EDGE Technology : GPRS EDGE Networks

Ever since Marconi demonstrated the possibility of continuous communication over wireless links in 1897, newer methods to push more information over the medium have been rigorously developed and adopted. But like every good thing, the medium of transmission of this information is limited. Bandwidth has always been a bottleneck in wireless communications. When the need to transmit digital media apart from voice data was felt in Europe with the GSM technology after 1990, a mere 9.4kbps per slot offered by GPRS services was found insufficient. As a primary step towards greater speeds, Enhanced Data-rate for GSM Evolution or more popularly EDGE was developed.

GSM technology rolled out for the first time in Europe in 1990 and had good market by 1992. However it was a voice-only service primarily with a few additional features like SMS and low-bit circuit switched data services. Improving upon this, High Speed Circuit Switched Data (HSCSD) service was introduced in 1998. Circuit switched services had their disadvantages and hence General Packet Radio Service (GPRS) was introduced in 1999 which allowed data transmission and reception in packet transfer mode within a PLMN without a permanent dedicated route between mobile and the external network. This was done keeping in view that the terminal equipment on the user end should remain unaffected and optimizing radio resources at the same time. It worked within GSM networks by taking more than one user slots in the TDMA frames.

However, it was capable of only 171.2 Kbps theoretically even when all time slots were allocated to data services. For multimedia services, a minimum throughput of at least 384 Kbps is required. As a result, Enhanced Data-rate for GSM Evolution was proposed to European Telecommunications Standards Institute in 1997. It was approved by Universal Wireless Communications Consortium (UWCC) and ETSI after feasibility studies conducted in ETSI and then standardized in the ETSI/3GPP Release 99 and later the EDGE Evolution by 3GPP in its Release 7. Most countries now use this standard with an exception of a few like South Korea and Japan which rely exclusively on other standards. EDGE is often called Enhanced GPRS (EGPRS) and concurrent work by ETSI and UWCC ensures that the framework developed works fine with both GSM and TDMA/136 systems. The roadmap was laid in two phases, the first emphasized on developing EGPRS and ECSD and the second focused on improving multimedia and real time support.

Fig. 1: Graphical Presentation of GSM Edge Evolution in Last Few Years

EDGE is a high speed mobile data standard which can work with GSM/GPRS and IS-136 networks over a radio interface and hence also known as Enhanced Data Rate for Global Evolution. This is achieved over the same GSM network bands of 800, 90, 1800 and 1900 MHz. EDGE manages to increase data speeds using the same 200 kHz GSM radio carriers by means of a different type of modulation. Along with the old Gaussian Mean Shift Keying, EDGE also uses the Eight State Phase Shift Keying (8-PSK). This is a linear, higher level modulation scheme which provides higher spectral efficiency, higher data rates and moderate implementation complexity. In both GMSK and 8 PSK Modulation Schemes, the symbol rate stays the same at 271 kb/s. However in 8-PSK, one symbol has 3 bits and hence bit rate per time slot amounts to 69.2 kbps which almost three times as that of 22.8 kbps in GMSK. Following is a brief overview of EDGE technology:

Data Rate	384 kbps
Channel Coding	Outer block coding, inner Convolutional Coding
Modulation Scheme	GMSK, 8-PSK
Access	TDMA and FDMA
Frequencies	Uplink: 890- 915 MHz Downlink: 935-960 MHz
RF Carrier Bandwidth	200kHz
TDMA Slots	8 (each of .577 ms)
Channel Coding Schemes	CS-1 to CS-4 and MCS1 to MCS-9

Though proposed as an evolution to GSM initially, it is now more of a generic air interface for providing high data rates efficiently facilitating a migration path from the second generation to the third as described in the Release 99, hence often termed as a 2.5G Technology, and also to complement UTRAN as fully capable Radio Access Network (RAN) integrable in the UMTS core and maintain backward compatibility at the same time. This means that no new licenses are required explicitly for EDGE services. It is not only aimed at the Packet Switched GPRS networks, but also at the global evolution of Circuit Switched GSM and the D-AMPS (IS136) networks.

Technical Details I

EDGE’s radio protocol strategy has been such that it reuses GSM/GPRS protocols as frequently as possible such that the need to implement newer protocols is minimized. Only a few of the protocols have been changed to optimize performance keeping in mind the increased bit rates and the newer data on radio protocol field after GSM. The physical layer parameters are also derived from GSM architecture. The carrier spacing of 200 kHz and the TDMA 51 frame structure are still the same. The 8-PSK burst format which includes a training sequence of 26 symbols in the middle, 3 tail symbols on both sides and 8.25 guard symbols at the trailing end are similar to GSM format. EDGE has been developed with two key technologies, Circuit switched and Packet Switched.

On GSM networks, EDGE was developed on top of HSCSD as Enhanced Circuit Switched Data (ECSD) which supported various GSM data rates like 2.4, 4.8, 9.6 and 14.4 Kbps on the 4 older GMSK modulation scheme and 28.8, 32 and 43.2 Kbps per time slot over three new 8-PSK modulation schemes which amounts to 172.8 Kbps on 4 time slots. This also has dynamic link adaptation by which the throughput is adapted to the prevailing radio conditions. GSM standards support both transparent and non-transparent types of bearers. There are 8 transparent bearers offering constant data rates between 9.6 and 64 kbps and 8 non transparent bearers which employ Radio Link Protocols to ensure error free data delivery at bit rates in between 4.8 to 57.6 kbps. EDGE did not change the definition or function of bearers, but only improved them. In case of GSM, a 57.6 kbps non transparent bearer required 4 time slots which can be done using ECSD TCS-1 modulation scheme in 2 slots in case of EDGE. This increases efficiency and also improves capacity of the system.

Fig. 2: Table Showing Overview of GSM EDGE Technology

Protocol stack for EGPRS-136 and TIA/EIA-136 CSN

Technical Details II

On GPRS, the evolution towards EDGE was called Enhanced GPRS or EGDE Classic. This supports throughputs of up to 475 Kbps over 8 time slots with slight changes on both sides of the radio interface, the BSS and the Mobile Station. The need for data protection and higher bit rate led to change in the Radio Link Control Protocol, the main changes being focused on link quality and adaptability. It employs 200 kHz control channel structure with 4/12 frequency reuse pattern. It incorporated methods like estimating link quality and selecting the most appropriate bit rate for user, or the incremental redundancy scheme (ARQ Technique) where the coding is selectively increased thus increasing redundancy and reducing effective bit rate until the user can successfully decode it.

The initial code rate is obtained by punctured convolutional codes and then redundancy is provided by puncturing a different set of bits on each retransmission. This helps provide IP connectivity from the mobile station to a fixed IP network. In the Quality of Service (QoS) profile of a bearer, parameters like priority, reliability, maximum and minimum bit rates and delays are defined, and a suitable bearer may be chosen according to the application and need. For Example, the profile may state that the maximum bit rate would be at least 384 kbps for speeds up to 100kmph and 144 kbps upto 250 kmph.

EDGE also supports the US TDMA IS136 standard. They however wanted to have EDGE services on the same frequency bands that D-AMPS was using which meant that some 30 kHz carriers of D-AMPS were released to accommodate 200 kHz carriers as mandated by EDGE which then cannot be used for voice services. This led to the development of the concept called EDGE Compact jointly by ETSI, 3GPP and UWCC. It operates in spectrum limited networks and is solely packet based. This is also one of the reasons that EDGE acronym is often called Enhanced Data Rate for Global Evolution and not GSM evolution (though in 3GPP specifications it still is GSM Evolution). It uses time synchronization between base stations and time groups within different frequencies on different sectors to enable the allocation of the same common control channels to different time groups on rotation basis and preventing simultaneous transmission and reception at the same time. This service could never be successfully deployed in the field because most service providers in US decided to move from D-AMPS to GSM/GPRS/EDGE technology.

Fig. 3: Figure Presentinng Protocol Stack for EGPRS-136 and TIA/EIA-136 GSM

Technical Details III

The introduction of a new modulation scheme also had effects on the requirements of the Mobile Station (MS). For a MS, the enhancement of EDGE comes at a price of cost, size and battery life which are quite dear to a user. Thus ETSI has standardized two mobile classes to reach this trade off: MS has only GMSK in uplink and 8-PSK in downlink or MS has 8-PSK in both uplink and downlink. The first reduces the complexity of design and suits users as upload is very less as compared to downloads. Also, the number of slots available in uplink and downlink may be asymmetric too. Thus EDGE has added more classes to the existing framework of GSM standard with different combination of modulation and multislots.

Due to inclusion of EDGE into GSM/GPRS system, there are slight changes that are introduced into the framework. The A-bis interface between a BTS and BSC offer 16 kbps data rate only, which would have been a bottleneck to EDGE channels which use 64 kbps. Thus multiple A-bis slots needed to be used. Other alternatives included ATM or IP based solutions. This problem was also been tackled under GPRS when CS3 and CS4 coding schemes were included. There are frequency planning and reuse issues as EDGE ha to coexist with GSM and GPRS until complete shift is accomplished through out.

Fig. 4: Image Showing the Areas Influenced by EDGE GSM Technology

(From top left clockwise) GMSK, 8-PSK, 32QAM, 16QAM

The indispensability and flexibility of EDGE is evident as service providers still compliment the HSPA/3G networks with an EDGE network. No matter how much population coverage 3G provides, it does not still provide as wide geographical coverage as GSM/EDGE due to huge investments involved. EDGE and its successor EDGE Evolution have helped mobile service providers in providing a profitable broadband service with minimal investments. Important enhancements like reduced latencies through reduced Transmission Time Interval and protocol enhancements, scope for dual carriers in downlink thus increasing the bandwidth from 200 to 400 kHz, use of turbo codes, added 3 more Modulation Schemes, dual antenna terminals for interference rejection etc have constantly evolved EGDE technology into a mature technology.

The mobile telephony world is moving at a fast pace and we are living in interesting times. EDGE has run a fast race and yet is the main support in almost all networks where 3G is yet to gather full steam. Also in areas where fixed network infrastructure hasn’t yet been established, EDGE has been deployed over the existing GSM infrastructure to provide fast internet access. With more than 180 countries having GSM/ EDGE infrastructure and more than 3 billion users hooked on to GSM networks, the basic framework for evolution from 2G to 3G has been well laid. In the times where many nations are talking about 4G and LTE Advanced, EDGE is still spearheading towards a greater coverage to have a richer multimedia experience anywhere, anytime.