Radio Propagation | WiMAX Radio

Radio propagation is the process of transferring a radio signal ( electromagnetic signal) from one point to another point. Radio propagation may involve a direct wave (space wave) or a wave that travels along the surface (a surface wave). Radio propagation characteristics typically vary based on the medium of transmission (e.g. Air) and the frequency of radio transmission. WiMAX systems may be operated as line of sight (direct transmission path) or non line of sight (indirect transmission path) systems.

WiMAX systems are typically designed with a radio link budget. A link budget is the maximum amount of signal losses that may occur between a transmitter and receiver to achieve an adequate signal quality level. The link budget typically includes cable losses, antenna conversion efficiency, propagation path loss, and fade margin.

Due to transmission impairments, a fade margin is budgeted in a communication link. Fade margin is the amount of signal loss, usually expressed in decibels, that a radio signal in a communication path is anticipated to change (or budgeted to change). This helps to ensure that typical signal fading periods do not result in a lower than expected quality of service.

One of the key types of signal fades that occur on microwave systems is a rain fade. A rain fade is the signal loss that results from signal absorption and scattering in water droplets (rain).

Line of Sight (LOS)

Line of sight (LOS) is a direct path in a wireless communication system that does not have any significant obstructions. WiMAX systems that operate in the 10-66 GHz range are LOS systems.

Non Line of Sight (NLOS)

Non line of sight (NLOS) is a wireless communication system that does not have a direct path (can have significant obstructions) between the transmitter and receiver. NLOS systems can use radio signals for transmission.

Figure 1 shows how non line of sight (NLOS) radio propagation can allow a radio signal to reach its destination in congested areas. A radio tower is transmitting through an urban area which does not allow a radio signal to travel a direct path from the tower to the receiver. Accordingly, multiple alternate paths are reflected off a building to reach its destination. A main signal (shortest reflected signal) and another signal (delayed signal) become part of the received signal.

Figure 1: Near Line of Sight Radio Propagation

WiMAX Protocol Layers

Protocol layers are a hierarchical model of network or communication functions. The divisions of the hierarchy are referred to as layers or levels, with each layer performing a specific task. In addition, each protocol layer obtains services from the protocol layer below it and performs services to the protocol layer above it. The WiMAX system has four key layers; MAC convergence, MAC layer, MAC privacy and the physical layer.

MAC Convergence

The MAC convergence layer is a functional process within a communication device or system that adapts one or more transmission mediums (such as radio packet or circuit data transmission) to one or more alternative transmission formats (such as ATM or IP data transmission).

MAC Layer

The MAC layer is composed of one or more logical communication channels that are used to coordinate the access of communication devices to a shared communications medium or channel (microwave radio). MAC channels typically communicate the availability and access priority schedules for devices that may want to gain access to a communication system.

MAC Privacy

The MAC privacy layer is associated with authenticating and encrypting information over the communication link (inner coding).

Physical Layer

The physical layer performs the conversion of data to a physical transmission medium (such as copper, radio, or optical) and coordinates the transmission and reception of these physical signals. The physical layer receives data for transmission from an upper layer and converts it into physical format suitable for transmission through a network (such as frames and bursts). An upper layer provides the physical layer with the necessary data and control (e.g. maximum packet size) to allow conversion to a format suitable for transmission on a specific network type and transmission line.

Security Sub-Layer

A security sub-layer is the functional process within a communication device or system that performs access controls, identity validation and/or encryption of data.

Figure 1 depicts the multiple layers of WiMAX. The physical layer is responsible for converting bits of information into radio bursts while the MAC security layer is responsible for identifying the users (authentication) and keeping the information private (encrypting). The MAC layer is responsible for requesting access and coordinating the flow of information. The MAC convergence layer is used to adapt the WiMAX system to other systems such as ATM, Ethernet or IP data systems.

Figure 1: WiMax Protocol Layers

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