Semiconductor suppliers and test firms such as Qualcomm and MediaTek, Rohde and Schwarz, Aeroflex and Anritsu, will be demonstrating LTE-A capabilities this week in Barcelona.
What will make LTE-A different and better than LTE are carrier aggregation, higher-order MIMO, and heterogeneous networks.
“LTE-A is 4.5G, very much a stepping stone between 4G LTE and fifth generation. It isn’t fifth generation, which is off in something like 2020″, says Steve Edwards of Rohde and Schwarz.
The chosen modulation techniques mean that a basestation can send data out at around 150Mbit/s in a 20MHz band of spectrum, or 300Mbit/s in 40MHz, and so on up to 750Mbit/s in 100MHz of spectrum.
However, no network operator has a single contiguous 100MHz chunk of spectrum so, if this was required, five 20MHz bands (or more smaller bands) called ‘component carriers’ would have to share the data load.
Carrier aggregation is the capability required at both basestation and handset to split and re-combine one stream of data packets across multiple component carriers, or many streams of packets to many handsets across multiple carriers. Carriers will be re-allocated once every 1ms.
By the time 5x20MHz carrier aggregation is possible, other improvements are predicted to have pushed the data rate of 100MHz to 1Gbit/s or more.
In initial LTE-A implementations – as even LTE-A is not one single thing, but a series of staged feature introductions – only two component carriers will be aggregated.
The data rate and quality of a point-to-point radio link can be improved if the link is shared between more than one antenna at one or both ends of the link. This is MIMO (multiple-input multiple-output).
It requires antennas to be separated physically, uses various techniques to drive and demodulate the different antennas, and can for example strengthen a link against multi-path destructive interference in a reflective environment.
LTE allowed 2×2 MIMO – two antennas at either end. LTE-A allows for more. “You can have eight at the base station and four at the mobile. It uses the same spectrum and adds a small amount of diversity, in time or phase, which can be summed to find missing bits,” says James Goodwin, director of product management at Anite.
“One of the challenges of MIMO is that you need antennas sufficiently far apart on the mobile, so four antennas might suit tablets, not miniature handsets,” added Goodwin.
In an LTE system, all cells have to be standard cells (‘macrocells’) – covering the usual one or few kilometres. LTE-A allows small cells, for example femto cells, to be integrated in the network where increased capacity is required. A network with different kinds of cells is a heterogeneous network (HetNet).
“Higher data rate is one big trend in the industry, another challenge is capacity,” says Jonathan Borrill, director of marketing at Anritsu. This is where femto cells can help.