GSM EDGE RF Interface: Modulation, Slot, Burst

The 2G GSM EDGE radio interface utilised a different form of modulation to achieve its comparatively high data rates.

GSM EDGE includes:
GSM EDGE introduction     Network architecture     RF interface     Modulation coding schemes     Evolved EDGE    

The 2G GSM EDGE radio interface built on that of GSM utilising the same slot and burst structure used for GSM, but adopted a different form of modulation to achieve its higher data rates.

The EDGE radio interface was designed to be compatible with and work alongside the GSM radio interface, so that the same base station could run basic GSM, GPRS and EDGE simultaneously.

The adoption of this approach for the radio interface also provided a much lower upgrade cost than adopting a completely new interface, although users did need to buy new phones to run EDGE and benefit from the improved data capabilities

GSM EDGE modulation

One of the ways in which 2G GSM EDGE evolution 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 signal 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. It gains its name from the fact it is filtered using a Gaussian filter.

Read more about GMSK.

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:

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.

Read more about Phase Shift Keying, PSK.

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

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.

The GSM EDGE radio interface or air interface is very similar to that of GSM, but adopts the different modulation scheme to enable it to carry the packet data more effectively. Being based upon the basic GSM slot and burst format, the EDGE interface is able to operate alongside GSM and GPRS signals.

Ian Poole   Written by Ian Poole .
  Experienced electronics engineer and author.



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