This blog entry is the third in a row about my thoughts on the current development of 4G wireless standards. You might want to take a look at the introduction before reading on.
There are two main goals of 4G wireless systems. First of all, more bandwidth will be required for the reasons explained in the previous blog entry on this topic. Secondly, 4G networks will no longer have a circuit switched subsystem as current 2G and 3G networks. Instead, the network is based purely on the Internet Protocol (IP). The main challenge of this design is how to support the stringent requirements of voice calls for constant bandwidth and delay. Having sufficient bandwidth is a good first step. Mobility and Quality of Service for a voice connection is clearly another and taking a look at these topics is better left to another article series. So let’s focus on the additional bandwidth 4G networks are to deliver. Before taking a closer look at individual technologies, here is what they will all have in common:
Currently, 3G networks are transforming into 3.5G networks as carriers add technologies such as High Speed Data Packet Access (HSDPA) and High Speed Uplink Packet Access (HSUPA) to UMTS. Similar activities can be observed in the EVDO world. Staying with the UMTS example, such 3.5G systems are realistically capable of delivering about 6-7 MBit/s in a 5 MHz band. Numbers which are twice as high are circulating as well. However, these speeds can only be reached under ideal conditions (very close to the antenna, no interference, etc) which are rarely found in the real world.
4G networks will go far beyond this by mainly improving three things:
- Air Interface Technology: 2G networks such as GSM use Time Division Multiple Access (TDMA) on the air interface. 3G networks made a radical change and use Code Division Multiple Access. 4G standards will make another radical change and will use Orthogonal Frequency Division Multiplexing (OFDM). The new modulation itself will not automatically bring an increase in speed but very much simplifies the following two enhancements:
- Channel Bandwidth: 2G systems such as GSM use a channel bandwidth of 0.2 MHz. UMTS made a great leap forward and uses 5 MHz. 4G systems will use a bandwidth of up to 20 MHz, i.e. the channel offers four times more bandwidth than channels of current systems. As 20 MHz channels might not be available everywhere, most 4G systems will be scalable, for example in steps of 1.25 MHz. It can therefore be expected that 4G channel sizes will range from 5 to 20 MHz.
- MiMo: The second method to increase throughput on the air interface is to use a technology called Multiple Input Multiple Output, or MiMo for short. The idea itself is simple, the maths behind is everything but. The idea of MiMo is to use the phenomena that radio waves bounce of objects like trees and buildings and thus create several wave paths from sender to receiver. While this behavior is often not desired, MiMo makes active use of it by using several antennas at the sender and receiver side, which allows the exchange of multiple data streams, each over a single individual wave front. Two or even four antennas are foreseen to be used in a device. How well this works is still to be determined in practice but it is likely that MiMo can increase throughput by a factor of two in urban environments.
Increasing channel size and using MiMo will increase throughput by about 8-10 times. Thus speeds of 40 MBit/s per sector of a cell are thus possible. Other articles will claim even more but again these numbers can only be reached under ideal conditions which are usually not found in a real environment. Sophisticated base stations use three or even four individual sectors which results in a total throughput of a single base station of up to 120 to 160 MBit/s. Not bad by today’s standards.
So much for the technical details for the moment. Let’s look at who’s going to put the nice numbers above into real products. Three different standards are being put together at the moment:
- WiMAX, aka IEEE 802.16e: Air interface specs are already pretty well put together and the technology definitely has a technical lead over the competition as far as this is concerned. The WiMAX forum [LINK] however is still working on standards for the radio access network and the core network which narrows its lead over other technologies.
- UMTS Long Term Evolution (LTE): This standard is developed by the Third Generation Partnership Project (3GPP), the same standards body already responsible for the GSM, GPRS, UMTS and HSDPA standards.
- EVDO Rev C. (also dubbed DORC): This standard is developed by the Third Generation Partnership Project 2 (3GPP2), the body responsible for CDMA and EVDO.
So why are there three standards, wouldn’t a single standard be enough? The short answer is “Well of course not”. The long answer is somewhat more complicated so I’ll leave this to part four of this mini series.