In the first few months of 2013 there have been signs that LTE Carrier Aggregation (CA) is coming to fruition, with a series of announcements of demonstrations, trials and launch plans by network operators and equipment manufacturers. Here are some examples:
- Ericsson, Qualcomm Technologies and Sierra Wireless provided the first public demonstration of LTE Carrier Aggregation at Mobile World Congress (MWC) 2013.
- Ericsson and Telstra will trial and deploy LTE Carrier Aggregation in Australia at 900MHz and 1800MHz.
- Huawei and Lebanon Touch have completed Carrier Aggregation trials in a live LTE network in Lebanon, operating at 800MHz and 1800MHz.
- EE will trial Carrier Aggregation in the UK before the end of 2013.
Carrier Aggregation is an important new feature of LTE Advanced, first introduced in 3GPP Release 10 (frozen 2011) and enhanced in 3GPP Release 11. Basic LTE, originally defined in 3GPP Release 8, is able to achieve peak data rates of 300Mbit/s downlink and 75Mbit/s uplink (using 4×4 MIMO antennas and the maximum 20MHz spectrum). This is a significant improvement on previous mobile systems and is achieved partly by improvements in spectral efficiency and partly by operating in a wider bandwidth. Even so, LTE is unable to achieve the ITU target of 1Gbit/s downlink data for low mobility communication in 4G systems.
The key to achieving higher data rates with LTE is to enable network operators to use the technology in bandwidths wider than 20MHz. Some network operators may be lucky enough to have contiguous spectrum allocations of more than 20MHz. However, the nature of spectrum allocation over the years is such that most operators have a mix and match of spectrum within and between frequency bands. Following the redistribution of analogue TV spectrum and the provision of higher frequency spectrum, an operator might have LTE spectrum at one or more of 700MHz, 800MHz, 900MHz, 1800MHz, 1900MHz, 2100MHz, 2500MHz and 2600MHz.
LTE Carrier Aggregation enables a network operator to combine radio channels within the same frequency band or across different bands to achieve much higher data rates and lower latency than otherwise would be possible. In principle the LTE Advanced standard will allow for the aggregation of up to five carriers, each of up to 20MHz, to achieve a total effective bandwidth of 100MHz, although early implementations will be limited to two carriers and therefore a maximum of 40MHz.
Carrier Aggregation can be applied to both Frequency Division Duplex (FDD) and Time Division Duplex (TDD) variants of LTE and it allows the combination of different carrier bandwidths in number of ways, as illustrated by the examples in the figure below.
The simplest form of carrier aggregation, shown in example (a), is where the carriers are contiguous and lie within the same frequency band. In this case it is feasible for a mobile device to handle the signals using a single transceiver, providing it is able to operate efficiently over the aggregate bandwidth.
Example (b) shows intra-band non-contiguous carrier aggregation, in which the carriers lie within the same frequency band, but they are not adjacent. In this case it is necessary for the mobile device to use a separate transceiver for each carrier.
The final example of carrier aggregation is based on inter-band non-contiguous carriers, as shown in example (c). In this case the carriers fall in different parts of the radio spectrum, such as 900MHz and 1800MHz. The ability to combine such carriers is particularly useful for network operators with fragmented spectrum allocations, although it does bring challenges for the mobile device. As in example (b) it is necessary to include a transceiver for each carrier and there is a need for careful design to ensure that the device can operate effectively in two (or more) different bands simultaneously.
Each individual component carrier used for aggregation adopts one of the standard LTE bandwidths defined in 3GPP Release 8 (1.4MHz, 3MHz, 5MHz, 10MHz, 15MHz or 20MHz), to achieve backward compatibility with existing mobile equipment. Legacy devices can be allocated capacity on any one of the component carriers, while mobiles supporting carrier aggregation can be allocated capacity aggregated across two (or more) carriers.
LTE Carrier Aggregation will provide mobile network operators with even greater scope to support services that hitherto would have been restricted to fixed networks, and may open up the possibility of providing a viable alternative to fixed network broadband services, particularly in rural locations where fixed broadband provision may be poor.