Deep Inside the Network, Episode 4: OFDMA Fractional Frequency Re-Use (FFR)

The air interface of next generation wireless networks such as WiMAX is based on orthogonal frequency division multiple access (OFDMA). This technology divides a channel into many sub-channels which can be used by several terminals independently at the same time. Fractional frequency re-use, which I describe in more detail below, can reduce interference caused by neighboring base stations using the same frequency block and can thus help to increase user throughput and  overall network capacity.

Due to the limited frequency bands available and the high bandwidth per cell (e.g. 5 or 10 MHz) of future wireless broadband networks, base stations of an operator use the same frequency band. Using the same frequency band for all base stations, however, creates interference for subscriber terminals when they receive signals from more than one base station at a time. This is the case for example for a subscriber located just in the middle between base station A and base station B. If the subscriber listens to base station A, the signal of base station B is seen as unwanted interference. Thus, from the point of view of the subscriber, the output power of base station B should be as low as possible in order to create as little interference as possible. For another subscriber in a similar location but listening to base station B the situation is just the opposite. Thus, a compromise has to be found to adjust the output power of the base stations to a value which enables subscribers at the cell edge to still communicate at a decent speed while not creating too much interference in neighboring cells.

For distant subscribers a base station has to use more transmission power in order to reach them. Subscribers close to the base station on the other hand require much less transmission power to receive the signal. As client devices only transmit and receive on some but not all sub-channels of the frequency band, transmission power of sub-channels used by clients close to a base station can be lower than the transmission power of sub-channels used by clients at the cell edge. In practice, the reduced transmission power for sub-channels used by clients close to a base station thus creates less interference for users close to other base stations.

Using a combination of high and low power sub-channels can be exploited to increase the overall capacity of the network compared to networks which use the same transmission power for all sub-channels. Base stations can be organized in a way to use the same set of sub-channels to serve subscribers close to them with a low transmission power. The rest of the sub-channels are used with a higher transmission power and can be used by both distant and close subscribers. To minimize interference of high power sub-channels for clients of neighboring base stations the cells are further organized in a way that two adjacent cells do not use the same high power sub-channels. Thus, both close and distant clients of a base station will not see the high power sub-channels of a neighboring base station as interference. This approach is known as “fractional frequency re-use” (FFR) as all base stations use the same frequency band, the same low power sub-channels, but only a fraction of the high power sub-channels.

It is worth to note that fractional frequency re-use can not be used in UMTS, as all subscribers use the complete bandwidth instead of sub-channels. Thus, OFDMA networks implementing FFR suffer less from interference problems which translates into higher spectral efficiency compared to UMTS, i.e. the overall bandwidth available in the network is higher.

Currently the only indication the 802.16e WiMAX networks will use this technique is a whitepaper written by the WiMAX forum (changes place frequently so no link given here). I performed a search on the net but found no vendor who is talking about it yet. A bit strange as FFR seems to be an interesting technology.

Further information on next generation wireless networks such as LTE, WiMAX and EVDO Rev C can be found here.