Mobile Stations

INTRODUCTION

The MS is the equipment used to access the network. The MS consists of two independent parts:

• Subscriber Identity Module (SIM) card

• Mobile Equipment (ME)

A SIM card is an electronic smart card which stores information about the subscription. The ME is the actual telephone terminal.

Mobile station

MOBILE STATION FUNCTIONS

TRANSMISSION AND RECEIPT

As described previously, the transmission and receipt process in an MS includes the steps shown in Figure.

Transmission and receipt in an MS

MEASUREMENTS MADE BY THE MOBILE STATION

The measurements made by an MS are used in making decisions about signal strength and handovers. Measurements are taken in both idle and active mode.

Idle Mode

Cell selection is made at “power on” of the mobile:

1. The mobile scans all radio frequency channels in the GSM system and calculates average levels for each. The mobile tunes to the strongest carrier and determines if it is a BCCH carrier. If so, the mobile reads the BCCH information to find out if the cell is locked (e.g. chosen PLMN, barred cell, etc.). Otherwise the mobile tunes to the second strongest carrier, and so on until a valid BCCH carrier is found.

2. The mobile may optionally include a BCCH carrier memory of valid BCCH carriers in the home PLMN. In this case it only needs to search these carriers. If this ends unsuccessfully, the mobile performs as in 1.

3. If no valid BCCH carrier is found, but a BCCH carrier belonging to another GSM network operator is found, the mobile will display the message “Emergency calls only”. Every emergency call is permitted onto a GSM network, even if the subscriber has not subscribed to the network the MS chooses.

Alternatively, if no valid BCCH carrier is found, and no other network can be found, the MS will display the message “No network”. In this case no calls (including emergency calls) can be made. Once it has tuned to a valid BCCH carrier, the mobile is informed which BCCH carriers it is to monitor for cell re– selection purposes. These are the BCCH carriers in neighboring cells. A list of the strongest carriers is updated regularly by the MS as a result of the measurements.

Active Mode

During a call, the mobile continuously reports (via SACCH) to the system how strong the received signal strength is from BTSs. Both signal strength and quality are measured on the MS’s “own” BTS. These measurements are used by the BSC to make fast decisions about target cells when a handover is required.

SS=  Signal strength

BSIC= Base Station Identity code

Measurements on neighboring cells during a call takes place when the mobile is idle, i.e. when it is not transmitting or receiving.

POWER SAVING FUNCTIONS

Discontinuous Transmission (DTX)

Discontinuous Transmission (DTX) is a method of saving battery power for the MS. An MS with the DTX function detects the input "voice" and turns the transmitter ON only while "voice" is present. When there is no voice input, the transmitter is turned OFF.  When the MS detects that speech is absent during the conversation, it sends out a signal called “Post” to report a transmission output state OFF for the TCH. Conversely, when the MS detects that speech is present again, it sends out a signal called “Pre” to report the transmission output state ON for the TCH. The Post signal incorporates background noise information from the MS, which enables the TRAU to generate background noise. This ensures that the other subscriber on the call hears something and does not think that the mobile subscriber has ended the call. The MS transmits the Post signal periodically during a speech pause, to enable the BTS to update the background noise.

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Another method used to conserve power at the MS is Discontinuous Reception (DRX). The paging channel, used by the BTS to signal an incoming call, is structured into subchannels. Each MS is assigned one of these sub-channels and needs to listen only to its own sub-channel. In the time between successive paging sub-channels, the mobile can go into “sleep mode”, when almost no power is used.