VAMOS (Voice services over Adaptive Multi-user channels on One Slot) IN GSM part1

VAMOS

BY engineer :abdallah Saleh 

VAMOS stands for Voice services over Adaptive Multi-user channels on One Slot. The idea here is to increase the voice calls capacity supported by GSM network. It is possible to use one time slot for four voice calls/services
The feature VAMOS is specified in 3GPP release 9.
VAMOS assign the same GSM physical channel (ARFCN-TDMA frame number-Time Slot) into two users simultaneously.
The GSM channel could be:
-Full Rate Channel
-Two Half Rate Channels
-Two VAMOS Full Rate Channels
-Four VAMOS Half Rate Channels
-One Half Rate Channel and Two VAMOS Half Rate Channels
-One VAMOS Full Rate Channel and Two VAMOS Half Rate Channels

VAMOS Advantages:

1-Doubling of voice calls per transceiver 

Increased call capacity per transceiver gives operators an efficient means to handle voice traffic growth in their networks without adding more TRXs.

Avoiding additional TRX’s results in savings in BTS HW investments, energy consumption and BTS foot print.

2-Free up capacity for EDGE data services

VAMOS reduces the number of time slots needed for voice services. This allows more time slots to be allocated for EDGE services.

Note: EDGE can carry a bandwidth up to 236.8 Kbit/s for 4 timeslots (theoretical maximum is 473.6 Kbit/s for 8 timeslots) in packet mode.

3-Free up spectrum for new technologies

For example UMTS900 (reframing 25 GSM 200 KHz frequency channel into 5 MHz UMTS Carrier) or LTE which allow for flexible operations in different spectrum bands.

VAMOS Disadvantage:

1-The parallel signal transmission of the two multiplexed users causes interference for one another, affecting speech quality if not properly controlled.

2-Call Drop Rate increased due to multiplexing of different MSs types.

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How VAMOS can differentiate between two users?

VAMOS transmits the combination of two signals at the same time over the same channel, each with a different orthogonal TSC’s (Training Sequence Code). 

Each of the two MSs that receive the data stream at the same time use their knowledge of their individual TSC to reconstruct their own part of the signal, effectively filtering away the second data stream as noise. 

Up-Link Operation:

Transmitter (MS): use the existing GMSK modulation scheme. In other words, no new transmitter elements are required in mobile devices. 

Receiver (BTS): different receiver algorithms may be used, that is Space Time Interference Rejection Combining (STIRC), Successive Interference Cancellation (SIC) or Joint Detection (JD) to receive both orthogonal sub-channels distinguished by their individual training sequences. Another option is to use two independent GMSK receivers for each sub-channel.

Down-Link Operation:

Transmitter (BTS): use AQPSK modulation technique to be able to transmit two calls at the same time.

Receiver (MS): use 3GPP Downlink Advanced Receiver Performance (DARP) which is also known as Single Antenna Interference Cancellation (SAIC) algorithm to correctly demodulate downlink Signal.

Training Sequences 

The training sequence code (TSC) or Channel Sounding Bits is a known 26-bit pattern placed in the middle of normal burst. TSC has eight fixed formats, which are represented by TSC ranged 0:7 respectively. The eight sequences are stored in all MS receivers to be used for Bit Synchronization and for Channel Estimation.

Because of TSC at the middle of time slot it also called Midamble. By having TSC there, the chances are better that the channel is not too different when it affects the training sequence compared to when the information bits were affected. If TSC was at the start of a burst, the channel might have changed by the end of the burst, and the same thing if it was at the end.

If MS have read SCH, it must get the TSC (Training Sequent Code) to correctly read the information on the downlink common signaling channel. TSC number is linked to the Base Station Color Code (BCC) of the cell. So one of the functions of BSIC is to inform MS of the TSC adopted by the common signaling channel of the cell.

to understand the VAMOS read other parts 

Part 1

part 2

Part 3

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Author : telecom engineer

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