1. Introduction

With the great advances of communication technologies, the communication paradigm has widely been shifted from point-to-point to multi-user wireless systems to support ever-increasing number of mobile devices being introduced in the market. The proliferation of mobile devices has necessitated an elaborate mechanism to serve multiple users over a shared communication medium. The most important building block in this mechanism is multiplexing approach. The multiplexing refers to a method which aims at combining multiple signals into one signal such that each user would be able to extract its desired data upon receiving the multiplexed signal. Figure 1 shows a communication system with three sources and corresponding destinations at system level. As shown in Figure 1(a), the system without multiplexing requires three different communication links each of which carrying the signal of single source toward its destination exclusively. Such a system is inefficient since it demands triple times more communication resources than the same system with multiplexer/de-multiplexer shown in Figure 1(b). With the aid of multiplexing, signals of all sources will be superimposed into one signal and sent over single available communication link. Establishing a successful transmission over the single link preserves valuable resources and decreases communication costs. Furthermore, serving multiple users through a channel result in massive connectivity, which paves the way for current and next generation of wireless networks designed for crowded urban areas. In today's communication, multiplexing has penetrated many communication applications ranging from digital broadcasting to Wi-Fi networks. Multiplexing brings the following advantages to wireless communication systems.


**28**

2013

9788120319561

*Multiplexing*

[1] Stallings W. Data and Computer Communications tenth edition. Upper Saddle River: Pearson Education, Inc. Pearson Prentice Hall Pearson Education, Inc; 13 September 2013

[2] Forouzan BA. Data Communications and Networking. 4th edition. McGraw

[3] Schiller J. Mobile Communications. tenth edition 13 September 2013 Pearson Education, Inc. Pearson Prentice Hall Pearson Education, Inc. Upper Saddle River, NJ 07458 Second Edition

[4] Schiller JH. Mobile Communications 2nd edition Pearson Education India. 2008;**2**. Available from: https://nptel. ac.in/courses/106105080/pdf/M2L7.pdf

[5] Pechetty TR, Vemulapalli M. An implementation of OFDM transmitter

[6] Floyd TL. Digital Fundamentals. 11 th Edition 2015. Upper Saddle River, NJ, USA: Pearson publication Prentice-Hall Inc; ISBN: 9780132737968, 0132737965.

[7] Nair BS. Digital Electronics and Logic Design. PHI Learning Private Limited. 1 January 2002; ISBN:

[8] Yarbrough JM. Digital Logic Application and Design. 1 edition Cengage Learning; ISBN-10: 9788131500583. 2006

[9] Available from: https:// www.tutorialspoint.com/ principles\_of\_communication/

modulation\_techniques.htm

principles\_of\_communication\_digital\_

and receiver on reconfigurable platforms. International Journal of Advanced Research in Electrical, Electronics and Instrumentation

Engineering. November 2013;**2**(11):5486-5490

Hill Higher Education

Figure 1.

A communication system comprising three sources and destinations: (a) without multiplexing and (b) with multiplexing.


Figure 2 depicts an example of synchronous TDM for three independent sources

in three cycles. The switch rotates between states (sources) with the rate of 1000 cycle per second. Hence, the cycle time is 1 ms, and each time slot equals to 333:3 μs. In the jth cycle, the ith source may have message Mij or nothing to be transmitted. For instance, in second cycle, source one remains idle while source two and source three have M<sup>22</sup> and M<sup>32</sup> for transmission, respectively. Therefore, these messages occupy the second and third time slots in the second cycle while the second time slot will be wasted without conveying any message. An unused time slot is shown with hatched rectangular. Each cycle could be preceded/terminated with a preamble/postamble enabling destinations to detect beginning/end of a cycle. The detail of each cycle is shown in the figure. Obviously, in each cycle, one third of the airtime will be wasted which drastically degrades the throughput of the

system. To prevent squandering airtime, asynchronous TDM has emerged.

Asynchronous TDM, also known as statistical TDM, pursues a more dynamic approach by giving the airtime to sources that have data for transmission. In this manner, the messages of different sources occupy all subsequent time slots, which yield improvement of spectrum utilization in turn. As a well-known application, asynchronous TDM is used in asynchronous transfer mode (ATM) networks [5]. To demonstrate the possible gain of the asynchronous TDM over synchronous TDM, let us consider the sources previously shown in Figure 2. This time, the asynchronous TDM is applied on the system, as shown in Figure 3. In each cycle, the switch passes through all sources and transfers the existing messages to frame assembler. The frame assembler tags a preamble to each message. The preambles include an ID or address field to notify the origin or intended destination of the message attached to the preamble. Then, the frame assembler aggregates the tagged messages and disregards the idle time intervals related to silent sources. The tagged messages occupy subsequent time slots and are transmitted sequentially. As shown in the figure, each cycle carries messages of sources that have something to transmit; hence, the cycles include two-time slots which improves the spectral utilization by 33%: Although the cycles duration are equal in the example, the transmission duration may vary depending on the number of messages to be carried. Clearly, asynchronous TDM demands more processing capability at multiplexer and de-multiplexer, and it may cause a delay up to one cycle for buffering and aggregating the existing messages. However, it is worthwhile since it yields higher spectral efficiency and saving valuable resources. As another advantage,

2.2 Asynchronous time division multiplexing

Figure 2.

31

Synchronous time division multiplexing.

Overview of Multiplexing Techniques in Wireless Networks

DOI: http://dx.doi.org/10.5772/intechopen.85755

Based on the application requirements, available spectrum, and users' hardware capability, the appropriate multiplexing approach will be designed over the required domain which could be time, frequency, power, code, wavelength or delay-Doppler domain. For example, time division multiplexing is not suitable for a delay-sensitive application, and spatial multiplexing is very amenable for a network with multi-antenna nodes. Among all of the aforementioned domains for multiplexing, only wavelength division multiplexing exclusively targets communication over the fiber cables while others suit wireless communication. In this chapter, the most important multiplexing approaches are studied.
