News of LTE-Advanced is making headlines. SK Telecom aggregated two 10 MHz carriers in 800 and 1800 MHz to achieve 150 Mbps downlink throughput with a version of the Samsung Galaxy S4 handset built upon Qualcomm’s Snapdragon 800 SoC. Verizon announced that its LTE network is nearly complete and suggested carrier aggregation (CA) is the next step. AT&T on the other hand has plans to use carrier aggregation over its 700 MHz unpaired lower D and E blocks.
Carrier Aggregation (Source: Qualcomm)
While LTE-Advanced has many features aside than carrier aggregation, such news is significant because they indicate how carriers are moving to address the demand for capacity. Implementing carrier aggregation has in my opinion the best cost/benefit of LTE-Advanced features, provided spectrum is available, which is the case with many operators. To put the issue into perspective, consider for instance other highlight features of LTE-Advanced:
* High order MIMO: today’s LTE systems use two transmit antennas at the base station and the handsets are equipped with two receive antennas (2×2). LTE-Advanced supports higher order MIMO such as 4×4, but the gain from this implementation will be limited as capacity cannot increase beyond the minimum number of transmit or receive antennas (so 4×2 results in doubling the capacity, similar to 2×2). Increasing the base station antennas to 4 while the handset remains at 2 antennas will not result in doubling of capacity, but there will be improved service nonetheless as the link between base station and mobile becomes more robust.
* Small cells: much discussed in recent years, small cells remains encumbered by the business case, interference management such as eICIC, SON, site location & expense, backhaul and other challenges. The advantage of small cells is that they can be deployed selectively to increase capacity in certain areas.
* Relays: have not been at the forefront of features as they are considered mainly as a coverage extension tool. Also, there is aversion to using access spectrum for backhaul even on a limited basis. I expect that spectrum sharing techniques can open up low cost spectrum where relays become viable.
* Coordinated Multipoint (CoMP): CoMP requires tight synchronization of transmitters and places a burden on the backhaul network. Many issues remain to be resolved in CoMP but ultimately operating this feature would require fiber connectivity to sites, which is expensive and not all carriers might have that capability.
Considering the cost/benefit equation associated with other major LTE-Advanced features, I expect carrier aggregation to gain traction quickly particularly as many operators now posses TDD spectrum. For example, Sprint’s acquisition of Clearwire can pave the way to use some of the 2.5 GHz spectrum in CA mode to augment downlink capacity. In Europe and other areas of the world, 3GPP Band 38 (2570-2620 MHz) is available for carrier aggregation. This is further supported on the handset site by new SoCs such as the Snapdragon 800. However, we may still have to wait for a short while before CA becomes a mainstream technology as I expect it will.
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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.