Write notes on channel combiners

9. Write notes on channel combiners.

Channel Combiner:

1. A Fixed Tuned Channel Combiner: At the travelling side, a fixed tunable combined unit is used. In every cell site, a channel combiner circuit is installed. The transmitted channels have to be combined based on the following two criteria,

a) The signal isolation between the radio channels must be maximum

b) The insertion loss should be minimum. However, the usage of channel combiner

can be avoided by feeding each channel to its corresponding antenna.

But, if there are I6 channels available in a cell site, there will be requirement of 16 antennas for operation which is bottle neck for real time functionalities. It is not economical to hive huge hardware setups. Thus, a conventional combiner can he used, which has 16 channel combining capacity and it is based on the frequency subset of 16 channels of cell site.

The channel combiner would be responsible for each of the 16 channels to exhibit a 3 dB loss due to the signal insertion in to the channel combiner. The signal isolation would be 17dB, if every channel is separated from its neighboring channels by 630 kHz frequency.

2. Tunable Combiner: Tunable combiner is also referred as frequency agile combiner. The frequency agile combiner is an advanced combiner circuit with additional features. It can return any frequency in real time by remote control device, namely microprocessor. This combiner is essentially a waveguide resonator with a tuning bar facility. A motor makes the tuning bar to rotate and once the motor starts rotating, the Voltage Standing Wave Ratio (VSWR) can be measured.

The controller unit has self-adjusting feature and it accepts an optimum value of VSWR as the motor complete, a full turn. The controller is compatible only with dynamic frequency assignment.

The cell-sites should be flexible to change their operating frequency ‘f’ that is controlled by MTSO/MSC. Thus, we can use this frequency agile combiner in the cell site transceiver setup.

3. Ring Combiner: Ring combiner is used to combine two groups of channels to give one output. This combiner has an insertion loss of 3 dB. For example, using a ring combiner two 16 channel groups into one 32 channel output. Even 64 channels can be used with this combiner if two antennas arc available in the cell site. In case of low transmitter power more than one ring combiner can be used for combining. However, the demerits of ring combiners are.

a) It reduces adjacent-channel separation.

b) They may be affected from the problem of power limitations.