Wireless Network Design: Optimization Models and Solution ...

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2 Introduction to Wireless Communications 31 the node backs off from transmission for a random amount of time and then senses the channel again. However, CSMA still suffers from collisions if multiple nodes are constantly monitoring a busy medium waiting for it to become free. As soon as the medium becomes free all these nodes might begin transmission simultaneously resulting in a packet collision. While CSMA offers significant improvement in performance over Aloha type protocols, it suffers from the hidden node and exposed node problems. Consider the scenario shown in Figure 2.13, where nodes B and C are trying to communicate with node A. If nodes B and C are not within range of each other, they cannot detect the presence of the other node’s signal and might try to simultaneously communicate with A. These transmissions would likely result in a collision and node A may not be able to decode either of these signals. This problem is commonly referred to as the hidden node problem. Now consider, the same scenario as shown in Figure 2.13. If node A is trying to send data to node B, then node C is prevented from transmitting due to carrier sense aspect. However, it is possible for node C to simultaneously transmit data to a different node since its signal is not received at node B and hence will not cause any interference. This problem is called the exposed node problem. A related problem that sometimes occurs in wireless networks is the issue of capture. For example, if signals from two different transmitters are received at a particular node at significantly different power levels, then the transmitter with the stronger signal will capture the medium. This capture will prevent the weaker transmitter from sending data and lead to severe unfairness in the resource allocation. Virtual carrier sense is proposed as a solution to overcome the hidden node problem [19]. In virtual carrier sense, each node that wants to transmit a packet first sends a short request-to-send (RTS) packet to the intended destination. If the destination node receives this packet successfully and determines that there are no other active signals in its vicinity, it replies with a short clear-to-send (CTS) packet. These RTS and CTS packets also include a field indicating the amount of time that the medium is expected to be busy. All nodes that receive either the RTS or CTS packet will not attempt transmission during the period of time specified in these packets. The use of RTS/CTS mechanism can also solve the exposed node problem under certain scenarios; for instance, using the MACAW protocol [8]. The fairness issues associated with capture can also be solved using more sophisticated back-off mechanisms. While the use of RTS/CTS coupled with a CSMA variant scheme reduces collisions significantly, it can add significant overhead especially if the packet size is small. The other problem is that in some scenarios nodes might back-off unnecessarily thereby reducing the number of simultaneous transmissions in the network and effectively reducing the network throughput. The popular 802.11 based standard uses a combination of CSMA with the RTS/CTS mechanism for multiple access.
32 Dinesh Rajan 2.5.2 Physical layer Multiuser Signaling Schemes Traditional cellular networks which primarily support voice tend to also build multiple access capabilities in the physical layer signaling. Next generation wideband networks tend to uses a variation of CDMA or OFDM, which are described next. 2.5.2.1 Direct Sequence Code Division Multiple Access DS-CDMA In a direct sequence CDMA system, all users use the same frequency and time to communicate. User separation is achieved by modulating the bits of each user with a distinct spreading sequence. By a careful selection of these spreading sequences, it is possible to recover the bits of each user from a linear combination of their transmit sequences. In many systems, these spreading codes are generated to form an orthonormal set. Although we only discus uplink communications in this section, CDMA can also be used in downlink channels. The CDMA2000 standard specified by the International Telecommunication Union (ITU) is based on DS-CDMA technology. Fig. 2.13 Topology showing the hidden node problem. If both nodes B and C transmit at the same time, their signals collide at node A.
Page 2: International Series in Operations
Page 5 and 6: Editors Jeff Kennington Eli Olinick
Page 7 and 8: vi Preface assistance. The work of
Page 9 and 10: viii Contents 3.4.1 Experiments to
Page 11 and 12: x Contents 8.3.2 The Solution Proce
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Page 16 and 17: List of Contributors Khaldoun Al Ag
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Page 37 and 38: 16 Dinesh Rajan of SNR is given in
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Part II Optimization Problems for N
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Chapter 6 Optimization Based WLAN M
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6 Optimization Based WLAN Modeling
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Chapter 7 Spectrum Auctions Karla H
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7 Spectrum Auctions 175 21. Dunford
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Chapter 8 The Design of Partially S
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Chapter 10 Compact Integer Programm
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302 Hang Jin and Li Guo 13.3 Radio
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304 Hang Jin and Li Guo Table 13.2
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