Evolved EDGE – The New Kid On the Block

I couple of years ago I thought EDGE (Enhanced Data Rate for GSM evolution) would not have a big chance on the market with new technologies like 3G and later on HSDPA entering the market. An yet, EDGE has made it into many networks around the world. In some countries, 3G hasn’t made it beyond big cities yet and I have come to value EDGE quite a lot in my frequent travels which often bring me to smaller towns or even the countryside. Interesting to see that work is underway in 3GPP to push EDGE forward once again.

Evolved EDGE

Today, EDGE for GPRS mainly increases user data rates by using new modulation and coding schemes which go far beyond the original GPRS specification. Effectively, EDGE increases GPRS speeds about four times. In practice, speeds of about 220 kbit/s can be reached under good radio conditions. Evolved EDGE sets out to increase the user data rate once again to a level around 1 MBit/s with the following enhancements:

Multiple Receiver Chains: Today, GSM mobile phones use once receiver chain. By adding a second one which analyzes the incoming signal (with a different polarization or different phase I am not sure) independently, chances to decode the data stream correctly increases. This means that higher modulation and coding schemes can be used in the same conditions as if the mobile phone only had one receiver chain. Note that this is a receive diversity scheme and not a MIMO (multiple input, multiple output) which will be used in 4G systems like LTE, WiMAX and also in the next generation of Wifi (802.11n).

Higher Order Modulation: EDGE uses 8PSK modulation which encodes 3 bits per transmission step. Evolved EDGE introduces 16QAM modulation which can encode 4 bits per transmission step and 32QAM modulation which encodes 5 bits per step. In practice, however, 32QAM is difficult to use for average transmission conditions on the air interface.

Two Simultaneous Radio Channels: Since 1992 the principle of GSM has been to use only a single carrier frequency to receive data. With E-EDGE, mobiles can now receive data on two frequencies. This could in effect double the data rate available to a single user.

Independent Transmission and Reception Chains: Another principle that E-EDGE is about to lift is the use of only a single transmission and reception chain which so far restricts mobiles to only sending or receiving at a time. By introducing independent transmission and reception chains, data rates are increased as the mobile phone does not have to switch between transmission and reception. This doesn’t only free up the timeslots used for the reverse direction but also frees up adjacent timeslots which are not usable with a combined transmitter and receiver which needs some time to switch between transmission and reception mode. While most EDGE mobiles on the market today are still restricted to four timeslots per carrier due to this phenomenon, having independent transmission and reception chains could allow mobiles to use all eight timeslots of a carrier.

Speed Calculation

When putting it all together, speed can be increased as follow: Higher order modulation can increase transmission speed by 1/4, so 220 kbit/s become 293 kbit/s. Use of twice the number of timeslots per carrier increases the top speed to 2 * 293 kbit/s = 586 kbit/s. Using two carriers could again double the speed to about 1.173 kbit/s. Not bad for a 15 year old technology that was originally intended for a transmission speed of 12 kbit/s for voice communication.

While this all sounds quite fascinating, there will be a number of downsides in practice as well:

Availability

E-EDGE will not be around for quite some time. Taking past developments as a reference, I expect that it will take at least another two to three years before networks are upgraded and for mobiles to be available (if such a decision is made).

Spectrum Efficiency

E-EDGE, in contrast to EDGE, only modestly increases spectrum efficiency. Thus, the total available bandwidth per sector per cell will still be in the range of "only" 1.5 MBit/s for a typical base station which uses three carriers per sector. In addition, the base station is also used for voice communication which further limits transmission speeds. Compare this to a full blown UMTS 3.5G HSDPA base station which uses 2 carriers per sector to reach a total bandwidth beyond 20 MBit/s and the difference becomes quite obvious. Thus, to say that E-EDGE drives data rates up to HSDPA levels is true as far as per user speeds are concerned but certainly not as far as the overall base station capacity is concerned. In addition, data rates of HSDPA in two to three years from now will have certainly moved on to beyond 3.5 MBit/s per user. Therefore, statements saying that E-EDGE is en par with HSDPA is pure marketing nonsense… Additionally, the number of E-1 links (2 MBit/s each) which connects the base station to the network also has to be increased to support the new modulation and additional timeslots which will be usable for voice.

Terminal Support

Most of the enhancements of E-EDGE will have a strong impact on current GSM terminal design. Independent receiver and transmitter chains have been standardized already since the first days of GPRS (GPRS class A). However, up to this date there are no such mobiles on the market. This seems to be a hard nut to crack. I am not sure if this is for technology reasons or simply due to the price or increased packaging requirements.

Other radio systems

Today, one of the main issues that keeps many operators from deploying 3G in rural areas is the use of the 2100 MHz spectrum which requires a higher number of base stations due to the smaller range compared to what can be achieved in the 900/850 MHz band used currently used by many GSM operators. At least in North America it looks, though, as if UMTS 3G will also be used in the 850 MHz band in the near future thus leveling this advantage of GSM/EDGE today. Furthermore, UMTS has now also been standardized for the 900 MHz band but it remains to be seen if regulators in Europe will allow deployments in that band in the near future. Also, other technologies such as WiMAX should also not be underestimated as a competition in the rural area in three or four years from now. With data rates over ten times higher per base station site then what an E-EDGE cell could deliver it seems doubtful to me that the technology would compete very well.

Summary

So will this technology get deployed and will it be successful? With this one, my crystal ball remains clouded. E-EDGE has many good ideas and will certainly increase data rates somewhat but it will not compete very well with other 3.5G and 4G technologies, which are also evolving to higher data rates. The best I therefore expect is that todays gap between 2.5G on the one hand and 3.5G and 4G networks on the other hand does not further increase.

Recently, Peter Rysavy has also written a good column about this subject. Take a look here, his articles on wireless network technologies are among the best to be found on the web!

For the details take a look at the 500 pages of the 3GPP Technical Report TR 45.912.