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15 - Spectrum assignment and fairness in femtocell networks
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- By Mustafa Cenk Erturk, University of South Florida, İsmail Güvenç, Florida International University, Sayandev Mukherjee, NTT Docomo USA, Huseyin Arslan, University of South Florida
- Edited by Tony Q. S. Quek, Singapore University of Technology and Design, Guillaume de la Roche, İsmail Güvenç, Florida International University, Marios Kountouris
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- Book:
- Small Cell Networks
- Published online:
- 05 May 2013
- Print publication:
- 02 May 2013, pp 357-382
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Summary
Introduction and prior art
Frequency allocation for heterogeneous networks
In heterogeneous networks, frequency resources can be allocated to different tiers in a co-channel (shared-spectrum) or dedicated channel (split-spectrum) manner, or through a hybrid technique, which is a combination of the two approaches. In the co-channel approach shown in Figure 15.1(a), while the spectrum resources are fully reused in different tiers, cross-tier interference may cause crucial setbacks to the system. For example, macrocell users in the vicinity of closed subscriber group (CSG) femtocells are not allowed to connect to the femtocells, even if their link quality with these femtocells is good. Therefore, such macrocell users receive strong downlink interference from CSG femtocells and may fall into outage.
The split spectrum approach shown in Figure 15.1(b), on the other hand, partitions the allocated spectrum between multiple tiers. Each tier can use its own segment of resource and therefore there is no cross-tier interference [1]. However, the amount of bandwidth available to each tier is reduced. Hybrid methods as shown in Figure 15.1(c) use a mixture of co-channel and dedicated channel methods, and aim to reuse the spectrum resources whenever feasible. For example, in [2], the macrocell users are dedicated a component carrier (CC), referred as the “escape carrier,” which is not used by the femtocell network. Any macrocell mobile station (MMS) that is close by to a femtocell is scheduled within this escape carrier, if the interference observed from the femtocell network is above a threshold.
8 - Interference mitigation and awareness for improved reliability
- from Part II - Low-rate systems
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- By Huseyin Arslan, University of South Florida, Florida, USA, Serhan Yarkan, Texas A&M University, Texas, USA, Mustafa E. Sahin, University of South Florida, Florida, USA, Sinan Gezici, Bilkent University, Turkey
- Edited by Ismail Guvenc, Sinan Gezici, Bilkent University, Ankara, Zafer Sahinoglu, Ulas C. Kozat
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- Book:
- Reliable Communications for Short-Range Wireless Systems
- Published online:
- 01 June 2011
- Print publication:
- 24 March 2011, pp 190-233
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Summary
Wireless systems are commonly affected by interference from various sources. For example, a number of users that operate in the same wireless network can result in multiple-access interference (MAI). In addition, for ultrawideband (UWB) systems, which operate at very low power spectral densities, strong narrowband interference (NBI) can have significant effects on the communications reliability. Therefore, interference mitigation and awareness are crucial in order to realize reliable communications systems. In this chapter, pulse-based UWB systems are considered, and the mitigation of MAI is investigated first. Then, NBI avoidance and cancelation are studied for UWB systems. Finally, interference awareness is discussed for short-rate communications, next-generation wireless networks, and cognitive radios.
Mitigation of multiple-access interference (MAI)
In an impulse radio ultrawideband (IR-UWB) communications system, pulses with very short durations, commonly less than one nanosecond, are transmitted with a low-duty cycle, and information is carried by the positions or the polarities of pulses [1–5]. Each pulse resides in an interval called “frame”, and the positions of pulses within frames are determined according to time-hopping (TH) sequences specific to each user. The low-duty cycle structure together with TH sequences provide a multiple-access capability for IR-UWB systems [6].
Although IR-UWB systems can theoretically accommodate a large number of users in a multiple-access environment [2, 4], advanced signal processing techniques are necessary in practice in order to mitigate the effects of interfering users on the detection of information symbols efficiently [6].
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