Optimizing WiMAX: Mitigating Co-Channel Interference for Maximum Spectral Efficiency

The efficient use of radio spectrum is one of the most important issues in wireless networks because spectrum is generally limited and wireless environment is constrained to channel interference. To cope up and for increased usefulness of radio spectrum wireless networks use frequency reuse technique. The frequency reuse technique allows the use of same frequency band in different cells of same network considering inter-cell distance and resulting interference level. WiMAX (Worldwide Interoperability for Microwave Access) PHY profile is designed to use FRF (Frequency Reuse Factor) of one. When FRF of one is used it results in an improved spectral efficacy but also results in CCI (Co-Channel interference) at cell boundaries. The effect of interference is always required to be measured so that some averaging/ minimization techniques may be incorporated to keep the interference level up to some acceptable threshold in wireless environment. In this paper, we have analyzed, that how effectively CCI impact can be mitigated by using different sub-carrier permutation types presented in IEEE 802.16 standard. A simulation based analysis is presented wherein impact of using same and different permutation base in adjacent cells in a WiMAX network on CCI, under varying load conditions is analyzed. We have further studied the effect of permutation base in environment where frequency reuse technique is used in conjunction with cell sectoring for better utilization of radio spectrum.

WiMAX Forum declared OFDMA [2] as the basic multiple access technology.

ORTHOGONAL FREQUENCY DIVISION MULTIPLE ACCESS
Broadband wireless access requires high order modulation schemes with better spectral efficiency to enable wired equivalent transfer rate/connectivity and quality. In comparison to widely used analog modulation OFDM assigns all Sub-Carriers of a symbol to one specific user; however OFDMA allows assignment of subcarriers in a symbol to multiple users. OFDM distributes the signal into various narrow-band signals also called sub-channels or subcarriers of different frequencies [3,4] and a subscriber is assigned one or more sub-channels for transmission. In OFDMA allocation of transmission resources is based on OFDM symbol and sub channels; thus providing high granularity bandwidth allocation using both time and frequency multiple access [5,6]. Data Subcarriers, Pilot Subcarrier and Null Subcarrier are three subcarrier types defined with subcarrier spacing of 10.9375 kHz [5].
The IEEE-802.16e-2005 PHY standard offers two different schemes for sub-channel allocation [7].

(a) Distributed Subcarrier Permutation:
In distributed subcarrier permutation a sub-channel uses subcarriers randomly distributed across the channel bandwidth. There are again two variants of distributed cub-carrier scheme (Fig. 1).

(a) Subcarriers are Distributed over the Entire
Frequency of Channel. This is stated to as FUSC (Fully Utilized-Sub-Channelization).

(b) Some Distributed Clusters of Subcarriers
Constitute a Sub-Channel. This is stated to as PUSC (Partially-Used-Sub-Channelization).

(b) Contiguous Subcarrier Permutation:
In contiguous subcarrier permutation type, a subchannel uses neighboring subcarriers that are carefully chosen by the scheduler (Fig. 1). This is called AMC (Adaptive Modulation and Coding) which is very useful in meeting high bandwidth requirements often constrained due varying channel conditions in wireless transmission.

Fully-Utilized-Sub-Channelization
In FUSC all of the SCs (Sub Channels) are allocated to the transmitter. That is why it is called fully utilized sub channel. FUSC can be used in DL only [3].

Sub-Channel Formation in FUSC
Prior to assignment of subcarriers to SC, pilot subcarriers are divided into fixed and variable type. FUSC groups are formed from data subcarriers and then after permutation is applied using equation 1 to allocate subcarriers to SC [3]. In a sub-channel there could be 48 data subcarriers; also it is important to note that number of SC is equal to subcarriers in a FUSC group alternatively it may be stated that using 2048-FFT, there could be 32 Sub-channels in total.
During the permutation process of subcarrier distribution, one subcarrier is selected from every FUSC group and is being allocated to a SC. The Equation (1)

Partial Usage of Sub-Channels
In PUSC sub-channels are distributed among different transmitters. That is why it is called partial usage of SC.
The SCs allotted to one transmitter make one segment.
Left behind sub-channels are ascribed to other different segments and thus to different sectors.
Two OFDM symbols by one SC for one PUSC DL slot; and 24 data subcarriers form one PUSC DL.  [7] of a group is carried to make SCs from the subcarrier.
Permutation base may be selected from 0-31 for the rest of PUSC zones (consist of OFDMA symbols with same permutation scheme).

Sub-Channel Formation in PUSC
The permutation process to form Sub-Channels is shown in Fig. 2. There are two intrinsic processes called Inner Permutation and Outer Permutation (Fig. 3). The Inner Permutation is used to make Sub-Channels from subcarriers in logical clusters of a group. As already stated Permutation base may be chosen from 0-31 for remaining PUSC zones. Then Physical clusters are renumbered to logical clusters using process of Outer-Permutation.
In DL PUSC available subcarriers are distributed as data Subcarriers of a group are assigned to sub-channels using permutation formula as given in Equation (1). UL PUSC Sub-channel is formed from six logical tiles (Fig. 5).
The permutation process to map data points on subcarriers of sub-channels is such that Physical tiles are renumbered as logical tiles using Equation (3) (Fig. 6).   (Fig. 7).    [16].

RELATED WORK
The WiMAX physical layer is in charge of multiplexing user and system data together with control signaling in order to ensure a proper utilization of the radio resources.  Network's composite/global response is recorded during this periodic rise in UL traffic and is presented in Fig. 10.

SIMULATION SETUP
It is anticipated that due to high load conditions an increased CCI affected the overall performance of network. From the observations recorded in Fig. 10 it is quite evident that global HTTP received traffic is decreased during the period of increased load in Cell_0 with spontaneous increase in HTTP page response time.
With the rise in traffic load the throughput is however increased.
To verify that this global impact on performance metrics, is due to FRF of 1 which resulted in CCI in the network and to further demonstrate the effect of using different permbase on CCI and overall network performance we developed network scenarios using different permutation base value assigned to different cells. Fig. 11