GSM EDGE Modulation, Slot, Burst and Air Interface

- summary, overview or tutorial about the basics of GSM EDGE air interface including the modulation scheme, slot, burst configurations.

The air interface for GSM EDGE, including the modulation, as well as the slot and burst structures, have been developed to be compatible with the overall GSM concept. In this way EDGE cellular technology is able to operate alongside the existing GSM systems by adding an EDGE upgrade.

In addition to this EDGE technology re-uses many of the features of the existing systems allowing both technologies to utilise the same base stations, etc. This provides a lower cost option to upgrade the network rather than having to deploy a completely new system.

With EDGE operating alongside GSM and GPRS, it has been necessary for the air interface to accommodate all signals, often catering for all three simultaneously. This approach, while proving some technical challenges has been very successful, as demonstrated by the number of operators whose networks are able to accommodate all three signals.

GSM EDGE modulation characteristics

One of the ways in which EDGE is able to provide higher data rates is to use a different modulation scheme for higher data rates. However the GMSK modulation scheme used for the basic GSM system is still used for the lower data rates.

GMSK was chosen for the original GSM system for a variety of reasons:

  • It is resilient to noise when compared to many other forms of modulation.
  • Radiation outside the accepted bandwidth is lower than other forms of phase shift keying.
  • It has a constant power level which allows higher efficiency RF power amplifiers to be used in the handset, thereby reducing current consumption and conserving battery life.

The GMSK modulation format is is used for the lower data rate transfers. The advantages mean that it is well suited for situations where lower data rates can be tolereated.

Note on GMSK:

GMSK, Gaussian Minimum Shift Keying is a form of phase modulation that is used in a number of portable radio and wireless applications. It has advantages in terms of spectral efficiency as well as having an almost constant amplitude which allows for the use of more efficient transmitter power amplifiers, thereby saving on current consumption, a critical issue for battery power equipment.

Click on the link for a GMSK tutorial

In order to enable data to be transmitted a form of phase modulation known as Octonary Phase Shift Keying, 8PSK was used. This form of modulation has a number of advantages that meant it was chosen for carrying high speed EDGE data:

  • Able to operate within the existing GSM / GPRS channel structure.
  • Able to operate within the existing GSM / GPRS channel bandwidth.
  • Able to operate within the existing GSM / GPRS channel coding structure.
  • Provides a higher data capability than the existing GSM GMSK modulation scheme.

The 8-PSK modulation scheme fulfils these requirements. It has the equivalent bandwidth and adjacent channel interference levels to GMSK. This makes it possible to integrate EDGE channels into the existing GSM / GPRS network and frequency plan as well as keeping the same channel coding structure.

Note on PSK:

Phase shift Keying, PSK is a form of modulation used particularly for data transmissions. If offers an effective way of transmitting data. By altering the number of different phase states which can be adopted, the data speeds that can be achieved within a given channel can be increased, but at the cost of lower resilience to noise an interference.

Click on the link for a PSK tutorial

The 8PSK modulation method is a linear method in which three consecutive bits are mapped onto one symbol in the I/Q plane as shown below

8PSK Modulation Constellation
8PSK Modulation Constellation

Using 8-PSK, the rate at which symbols are sent remains the same. However each symbol now represents three bits instead of one. This means that the actual data rate is increased by a factor of three.

The "distance" between the different positions on the constellation diagram is shorter using 8PSK modulation than when using GMSK. This means that there is an increased risk of any of the symbols being misinterpreted, especially in the presence of interference or noise. This occurs because it is more difficult for the radio receiver to detect which symbol it has received. To overcome this, additional error coding may be required to protect against the possibility of errors. However increased levels of error protection require additional data to be sent and this reduces the data throughput of the required data.

In view of this, it is found that when the signal is poor GMSK can be more effective than 8PSK, and as a result, the overall EDGE modulation scheme is a mixture of GMSK and 8PSK.

GSM EDGE time slots

EDGE, GPRS and GSM have to all operate along side each other in a network. It is a primary requirement that the evolutionary technologies are able to all operate on the same network. This ensures the service offered to existing customers using older phones along with those paying additional rates for the premium EDGE services. This means that the network has to support both services operating simultaneously.

Accordingly different slots within the traffic frames will need to be able to support different structures and different types of modulation dependent upon the phones being used, the calls being made and the prevailing conditions. It is quite possible that one slot may be supporting a GSM call, the next a GPRS data connection, and the third an EDGE connection using GMSK or 8PSK.

By Ian Poole

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