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APPLICATION OF POWER CONTROL AND CDMA IN ADHOC NETWORKS
Introduction
Mobile Adhoc Networks: Mobile adhoc networks (MANETs) have been a subject of extensive research for the past few years. These adhoc networks consist of autonomous nodes that collaborate in data transfer, acting both as end-systems and as routers simultaneously. In case of cellular networks there is single hop wireless connectivity to the wired world i.e., the space is divided into cells and the base station is responsible to communicate with hosts in its cell. Hand-off occurs when a mobile host start communicating via a new base station.
CDMA
In wireless networks, one of the fundamental challenges is the developing of efficient MAC (medium access control) protocols. MAC comprises all mechanisms that regulate user access to a medium so that many users may use the same medium with minimum interference. The base technologies used in MAC are FDMA (frequency division multiple access), TDMA( time division multiple access ) and CDMA (code division multiple access). Extensive research and deployment has fortified that, in the above three technologies, CDMA is the best technology for the power controlled multiple access systems providing high network capacity. So in this section we present some basic concepts related to CDMA. CDMA is based on spread spectrum techniques in which each user’s packet transmissions acquire the entire spectrum, and there is no allocation of bandwidth for any user. A digital signal of bandwidth R bits/s occupies a bandwidth of R Hz if baseband pulse transmission is used for actually transmitting packets. In CDMA, this signal is multiplied (or the XOR operation is performed which is discussed in the “system specification” section) using a pseudo-random sequence or PN code of bandwidth W bits/s (and hence occupies a spectrum of W Hz). These PN codes may be unique to a particular node or may be common between all the nodes. The resultant signal itself occupies a bandwidth of W Hz and this process is called spreading. Ideally this PN sequence must be a perfectly random noise, but this will be impossible to generate at the transmitter, and more-so impossible to regenerate at the receiver for reception. So, the pseudo-random sequences are the next-feasible option which approximate the binary random sequence sufficiently. The pseudo random codes (noise), with which the transmitted signal is multiplied, should have a good autocorrelation and should be orthogonal to other codes. Orthogonal in code space has the same meaning as in the standard space(i.e. the three dimensional space) that the inner product with the other codes should be zero. Having good autocorrelation imply that the inner product with itself produces large value but when it is shifted by one unit, inner product stays at the low value until the code matches itself again perfectly.Spreading the transmissions provides multi-faceted benefits and the transmitted signal is more “robust” than ordinary baseband pulse transmissions. Benefits of CDMA include increased received quality due to protection against multipath fading, protection against jamming, frequency diversity, interference cancellation and better performance with regard to mobility of the nodes.
CDMA based power controlled MAC protocol for Adhoc Networks: The channel access in adhoc networks is contention based due to its distributed nature which leads to a low throughout under high traffic loads on the system. Currently, the de facto mechanism for MAC protocol follows the “carrier sense multiple access with collision avoidance (CSMA/CA)” paradigm. It has been observed that throughput of a network is significantly low in CSMA/CA based MAC implementations. The low throughput is attributed to the contention-based nature of CSMA/CA. In CSMA/CA with fixed transmission power based schemes, the protocol executes a handshake mechanism to reserve the channel in order to avoid collision from nodes located within the transmission range of transmitter and receiver. This results in a “silence zone” around the source and the destination, where no node can transmit or receive packets until the data transmission is complete. This class of protocols do not take advantage of the relative distance between the transmitter and the receiver, which results in transmissions done with excess power than that required for correct reception at the destination node. Therefore these protocols are not efficient with respect to power consumption. But power controlled MAC protocols for adhoc networks makes use of this aspect by transmitting only at a power level required for correct reception. This concept of power control is also known as Interference Limited Access as by controlling power, Multiple Access Interference(MAI) is bounded. This power control scheme has been effectively used in cellular wireless systems to increase capacity and performance, especially in CDMA networks. In a cellular environment, the nature of any transmission is point-to-multipoint with Base Station acting as the coordinator to measure the MAI in the system. In contrast, the transmission in an adhoc network is multipoint to multipoint with lack of any coordination. Hence, the multiple access capabilities of CDMA are extremely useful in adhoc networks.
Motivation:
In wireless network, setting of fixed access points and backbone infrastructure is not always viable. For instance, infrastructure may not be present in the disaster area or war zone or in remote terrain. In such arenas, Mobile Adhoc Networks (MANETs) are extremely useful. Apart from such scenarios, the widespread availability of mobile computing devices and handheld devices like palm-tops, portables and personal digital assistants have increased the user base of Wireless Local Area Networks (WLANs) which has typical features of adhoc networks. They are easy to deploy and do not use backbone infrastructure support. But design of efficient media access protocols for such self-organizing networks is fraught with many challenges. There is randomness inherently associated with adhoc networks. It is really fascinating that these random attributes make them more useful and flexible as compared to infrastructured wireless design in their realm. The art of optimization of available resources displays how this chaotic nature of adhoc network wins over its counterparts. The flexibility they offer is admirable. These thoughts were a source inspiration and motivation to write about how their efficiency can be increased by the use of CDMA and Power control.
