Battery Life of a Cell Phone

One of the main factors which restrict reducing the size of a MS is the battery. A battery must be large enough to maintain a telephone call for an acceptable amount of time without needing to be recharged. Since there is demand for MSs to become smaller and lighter the battery must also become smaller and lighter.

Four features which can enable the life of a GSM MS battery to be extended are:

• Power Control
• Discontinuous Transmission (DTX)
• Discontinuous Reception (DRX)
• Use of Adaptive Antennas

Power Control: This is a feature of the GSM air interface which allows the network provider to not only compensate for the distance from MS to BTS as regards timing, but can also cause the BTS and MS to adjust their power output to take account of that distance also. The closer the MS is to the BTS, the less the power it and the BTS will be required to transmit. This feature saves radio battery power at the MS, and helps to reduce co-channel and adjacent channel interference.

Discontinuous Transmission (DTX): DTX increases the efficiency of the system through a decrease in the possible radio transmission interference level. It does this by ensuring that the MS does not transmit unnecessary message data. DTX can be implemented, as necessary, on a call by call basis. The effects will be most noticeable in communications between two MS. DTX in its most extreme form, when implemented at the MS can also result in considerable power saving. If the MS does not transmit during ‘silences’ there is a reduction in the overall power output requirement.

Discontinuous Reception (DRX): DRX allows the MS to effectively “switch off” during times when reception is deemed unnecessary. This allows the MS to ‘go to sleep’ and listen-in only when necessary, with the effective saving in power usage. DRX may only be used when a MS is not in a call.

Use of Adaptive Antennas: Since the antenna in cellular phones is an omni-directional one (this is done to enable signal reception from all sides), its gain is low. For a constant required EIRP (Pt*Gt), this increases the required transmit power, thus reducing battery life. If the antenna is made directional and if somehow it's main-lobe can be steered whenever the MS changes it's direction w.r.t. to the BS, the EIRP could be served with a lower transmit power.