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Cellular-Planning and Optimization

Capacity Calculations

Capacity CalculationsThe capacity of a GSM-based network is dependent on the frequency reuse scheme andthe amount of the operators spectrum. Frequency reuse schemes are used to divide thespectrum into unique groups of channels, so that the frequency planner canmethodically assign channels to a network of cells. The reuse scheme is chosen tominimize the amount of interference in a network of cells, but also has an impact on thecapacity of a particular site.

The amount of spectrum a license holder has must be considered as well. Since theGSM channel is 200 kHz wide, five channels can fit into 1 MHz of spectrum. Therefore,

a European operator, with 25 MHz of spectrum, will have 62 channels available;whereas, a PCS operator in the United States, with 30 MHz of spectrum, will have 75channels available. Remember that half of the spectrum is for downlink and half is foruplink.A GSM market that has just launched commercial service will likely use a 4/12-reusescheme. Using a 4/12-reuse scheme will have less capacity (not likely to be a problem asthe network turns on), but more importantly, the larger distance to reuse will minimizeinterference. As subscribers are added to the network, the capacity increases to thepoint that frequencies must be reused over shorter distances. Using frequency hopping,the GSM network may be frequency planned using all of the channels within three sites.

Frequency hopping is explained in a later section.The number of channels available per sector when considering the two reuse schemes iscalculated as follows:

To calculate the capacity of the network, the Erlang capacity of each sector must beidentified. The Erlang capacity of each sector is calculated by considering the desiredGrade Of Service (GOS) and the number of voice channels per sector. It is industry-accepted practice to design a network with 1 - 2% GOS. The number of voice channels

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per sector is calculated with respect to the number of RF carriers per sector. Generally,eight time slots are considered for each carrier except the first carrier, which dedicatesone or two time slots for control and messaging. By using an Erlang-B table, the Erlangsper sector are determined.The Erlang capacity of the network is calculated as follows:

Comparing the total capacity of networks using the two different reuse schemes isdepicted as follow.

Capacity of Network Using 3/9 Reuse SchemeTotal Number of Cells Required 100Number of Voice Channels/Sector 22Blocking Rate 1 %Erlangs/Sector 13.65Sectors per Site 3Erlangs/Site 40.95Erlang Capacity of Network 4095

Capacity of Network Using 4/12 Reuse SchemeTotal Number of Cells Required 100Number of Voice Channels/Sector 14Blocking Rate 1 %Erlangs/Sector 7.351681Sectors per Site 3Erlangs/Site 22.05Erlang Capacity of Network 2205

In the case of a network using a 3/9-reuse scheme, there would be 810 channels.Conversely, the same network of sites using a 4/12-reuse scheme would have 630channels; however, the capacity has almost doubled. This example is a bit oversimplified, and conservatively assumes that all sites in the network have the same

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distribution of traffic; however, it depicts the gains associated with a tighter reusescheme.