The research in this thesis investigates the effects of inter-cell interference in a FFR based on TDD-OFDMA environment. We develop an analytical approach to evaluate the effects of inter-cell interference on the design of FFR factors. Furthermore, we provide some sim-ulation results to clarify advantages of using FFR in a multi-cellular system with supporting asymmetric services.
The remaining chapters of this work are organized as following. Chapter 2 reviews some background of 802.16m WiMAX standard. Chapter 3 discuss literature surveys about the effects of cross-slot interference, fractional frequency reuse and problem formulation.
Chapter 4 analyzes the outage performance for FFR based on TDD-OFDMA systems with asymmetric traffics in a two-cell environment and investigates the size of cross time slot region to effect the system performance. Chapter 5 discusses advantages of FFR schemes in TDD-OFDMA multi-cellular environment. Chapter 6 provides some concluding remarks and suggestions for future research.
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CHAPTER 2
Background of 802.16m WiMAX System
2.1 Orthogonal Frequency Division Multiple Access (OFDMA)
OFDMA is a multicarrier modulation technique that has found wide adoption in a widespread variety of high-data-rate communication systems, including WiMAX and LTE standard in fourth generation cellular system. Several advantages of OFDMA can improve transmis-sion performance, such as orthogonal frequency divitransmis-sion multiplexing (OFDM) modulation technique, frequency diversity and multi-user diversity...etd.
In wireless transmission environments, multi-path delay interference is a serious problem. The orthogonality of OFDM subcarriers may be lost when the signal passes through a time-dispersive radio channel due to inter-OFDM symbol interference. However, a cyclic extension of the OFDM signal can be performed to avoid this interference. Using the cyclic prefix (CP) method is getting the last part of the OFDM signal is added in the beginning of the OFDM signal. The addition of the CP makes the transmitted OFDM signal periodic and helps in avoiding inter-OFDM symbols.
In a frequency-selective channel, different modulations symbols can be transmit-ted by using OFDM technique which can be affectransmit-ted by different channel fading. We can consider the channel fading gain and group subchannels with higher channel gain to each mobile station (MS). Thus, OFDMA can achieve frequency diversity in variety channel fading environment. The OFDMA system will allocate subchannels to each MS due to
Table 2.1: OFDMA scalability parameter in WiMAX system
Parameter Value
System bandwidth (MHz) 1.25 5 10 20 3.5 7 8.75
Samplink factor 28/25 8/7
Sampling frequency (Fs,MHz) 1.4 5.6 11.2 22.4 4 8 10 Sample time (1/Fs,nsec) 714.3 178.6 89.3 44.6 250 125 100
FFT size (NF F T) 128 512 1024 2048 512 1024 1024 Subcarrier frequency spacing (Δf ,kHz) 10.9375 7.8125 9.7656
Useful symbol time (Tb=1/Δf ,µs) 91.4 128 102.4
Guard time (Tg=Tb/8,µs) 11.4 16 12.5
OFDMA symbol time (Ts=Tb+Tg,µs) 102.8 144 115.2
the available subcarriers may divided into several groups of subcarriers which are called subchannels. If we suitable arrange these subchannels, the multiuser diversity can improv-ing the transmission quality by different subcarrier and subchannel permutation. However, subchannels form the minimum frequency resource-unit allocated by base station for up-link and downup-link scheme. Therefore, different subchannels may be allocated to different users as a multiple-access mechanism in uplink and downlink.
2.1.1 OFDMA Scalable Parameter
The architecture is based on a scalable subchannelization structure with variable Fast Fourier Transform (FFT) sizes according to the system bandwidth. However, coherence time, Doppler shift and coherence bandwidth of the channel are considered in a scalable structure where the FFT sizes scale with bandwidth to keep the subcarrier spacing fixed. The Table 2.1.1 shows the scalability range proposed in IEEE 802.16m WiMAX standard.
2.1.2 Subchannelizaion of the OFDMA System
In an OFDMA system, subchannels may be constituted using either subcarriers pseudo-randomly distributed across the frequency spectrum or contiguous subcarriers. Subchan-nels are formed by using distributed subcarriers provide more frequency diversity, which is particularly useful for mobile applications. In WiMAX system, the pseudo-randomly dis-tributed scheme is called partial usage of full usage of subcarriers (FUSC) and subcarriers (PUSC).
The subchannelization scheme based on contiguous subcarriers in WiMAX stan-dard is called band adaptive modulation and coding (Band AMC). Although this scheme must lost frequency diversity, Band AMC allows system designers to exploit multiuser diversity that is allocating subchannels to users based on their frequency response or con-sidering the co-channel interference from neighboring cell. In general, multiuser diversity can provide significant gains in system capacity by different allocation scheme.
Full Usage Subcarrier (FUSC) and Partial Usage Subcarrier (PUSC)
In the downlink transmission, the data subcarriers can be used to create the various sub-channels by FUSC and PUSC permutation that are called DL-FUSC and DL-PUSC respec-tively. However, uplink case only support PUSC permutation method called UL-PUSC.
They are formed subcarrier as a subchannel by each pseudorandom permutation approach.
In the case of DL-FUSC, each subchannel is made up of 48 data subcarriers, which are dis-tributed evenly throughout the en tire frequency band. The DL-PUSC is similar to FUSC except that the subcarriers are first divided onto six groups. Permutation of subcarriers formed subchannels is performed independently within each group. Thus, each group can be separated from each others and still keep the frequency diversity. In the UL-PUSC the subcarriers are first divided into various tiles consist of four subcarriers over three OFDM symbols. The tiles are renumbered and using a pseudorandom numbering sequency, and
divided into six groups. Each subchannel is created using six tiles from a single group.
Band Adaptive Modulation and Coding (AMC)
According to use the Band AMC permutation mode, each subchannels is constituted by adjacent subcarriers. Although frequency diversity is lost, implement of multiuser diver-sity is easier. The system can adaptive allocate subchannels to each user with the highest SINR/capacity or choice subchannels with less co-channel interference since the multiuser diversity provides improvement in link reliability, system capacity and throughput. In the WiMAX system, we can use Band AMC permutation. Nine adjacent subcarriers and one pilot subcarrier are used to form a bin. A Band AMC subchannel consists of six contiguous bins. Thus, a Band AMC subchannel can consist of one bin over six consecutive symbols, two consecutive bins over three consecutive symbols, or three consecutive bins over two consecutive symbols. Hence, the size of subchannel is dependent on the subcarrier permu-tation mode. For instance, each subchannel is 8, 16, or 24 sybcarriers by six, three, or two OFDM symbols.