Blog: WiMAX® Study Completed
FCL has recently completed a study to investigate whether WiMAX type solutions might be good contenders for a client's particular type of mobile communications system.
WiMAX® stands for Worldwide Interoperation for Microwave Access, not to be confused with WiFi® (Wireless Fidelity), the relatively short range wireless network standard which has developed into widespread usage today to replace Ethernet® cable based local area networks (LANs). WiMAX is broadly accepted as one example of a 4th. generation mobile communications system, or 4G. WiMAX is based on the IEEE 802.16 standard which has progressed through several versions over the past few years and is promoted through the WiMAX forum, set up in 2001. (WiFi is based on the IEEE 802.11 standard). Initial proposals for WiMAX defined its use over microwave frequencies, surprisingly, up to about 65 GHz. It was intended for ad-hoc or mesh type line of sight (LOS) links, using fixed external antennas in metropolitan areas, as a high bandwidth substitute for the 'last mile' of the public switched telephone service (PSTN) to residential and smaller commercial premises. The higher part of the frequency band was found to be very absorptive through most media thus making it ineffective for any attempted non-LOS (NLOS) use. The lack of NLOS capability was very limiting so it did not develop well at first.
In 2004 the so-called 'fixed WiMAX' or 802.16-2004 standard was passed. This defined RF carrier frequencies in the range 2.5 GHz to 11.0 GHz using 256 or 2048 sub-carriers configured for orthogonal frequency division multiple access (OFDMA). These much more manageable frequencies were not so heavily absorbed, producing some quite useful reflections. Each sub-carrier carries a relatively slow digital modulation, a small part of the full data channel. RF bandwidths were available from 1.25 MHz to 15 MHz, intended for fixed installations using stationary antennas. OFDMA modes such as these are attractive if there is potential for significant multipath propagation between the transmit and receive antennas, usually caused by reflections from nearby buildings, metallic structures, mountains etc. The number of sub-carriers is in fact an integer power of 2, a requirement for the optimum use of the 'fast' Fourier transform (FFT) algorithm discovered by Tukey and Cooley. These algorithms also mean that significant signal processing capability is required at both the receivers and transmitters. Although this technology has been available for many years, it was not until the last few years that it has become available relatively cheaply and in small battery powered portable devices for mobile operation. WiMAX is an efficient user of bandwidth at 3.74 bits per second per Hertz (bps/Hz) occupied bandwidth compared to 2.8 bps/Hz for 3G systems, in both cases at the same signal to noise ratio.
WiMAX includes several features that help it (a) cope with propagation through a multipath-rich environment and (b) simplify the digital signal processing (DSP) designs of WiMAX receivers and transmitters. It includes convolutional and Reed Solomon coding, types of forward error correction which are tailored to the particular types of error manifestations in these environments. The process of dividing the data channel up into many slow data channels, each used to modulate one sub-carrier, enables good tolerance of the differential delays caused by multipath reflections using a guard band or cyclic prefix. The orthogonal property, which requires that the frequency spacing between adjacent sub-carriers is the reciprocal of the sub-carrier symbol period, allows the use of the well-understood FFT and inverse FFT (IFFT) chipsets designed for these algorithms which are deployed at the receiver and transmitter respectively.
The lower microwave frequencies in 802.16-2004 allowed significant propagation under NLOS conditions. In fact, the internationally allocated WiMAX frequency bands are commonly around 2 GHz to 5 GHz . These frequencies do not differ significantly from those already allocated to 2G and 3G digital cellular mobile communications systems, for which many propagation studies have been published. These are useful to system designers to assist propagation predictions for the actual WiMax frequencies of interest.
The later 'D' (nomadic) and 'E' (fully mobile) versions of the 802.16 standard in 2005 have made it a strong contender for mobile broadband services. Although originally envisaged to compete for the last mile of PSTN networks, WiMAX is now considered by many to be a serious mainstream competitor with 3G (which used CDMA) for mobile data services. Its only drawback is, as yet, a very limited availability in some metropolitan areas compared to that of 3G which is spreading rapidly in most countries. The D and E versions specify enhancements including: scaleable OFDMA, seamless cell handover, spatial multiplexing, antenna beam forming, advanced encryption and authentication.
There are three main drawbacks of WiMAX compared to 3G:
- Transmissions have a high peak to mean power ratio, requiring tighter power amplifier (PA) linearity requirements at the transmitter.
- Some signal strength degradation towards the cell edges compared to the equivalent 3G requiring modification of the antenna beam patterns.
- Potential for distortion caused by Doppler shift because of the closely spaced sub-carriers if the relative velocity between the antennas is appreciable.
- Some susceptibility to phase noise again because of the relatively close sub-carrier spacing.