Channel Loading & Spectral Efficiency | WiMAX

Channel Loading

Channel loading is a ratio of the number of users authorized to operate on a particular channel or systems compared to the number of users that actively transmit on a system. An example of channel loading on a WiMAX system is the number of broadband subscribers that can be effectively served by a single WiMAX radio channel.

The amount of channel loading depends on a variety of factors including the type of use (e.g. bursty web browsing or watching continuous digital telephony). For many types of applications, the subscriber station does not usually continuously transmit data while it is connected to the WiMAX system. For Internet browsing, the typical data transmission activity is less than 10%. This could allow channel loading of 10:1 or more. For example, in a WiMAX cell or single radio coverage area that has a channel capacity of 70 Mbps, 700 broadband Internet customers could be provided with data rates of 1 Mbps each.

The service provider can affect the channel loading through their price plans. Price plans can range from usage based service (charge per megabyte transferred) to unlimited rate plans. In 2006, the services and rate plans offered by WiMAX service providers were similar to digital subscriber line (DSL) and cable modem services rate plans.

Spectral Efficiency

Spectral efficiency is a measurement characterizing a particular modulation and coding method that describes how much information can be transferred in a given bandwidth. This is often given as bits per second per Hertz.

Modulation and coding methods that have high spectral efficiency are typically very sensitive to small amounts of noise and interference and often have low geographic spectral efficiency. WiMAX was designed to use multiple types of modulation, which allows the system to offer very high spectrum efficiency when the signal quality permits. Because of the robust modulation type used for the GSM system, its spectral efficiency is approximately 1.0 to 1.5 bits per Hertz. More efficient modulation types are used in the WCDMA system providing 1.5 to 2.5 bits per Hertz. 802.11 Wi-Fi systems can use efficient modulation types when channel quality is acceptable (e.g. limited interference by other unlicensed devices), which can provide spectral efficiency of 2 to 3 bits per Hertz. The 802.16 WiMAX system can use very efficient modulation providing an efficiency of 3 to 5 bits per Hertz.

The approximate spectral efficiency for several different types of systems. This diagram shows that the spectral efficiency of early mobile telephone systems (e.g. GSM) is approximately 1.0 to 1.5 bits per Hertz and that newer cellular systems (such as WCDMA) have spectral efficiencies of 1.5 to 2.5 bits per Hertz. The spectral efficiency of 802.11 WLAN system can be 2 to 3 bits per Hertz and the spectral efficiency of the 802.16 WiMAX system is approximately 3 to 4 bits per Hertz.