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Thursday, August 11, 2011

4G RANs: The challenge of Coexistence and Spectrum Sharing

4G systems are expected to offer higher capacity, greater user bandwidth, and more reliable operation at the same or lower cost than second and third generation systems. At the same time they must operate with much higher spectral efficiency than current systems, since there is not likely to be an abundance of available frequency bands.
A significant ingredient in the solution of the spectrum availability problem is the ability of different wireless systems, users, and service providers to share spectrum with one another, on an as-needed basis.

The result will be a much more efficient use of common spectrum through sharing, as opposed to today’s situation in which most of the electromagnetic spectrum is dedicated to exclusive ‘‘owners,’’ some of whom use the spectrum only sporadically. Coexistence is the concurrent operation of different services or RANs in the same or adjacent frequency bands without causing degradation to any service, with emphasis on the indicated limitations in terms of, for example, frequency separation, physical separation, and transmission powers. Sharing is the use of the same frequency band by different RANs or services, either with coordination or possibly without any coordination between the systems, with emphasis on the spectrum access schemes and methods.

Efficient sharing capabilities are required because most frequency bands that are well suited for next generation emerging systems are already allocated and used to some extent by existing services. Therefore, future systems may not get sufficient dedicated frequency bands of their own, but will have to be able to use the same bands with other services. Of course, it must be guaranteed that existing services will not be significantly negatively affected. Sharing of the same frequency band by different services or technologies is only possible through well-defined limitations on the spectrum use, output power level, equipment density, transmission masks, and so forth. Technical requirements that facilitate sharing capabilities are the transmitter power control, low-level protocols, and dynamic frequency selection.

Spectrum sharing includes the common use of bands between new technologies with existing ones, as well as between several other new technologies that are deployed in parallel in the same bands. An example of a new technology that shares bands with many existing ones is UWB, whereas WLAN and Bluetooth are a typical example for technologies, which are deployed at the same time and make use of overlapping frequency bands. This spectrum sharing will be made possible by the concept of ‘‘cognitive radio’’; entities wishing to use part of a frequency band for transmission first scan to determine which portions of the band are currently unused, quickly establish temporary occupancy of suitable subbands and transmit on them, and finally vacate the subbands when the transmission is finished, or hop to another set of subbands if a higher priority user requires the original set. Cognitive radio becomes possible with recent advances in software-defined radios (SDRs), with intelligent spectrum-sensing techniques, and with transmitter and receiver digital signal processing (DSP) and RF technologies that accommodate signals whose spectra are split into several disjoint subbands and that can quickly change their transmitted spectra to adapt to the current multiuser environment without causing interference.

4g heterogeneous networks will very likely have to be able to coexist with some services and systems using the same or adjacent frequency bands. The deployment of these networks should be easy from the spectrum coordination point of view and, thus, the required coordination for transmitters and systems should be kept to a minimum. Coexistence of different services on adjacent bands is facilitated by grouping the services according to their power levels and interference sensitivity. The issue of coexisting systems is closely related to the concept of dynamic spectrum allocation (DSA), because it is concerned with different radio systems sharing the same block of spectrum. However, in coexistence the concern is how to find the effect on the performance of the systems when they are simply both operating in the same spectrum band. This can either be investigated for systems operating on exactly the same frequencies (i.e., the cochannel case), or when the systems are operating in adjacent frequencies (i.e., the adjacent channel case).

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