Frequency reuse

Frequency reuse is the process of using the same radio frequencies on radio transmitter sites within a geographic area that are separated by sufficient distance to cause minimal interference with each other. Frequency reuse allows for a dramatic increase in the number of customers that can be served (capacity) within a geographic area on a limited amount of radio spectrum (limited number of radio channels). Frequency reuse allows WiMAX system operators to reuse the same frequency at different cell sites within their system operating area.

The number of times a frequency can be reused is determined by the amount of interference a radio channel can tolerate from nearby transmitters that are operating on the same frequency (carrier to interference ratio).

Carrier to interference (C/I) level is the amount of interference level from all unwanted interfering signals in comparison to the desired carrier signal. The C/I ratio is commonly expressed in dB. Different types of systems can tolerate different levels of interference dependent on the modulation type and error protection systems. The typical C/I ratio for narrowband mobile radio systems ranges from 9 dB (GSM) to 20 dB (analog cellular). WiMAX systems can be much more tolerant to interference levels (possibly less than 3 dB C/I) when OFDM and adaptive antenna systems are used.

WiMAX systems may also reuse frequencies through the use of cell sectoring. Sectoring is a process of dividing a geographic region (such as a radio coverage area) where the initial geographic area (e.g. cell site coverage area) is divided into smaller coverage areas (sectors) by using focusing equipment (e.g. directional antennas).

The radio channel signal strength decreases exponentially with distance. As a result, mobile radios that are far enough apart can use the same radio channel frequency with minimal interference.

Without beamforming only reuse type 3x3x3 has acceptable value of outage probability. While using beamforming, while rest of the reuse patterns show acceptable results, network type of reuse 1 with loaded systems ( using all available subchannels) will result in significant system outage. However, method of partial usage of subchannels reduced the outage to acceptable level while still maintaining the average throughput at the highest level. By using 80% of total subchannels reuse type 1x3x1 will give good results for both the radio quality and throughput parameters. For loaded systems ( using beamforming), it has been concluded that reuse t ype 3x3x1 comes up with the best performance.

OFDM works well in the channels . In multi-cell deployments, in order to avoid intercell interference, basic OFDM requires directional antennas or relatively high frequency-reuse schemes and careful radio-frequency (RF) planning.

        OFDMA with its various subcarrier allocation schemes (FUSC and PUSC) improves performance in multi-cell deployments by averaging the interference across multiple cells. The interference becomes a function of cell loading and can be significantly reduced by efficient scheduling. OFDMA systems, on the other hand, are very flexible in terms of RF planning and support a variety of frequency reuse schemes (FRS). These FRS may be
described by denotation NcxNsxNf, where
  •  Nc is number of independent frequency channels in the WiMAX network
  •  Ns is the number of sectors per cell
  •  Nf is the number of segments in exploited frequency channel.
        Two of these FRS are for instance 1x3x1 and 1x3x3.
         Both schemes use three-sector base-stations and require only one RF channel for all sectors and BS, hence opening the door for operators who have limited amount of spectrum.FRS 1x3x1 eliminates the need for any frequency planning. That is a significant advantage especially for heavy urban areas where RF planning is very difficult. FRS 1x3x3 uses different (orthogonal) sets of tones (called “segments”) for each sector of a base-station thereby reducing inter-cell interference and minimizing outage area. This scheme also simplifies RF planning–one need only assign segments to sectors while using the same RF channel among all base-stations.

        Since the OFDMA PHY layer has many choices of sub-carrier allocation methods, multiple zones can use different sub-carrier allocation methods to divide each subframe. One benefit of using zone switching is that different frequency schemes can be dynamically deployed in a cell, forming a fractional frequency reuse scheme (FFRS). 
        The image here shows an example of deploying different FFRS in one frame. For the first half of each frame, the entire frequency band is divided by three and allocated in each sector. For the second half of each frame, the whole same frequency band is used in each sector.
        The benefits of deploying FFRS in one frame are:
  • edge users, who are receiving co-channel interference from other sectors in other cells, also have suppressed co-channel interference (CCI)
  • users around the cell center have the full frequency band because they are relatively less subject to co-channel interference.