3.2 Technology Capabilities
3.2.2 EDGE
functions like voice calls or the packet broadcast control channel and packet data channels.
The network can dynamically adjust capacity between voice and data functions and can also reserve a minimum amount of resources for each service (Ericsson, 2002a).
Ericsson (2002b), have demonstrated that achieving the theoretical maximum GPRS data transmission speed would require a single user taking over all 8 timeslots without error protection. Clearly, it is unlikely that a network operator will allow all timeslots to be used by a single GPRS user. The bandwidth available to a GPRS user will, therefore, be severely limited. The reality is that mobile networks are always likely to have lower data rates than fixed networks. Relatively high mobile data speeds may not be available to individual mobile users until EDGE or UMTS are introduced.
According to Rysavy (2002), EDGE leverages the knowledge gained through use of the existing GPRS standard to deliver significant technical improvements. Table 3.3 compares the basic technical data of GPRS and EDGE.
Feature Modulation
Symbol Rate
Modulation Bit Rate
Radio Data Rate per time slot User Data Rate per time slot User Data Rate (8 time slots)
GPRS GMSK 270 ksym/s
270 kb/s 22.8 kb/s 20 kb/s (CS 4)
160 kb/s
EDGE 8PSK/GMSK
270 ksym/s 810 kb/s 69.2 kb/s 59.2 kb/s (MCS 9)
473.6 kb/s
Table 3.3: Comparison of EDGE and GPRS Technical Data
Adapted From: Rysavy, P. (2002). Data Capabilities for GSM Evolution to UMTS. [Online].
Available at http:// www, rysavy. com/articles (Last accessed: February 2004).
Although GPRS and EDGE share the same symbol rate, the modulation bit rate differs.
EDGE can transmit three times as many bits as GPRS during the same period of time. This is the main reason for the higher EDGE bit rates. One of the key elements of EDGE is the form of modulation that is used. GPRS, being essentially a packet switched version of GSM uses Gaussian minimum shift keying (GMSK), along with GSM itself. This form of modulation limits the data rate that can be transmitted over the air interface. EDGE uses a new modulation scheme called Octonary Phase Shift Keying (8- PSK). This is a form of phase shift keying where 8 phase states are used (Rysavy, 2002).
The advantage is that it can transmit high data rates, although it is not as immune to interference and noise. The network, therefore, switches to 8PSK to allow the high data transfer rates when signal strengths are sufficient to permit the data transfer with a sufficiently low Bit Error Rate. By using 8-PSK, it is possible to transfer data at 59.2 kb/s per channel rather than 9.6 Kb/s that is possible using GMSK. By allowing the use of multiple channels, the technology allows the transfer of data at rates up to 384 Kb/s. However, it
should be remembered that these data transfer rates are only possible when the network is not highly loaded, as access to all the channels would not be allowed (Ericsson, 2002b).
For GPRS, four different coding schemes, designated CS1 through CS4, are defined. Each has different amounts of error-correcting coding that is optimised for different radio environments. For EGPRS, nine modulation coding schemes, designated MCS1 through MCS9, are introduced. These fulfill the same task as the GPRS coding schemes. The lower four EGPRS coding schemes (MSC1 to MSC4) use GMSK, whereas the upper five (MSC5 to MSC9) use 8PSK modulation (Ericsson, 2002b). Figure 3.2 shows both GPRS and EGPRS coding schemes, along with their maximum throughputs.
Kb/s
G M S K *~>* 8PSK
FIGURE 3.2: CODING SCHEMES FOR GPRS AND EGPRS
Adapted From: Ericsson. (2002b). Giving Operators a New EDGE. Sweden: Ericsson AB.
GPRS user throughput reaches saturation at a maximum of 20 Kb/s with CS4, whereas the EGPRS bit rate continues to increase as the radio quality increases, until throughput reaches saturation at 59.2 Kb/s. Both GPRS CS1 to CS4 and EGPRS MCS1 to MCS4 use GMSK modulation with slightly different throughput performances. This is due to differences in the header size (and payload size) of the EGPRS packets. This makes it possible to resegment EGPRS packets. A packet sent with a higher coding scheme (less error correction) that is not properly received, can be retransmitted with a lower coding scheme (more error correction) if
the new radio environment requires it. This re-segmenting (retransmitting with another coding scheme) requires changes in the payload sizes of the radio blocks, which is why EGPRS and GPRS do not have the same performance for the GMSK modulated coding schemes. Re-segmentation is not possible with GPRS (Rysavy, 2002).
The increased bit rates put requirements on the GSM/GPRS network architecture. Figure 3.3 illustrates the nodes of the GPRS network that are affected with the introduction of EDGE.
These are shaded red in the diagram.
FIGURE 3.3: NODES AFFECTED BY THE INTRODUCTION OF EDGE
Adapted From: Third Generation (3G) Wireless White Paper. [Online]. Available at http://www. triUium. com/pages/white papers (Last accessed: March 2004).
According to Ericsson (2001), the impact of EGPRS on the existing GSM/GPRS network is limited to the base station system due to the minor differences between GPRS and EGPRS.
The base station is affected by the new transceiver unit capable of handling EDGE modulation as well as new software that enables the new protocol for packets over the radio interface in both the base station and base station controller. An apparent bottleneck is the Abis interface (the air interface between the BSC and Base Station), which today supports up to 16 kb/s per traffic channel. With EDGE, the bit rate per traffic channel will approach 64 kb/s, which makes allocation of multiple Abis slots to one traffic channel necessary. The core
network does not require any adaptations. Due to this simple upgrade, a network capable of EDGE can be deployed with limited investments and within a short time frame.
The ability of EDGE to utilise existing resources, infrastructure and spectrum, is seen as a major advantage of the technology and a key driver. By deploying EDGE to enhance a GPRS network, efficiencies can be demonstrated with the support of greater data capacity with the existing spectrum. These efficiencies will also have an effect on voice capacity, by freeing-up channels, which can be then used for voice traffic. EDGE is also an essential complementary stepping-stone en-route to UMTS and integrated cellular networks.
Rysavy (2002), demonstrated that by using EDGE to provide high-speed coverage outside the first stage of UMTS enabled areas, such as main metropolitan areas, would increase the adoption of 3G services and applications in the market. Going forward using EDGE to provide enhanced services in support of UMTS coverage, perhaps in areas of lower traffic density, will ensure EDGE as a must technology and gives a network operator who deploys EDGE a distinct advantage within the marketplace. Being able to deliver standardised applications and services, from whatever technology platform, with controlled Quality of Service (QoS) is a shared industry vision. A unified, enhanced Radio Access Network (RAN), supporting the different technologies is the key to this goal.
The need to reduce business risk and make the best use of existing resources is of paramount importance within today's business environment. GSM based networks have become the standard within the cellular landscape. According to Hughes (2002), EDGE is a GSM based technology that provides an enhancement for GPRS at little additional cost and is considered the best way in which to capitalize on existing resources. GPRS enabled applications and services will generally not require any additional investment to become EDGE compatible.
The business benefits of deploying GSM based end-user devices, as opposed to other technologies is clear when economies of scale are taken into consideration.
Although the financial benefits of EDGE can be apportioned to individual network elements as outlined above, one of the main business drivers is that EDGE forms an essential part of the overall GSM evolution towards a seamless multi-radio GSM/GPRS/EDGE/UMTS network. As mentioned previously, GSM is the main standard for cellular communications
worldwide and the business benefits of deploying an industry standard technology can be seen in nearly every aspect of a network deployment, from end-user devices, to applications to hardware (Siemens, 2002).
Operator driven studies have indicated that for a GSM operator to upgrade a GPRS capable network for EDGE working is approximately 7 - 15% of the total initial GSM investment.
For a GSM operator there will always be a basic requirement to enhance their network infrastructure to improve service over time. This may include the introduction of a packet layer via GPRS and other enhancements, which will make best use of their existing infrastructure and investment. It is also worth considering that if new base stations are installed they will be delivered EDGE ready, especially if they are from a mainstream manufacturer. This program of improvements will, by definition, move the operator towards a network infrastructure, which can then be upgraded to EDGE at very little incremental cost (Rysavy, 2002).
MTN's GSM network is 10 years old with numerous outdated radio base station equipment still being used. For MTN to upgrade its GPRS network to EDGE will require a huge capital outlay in order to upgrade the base station to the latest EDGE capable equipment. Extra transmission and software costs will also be incurred.