WiMAX Radio Overview

A WiMAX radio channel is a communications channel that uses radio waves to transfer information from a source to a destination. It may transport one or many communication channels and communication circuits on a single RF channel.

WiMAX radio channels may operate within different frequency bands, have different radio channel bandwidths, dynamically change modulation types, use a variety of access technologies and other characteristics that allow WiMAX to reliably provide a variety of types of communication services.

The WiMAX radio systems can use a single carrier (SC) or multi-carrier (MC) transmission. Single carrier transmission is the use of a single carrier wave that is modified to carry (transport) all of the information. Multi-carrier is a communication system that combines or binds together two or more communication carrier signals (carrier channels) to produce a single communication channel. This single communication channel has capabilities (capacity) beyond any of the individual carriers that have been combined. When each of the carriers in a multi-carrier system is mutually independent (orthogonal) to each other, it is called orthogonal frequency division multiplexing (OFDM).

Figure 1 shows the key components of a basic WiMAX radio system. The major component of a WiMAX system include subscriber station (SS), a base station (BS) and interconnection gateways to datacom (e.g. Internet) and telecom (e.g. PSTN). An antenna and receiver (subscriber station) in the home or business converts the microwave radio signals into broadband data signals for distribution. In the example, a WiMAX system is being used to provide telephone and broadband data communication services. When used for telephone services, the WiMAX system converts broadcast signals to an audio format (such as VoIP) for distribution to IP telephones or analog telephone adapter (ATA) boxes. When WiMAX is used for broadband data, the WiMAX system also connects the Internet through a gateway to the Internet. The WiMAX system can reach distances of up to 50 km when operating at lower frequencies (2-11 GHz).

Figure 1: WiMax System

Adaptive Antenna System (AAS) | Technologies

An adaptive antenna system allows a transmitter to focus radio beams to increase the transmission range, reduce interference and increase signal quality. When an AAS system is used to allow multiple users to communicate with the same transceiver (multiple beams), it is called spatial division multiple access (SDMA). SDMA technology has been successfully used in satellite communications for several years. In some SDMA systems, radio beams may dynamically change with the location of the mobile radio.

The WiMAX system is designed with AAS capability. To support AAS, it is necessary to supplement the medium access control (MAC) protocol with additional commands so that base stations can better monitor subscriber stations which may be operating in a narrow focused beam area. If the subscriber station were to move out of the focused beam area, the system could loose control of the subscriber station.

Figure 1 shows an example of a WiMAX adaptive antenna system (AAS). The cell site can focus radio signals using the same frequency to multiple devices within the same cell site. Focusing of the radio signal allows for an increase in the distance that a cell site can have when communicating with devices. Using AAS technology, the system can adapt the direction of the focused beam to a specific device as it moves throughout the coverage area.

Figure 1: WiMax Adaptive Antenna System

Diversity Transmission

Diversity transmission is the process of using two or more signals to carry the same information source between a transmitter and a receiver. Diversity transmission can use the physical separation of antenna elements (spatial diversity), the use of multiple wavelengths (frequency diversity) and the shifting of time (time diversity).

Protocols on the WiMAX system are designed to take advantage of diversity transmission options and to allow for the use of multiple input multiple output (MIMO) antenna systems. MIMO is the combining or use of two or more radio or telecom transport channels for a communication channel through the user of multiple antenna elements. The use of MIMO to combine alternate transport links provides for higher data transmission rates (inverse multiplexing) and increased reliability (interference control).

Transmission Diversity

Transmission diversity is the process of sending two or more signals from the same information source so a receiver can select or combine the signals to produce a received signal of better quality than a single transmitted signal.

Receive Diversity

Receive diversity is used to select or combine a received signal to yield a stronger signal quality level. Receive diversity uses two antennas that are physically separated vertically or horizontally.

Receiver diversity can compensate for radio signal fading that may occur on a single antenna, and may be performed by maximum ratio combining (MRC) or selection diversity. Maximal ratio combining is the process of combining the signals from two or more antenna elements to increase the level and quality of a received signal. Selection diversity is the process of selecting one antenna from a set of receiving antennas to increase the level or quality of a received signal.

Frequency Diversity

Frequency diversity is the process of receiving a radio signal or components of a radio signal on multiple channels (different frequencies) or over a wide radio channel (wide frequency band) to reduce the effects of radio signal distortions (such as signal fading) that occur on one frequency component but do not occur (or are not as severe) on another frequency component.

Temporal (Time) Diversity

Time diversity is the process of sending the same signal or components of a signal through a communication channel where the same signal is transmitted or received at different times. The reception of two or more of the same signal with time diversity may be used to compare, recover, or add to the overall quality of the received signal.

Spatial Diversity

Spatial diversity is a method of transmission or reception employed to minimize the effects of fading by the simultaneous use of two or more antennas spaced a number of wavelengths apart.

Antenna diversity is a form of spatial diversity that improves the reception of a radio signal by using the signals from two (or more) antennas to minimize the effects of radio signal fading or distortion. Antenna diversity typically requires the antennas to be spaced a number of wavelengths apart.

Space time coding is the adding of time information to transmission carriers to allow diversity operation by identifying and processing multiple carriers of the same signal that may arrive at different times and or from different locations.

Figure 1 shows different types of diversity transmission and reception. The antenna (spatial) diversity utilizes the distance between antennas to improve signal performance. Frequency (spectral) diversity transmits the same or related information on multiple frequency signals to reduce frequency selective fading. Time (temporal) diversity overcomes the challenges of burst distortion by allowing the same information signal to be received at different times.

Figure 1: Diversity Transmission