WiMAX Enhancement

After discussing the threats to WiMAX technology, during this section we will introduce the enhancements that can reduce these threats. These enhancements includes power control, error detection techniques, sub-channelization, transmission diversity, antennas for fixed WiMAX applications, and adaptive modulation.

Power Control

Algorithms of power control are applied to enhance the general performance of the system, it is deployed by the base station sending power control information to every Customer Premise Equipments (CPEs) to control the transmit power level so that the level inward bound at the base station is at a fixed level. In a dynamical changing fading environment this pre-determined performance level indicates that the CPE only broadcasts sufficient power to meet this constraint. The communication would be that the CPE broadcast level is supported on worst case circumstances. The power control decreases the general power consumption of the CPE and the possible interference with other base stations. For Line-of-Sight (LOS) the transmission power of the CPE is approximately comparative to its distance from the base station, for Non-Line-Of-Sight (NLOS) it is also closely dependant on the clearance and barriers.

Error Detection Techniques

WiMAX have built-in error detection techniques to reduce the system Signal to Noise Ratio (SNR) obligations. Convolutional Encoding, Strong Reed Solomon FEC, and interleaving algorithms are used to identify and correct errors to enhance throughput. These strong error correction techniques assist to recover corrupted frames that may have been missing due to frequency selective fading or burst errors. To remove the errors, Automatic Repeat Request (ARQ) is used that cannot be corrected by the FEC by resending the error-ed information again. This notably improves the Bit Error Rate (BER) performance for a similar maximum level.

Sub Channelization

Another option within WiMAX in the uplink is Sub Channelization. Without sub channelization, narrow restrictions and requirement for cost effective Customer Premise Equipments (CPEs), usually cause the link budget to be unbalanced, for that reason the system range to be up link is restricted. Sub channeling allows the link budget to be objective such that the system gains are parallel for both the up and down links. Sub channeling focuses the broadcasting power into fewer OFDM carriers; this is what boosts the system gain that can either be applied to widen the reach of the system, overcome the infrastructure penetration losses, and or lessen the power consumption of the CPE. The use of sub channeling is promoted expansion in Orthogonal Frequency Division Multiple Access (OFDMA) to permit a more flexible use of resources that can maintain roaming or mobile operation.

Transmission Diversity

Diversity formats are used for improvement of multi-path and reflections signals that arise in Non-Line-of-Sight (NLOS) environment. Diversity is an optional element in WiMAX. The diversity algorithms presented by WiMAX in both the transmitter and receiver significantly enhances the system accessibility. The WiMAX transmit diversity option uses space time coding to offer transmit source freedom; this decreases the weaken margin requirement and resist the interference with other devices. For receive diversity, a variety of joining techniques are present to increase the accessibility of the system. For example, Maximum Ratio Combining (MRC) takes benefit of two separate receive chains to prevent fading and decrease path loss. Diversity has verified to be a helpful tool for coping with the challenges of NLOS transmission.

Antennas For Fixed WiMAX Applications

Directional antennas enhance the fade margin by adding together extra gain. This increases the link accessibility comparisons between directional and omni-directional antennas. Delay spread is further reduced by directional antennas at both the Base Station and Customer Premise Equipment (CPE). The antenna pattern restrains any multi-path signals that appear in the sidelobes and backlobes. The efficiency of these methods has been verified and demonstrated in booming deployments, in which the service operates under considerable NLOS fading.

Adaptive Modulation

WiMAX system supports adaptive modulation to regulate the Signal Modulation Scheme (SMC) depending on the Signal to Noise Ratio (SNR) state of the radio link. When the radio link is soaring in quality, the peak modulation scheme is used, offering the system additional capacity. During a signal fade, the WiMAX system can move to a lower modulation scheme to keep the connection quality and link permanence. This element allows the system to overcome time-selective fading. The key element of adaptive modulation is that it enhances the range that a higher modulation scheme can be used over, because the system can bend to the actual fading circumstances, as opposed to having a fixed scheme that is planned for the worst case situations.