Scott Reeves looks at the two main contenders for 4G and where they differ from the current 3G systems in operation.
The original use of mobile phones has been transformed over the last 10 to 15 years. The advent of different types of higher data rate technologies (like GPRS) began a shift in revenue from voice to data for telecommunication companies. The growing demand to be able to use the Internet anywhere, anytime, led to the development of higher bandwidth technologies, such as W-CDMA and WiMAX.
The first generation of mobile communications started with the Advanced Mobile Phone Systems (AMPS), which was an analogue system. AMPS can be thought of as 1G. From there, we progressed to GSM and CDMA-one (pretty much regarded as 2G) and then to UMTS and EV-DO, which are 3G technologies. The latest technologies that are regarded as candidates for 4G are LTE (from the 3GPP group) and 802.16m (from the IEEE). In the case of 802.16m, the candidate for 4G is also known as WirelessMAN Advanced, or WiMAX2. LTE progresses through versions known as releases. The latest release that qualifies as being 4G is release 10, often called LTE-Advanced.
The ITU specification
The group that designates technologies as 4G is the International Telecommunications Union (ITU). The ITU issued a press release on October 21, 2010, that qualified LTE-Advanced and WiMAX2 as meeting the requirements for 4G. The report produced by the ITU is "Report ITU-R M.2134." It's a fairly short report, but I'll pick out the main points, as these give some indication as to what constitutes 4G.
The first point relates to mobility. Generally, low mobility is a person walking. High mobility is usually around 100km/h or about 60mph; a typical speed when traveling on a train or a car. Mobility also means that a person should be able to move between base stations without losing a connection, so there is a handover component to 4G.
The second part of the ITU report relates to throughput. There's no mention of throughput specifically, as in "You'll have 1GB/sec throughput on the downlink." What it does have is the spectral efficiency target for each speed and the likely throughput for this. The report does have some concrete examples, which give an indicative level of the throughput. For instance, with 100MHz of bandwidth, a low mobility user should have a peak data rate of 1.5GBits/sec in the downlink. Under the same conditions, the peak uplink speed should be 675Mbits/sec.
This is significantly higher than current 3G rates, both in the downlink and the uplink. This is where you can quite definitely state that a network is 4G rather than 3G.
One of the other parts of 4G that sets it apart from its predecessor is that it's entirely packet switched. IP is used in the network layer to route packets. This sets 4G apart from earlier 3G technologies, which often use older circuit-switched networks for voice.
Wireless is more complicated in terms of transmission than a wired network. The inverse square law applies to wireless propagation; in fact, it's often greater than the inverse square. The radio signals can also take different paths to the receiver and interfere at the receiver end.
Then there's the limitation of spectrum. This can be very restrictive. The frequency bands currently available are quite small. To further complicate matters, many of the spectrum allocations around the world are in a somewhat fragmented state. Portions may be allocated to 2G, for example, and other parts to 3G — but these portions are often not in contiguous bands of the spectrum, leading to fragmentation.
A way around the problem of fragmentation of spectrum is Carrier Aggregation. This idea was probably meant more to increase potential throughput, but it has the side effect of also allowing operators to be able to use non-contiguous slices of spectrum. Carrier aggregation is a feature of both LTE-Advanced and WiMAX2 — it was not possible under earlier technologies.
At present, LTE-Advanced and WiMAX2 are still to be rolled out, although many of the technologies they'll use — such as Carrier Aggregation and MIMO — are currently in use or are in an advanced stage of development. What are being rolled out are WiMAX and LTE networks that have the capability of being upgraded to WiMAX2 and LTE-Advanced respectively in the future.