transmission of two antennas to accomplish spatial diversity (Pahlavan, K. & Levesque, A. H., 2005). In his explanation, he invented the simplest STBC. Although he never coined space time block in his research, he opted to design a two-transmit antenna system which can be denoted by the following matrix:
In this matrix, * is used to denotes complex conjugate
In the transmission of two symbols, there is a need to use two time slots. In such an optimal decoding scheme, the bit-error rate (BER) in the STBC can be regarded to be equivalent to -branch maximal ratio combining (MRC). In this decoding, the above matrix can be used to prove the level of perfect performance of orthogonality of all the received symbols (Gerard J. F. & Michael. J. G., 2008). Hence, it is evident that copies of the data are received. This leads t the transmission of two copies of each symbol. This shows that Alamouti's Code is a very special STBC and probably the only orthogonal STBC which attains rate-1. This implies that this type of coding is the only sub-set of STBC which has the ability to attain its optimal diversity gain.
Despite the fact that Alamouti's Code is characterized by an error-rate performance, it still stands as the most advantageous code over other STBCs. Unlike the rest of the STBCs, it has complex modulations which enable it to possess nearly all the constellation diagrams o which it relies. It stands a better chance of delivering quality performance because of its complex modulations (Krishnamurthy, P. & Pahlavan, K., 2002). In fact, its invention would revolutionize wireless communication since it emerged as the only encoding method that promotes a full diversity which incorporates linear processing at the receiver.
It represents a development from the earlier encoders because they were requiring processing schemes by involving the use of a variety of transmit antennas. Alamouti's Code is better as it shows the improvements in the encoding sector which was initially based on techniques that would not facilitate a high level of communication to satisfy people’s demands (Geier, J., 2001). It was the first type of open loop transmits diversity technique with a capacity to use linear processing at the receiver. As a result, its eminent contributions in the wireless communication industry are evident particularly following the subsequent generalizations of its concept.
The research done by this scholar was an invaluable one because it transformed wireless communication. One of the major concerns for the stakeholders in this sector is attain the maximum possible speed. Although it has been a very great challenge to attain a full speed of 1, Alamouti's Code offers a solution to the crisis. As a result of its invention, it has promoted the use of multiple antennas (Alamouti, S,M., 2008). The use of multi-antennas is a remarkable development because it allows for a full exploitation of the spatial diversity. It is very simple to implement it was compared to other encoding technologies. At the same time, it is becoming the popular Orthogonal Frequency Division Multiplexing (OFDM) enabling it to have right modulations for transmitting in selected frequency channels. It can easily convert them into a bunch of fading channels necessary for different types of combinations such as various efficient CP-OFDM and STC and schemes.
In conclusion, it is important to acknowledge that communication is a very powerful tool I the modern society. It is the only way through which people ca get an opportunity to send messages and share ideas. Wireless communication has greatly transformed the society. However, in order to make it perfect, scholars and experts should to conduct intensive research to discover new ideas to transform and improve STBC ns Alamouti's Code.
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