Spectrum sharing for wireless communication subject to regulatory constraints on power

Spectrum is or soon will be a scarce asset, and hence methods for effciently sharing spectrum are important. Concern about the possible effects of wireless radiation on health are also growing because of the widespread and growing use of devices that communicate wirelessly. Although some of this concern can be attributed to illinformed alarm, international agreements and industry standards recognise the need for prudence in managing exposure to electromagnetic fields (EMF). When efficient shared use of spectrum is investigated, it is necessary to consider why the power available for wireless transmission is limited, and how this limitation on available power is expressed, and therefore the issue of spectrum sharing cannot be addressed without taking into account safety-related constraints on power. EMF levels need to be regulated to levels well below levels where there might be harm and therefore below the internationally agreed EMF exposure limit standards. Hence, we do not expect to see any health effects at these levels. In Chapter 3 of this dissertation, it is argued that for the safety of human health, we should assume that there must be constraints on the power, or EMF, used at each device participating in the shared communication. These constraints on EMF affect the way we share the spectrum. The way these regulations are expressed needs great care because it will have an effect on the design of the wireless communication systems. In Chapter 4, a Spread Spectrum-Orthogonal Frequency Division Multiplexing (SS-OFDM) model is developed for efficient sharing of the spectrum among nearby users. Efficient sharing is shown to be consistent with nearby WiFi domains appearing as noise to each other (which is the characteristic property of spread-spectrum). In Chapter 5, we assume that there must be constraints on the power, or EMF, used at each device participating in the shared communication. This thesis considers five different forms of power/EMF constraint and compares the sum-throughput achieved by all devices, under these different constraints. Note that the five different approaches to meeting power/EMF constraints that are considered here vary slightly in the way the constraint is expressed, but also, and this is the more significant aspect, in the way in which the constraint is enforced. These five approaches are; Carrier-Sense Multiple Access (CSMA) method, Orthogonal Frequency-Division Multiple Access (OFDMA), EMF limited, SS-OFDM, and mutually interfering. In Chapter 6, cross-subchannel noise in OFDMA is modelled, which shows that nearby systems interfere with each other to a greater degree than might be expected. Conclusions are presented in Chapter 7

Sum-rate optimal communication under different power constraints

In this paper the problem of optimal allocation of power to different devices and spectrum when communication takes place in the same region, using shared spectrum, is investigated. We assume that there must be constraints on the power, or EMF, used at each device participating in the shared communication. We consider different forms of power/EMF constraint and compare the sum-throughput achieved by all devices, under these different constraints

A new approach to spread-spectrum OFDM

Orthogonal frequency division multiplexing (OFDM) systems are reviewed and the Shannon bound is dis- cussed as a criterion of efficient spectrum use and a design criterion. The problem of efficient sharing of spectrum by wireless communication systems is discussed and combined use of direct-sequence spread- spectrum (DSSS) coding and OFDM is proposed as an approach which can achieve efficient spectrum sharing. A system which enables DSSS, with codes from the Galois field of order f where f is a prime larger than 2, to be used efficiently in conjunction with OFDM is then defined, analysed, and implemented. Experiments with this system are described