Thursday, February 28, 2019

How channel are assigned to users in cellular System

Channel Assignment Strategies

As a communication student or network engineer, you must know that the radio spectrum is not enough to support the required cellular user. For this reason, a different technique like FDMA, TDMA, OFDMA, are adapted for efficient reuse of frequency having minimum interference. Channel created using these techniques also need to be assigned in proper manners. For this reason, a group of channels is allocated to each individual base station. Many methods are adapted to attach the given channel to users because it has a significant impact on network performance. Even as I said, this also helps in the reduction of co-channel interference. The channels assignment method is either fix or dynamic.
Channel Assignment Methods

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1. Fixed Channel Assignment

Using this technique, a set of voice channels is allocated to a cell. The user gets a channel when a call attempt is made. A user is always serving with unused channels. In case all channels are busy then the call attempt is blocked, and the user needs to wait until the channel gets free. This means blocking probability in fixed channel assignment is high, which degrades the network performance.

2. Borrowed Channel Assignment

This is the modified version of the fixed channel technique. In this approach, a channel is acquired from the near base station when all channels are busy. Later this channel is returned back to the original base station when the call is ended. The borrowing process is supervised by a Mobile switching center (MSC) while taking care of the frequency Reuse plan.

3. Dynamic Channel Assignment

In a dynamic channel assignment technique, no channel is permanently allocated to a cell. Each time MSC allocates channels whenever a base station receives a call request. MSC takes care of specific parameters like frequency reuse distance and cost during channel assignment processing to avoid co-channel interference and decreased the ratio of call blocking. In the dynamic channel, the allocation channel is only assigned if that channel is not used in the current cell. Also, MSC makes sure that the channel is not used within a restricted area because to avoid co-channel interference. This results in a decreasing call blocking ratio and increase the capacity of the cellular system. However, Dynamic channel allocation puts some load on MSC because this technique requires real-time data simultaneously like channel occupancy, traffic distribution, and RSSI (radio signal strength indication). Also, this information requires space to store data.

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