From LTE-U to LTE-DSA: Solving The Capacity Crunch

The proposal by Qualcomm to enable LTE operation in unlicensed band (LTE-U) received a warm response from some (e.g. Ericsson, Verizon) and not so warm from others especially incumbents with strong legacy in Wi-Fi in both the vendor and operator communities. The contentious issue center on co-existence of LTE and Wi-Fi in the same band as Wi-Fi implements ‘listen before talk’ or in technical terms carrier sense multiple access (CSMA) as opposed to LTE where transmissions are scheduled by the base station. This issue plagued WiMAX in unlicensed bands and was topic of much work at the IEEE during standardization activities of that technology. Still, while the proposal is not yet an approved work item for 3GPP LTE Release 13, the next few weeks will most likely see this feature approved to include in the standard with completion timelines by end of 1Q 2016, when we very possibly can see actual systems deployed.

LTE-U is an extension to the current carrier aggregation feature which has been deployed by a few operators around the world, starting with Korea where all three service providers rolled it out last year. In LTE-U, the primary cell remains one that operates in a licensed band while the secondary cell operates in the U-NII-3 5 GHz unlicensed band as a supplemental downlink channel. The control channel remains on the primary cell for robust operation and the secondary cell is used as an additional conduit for user data in the downlink. The 5.725 – 5.850 MHz allows 1 W RF transmit power (in the US), the highest of any other band in the 5 GHz spectrum. The U-NII-3 band is commonly used by wireless ISPs, especially in suburban and rural areas. With this proposal, it makes sense to use it in small cells particularly as the coverage of this frequency range is limited.

LTE-U: Operation of LTE in unlicensed 5 GHz spectrum (source: Qualcomm)

We’ll have to wait and see to what extent this feature will be commercially successful. Qualcomm says LTE-U complements Wi-Fi. Qualcomm already has strong capabilities in both Wi-Fi and LTE baseband. Combining the two in a SoC for small cells can be accomplished with reasonable ease. But as Wi-Fi gets more integrated into the wireless network, I have my doubts that this feature can actually stop the propagation of Wi-Fi into the wireless access network. Also, LTE-U does not help the uplink capacity. Finally, there is a regulatory framework that cannot be surpassed especially in the EU and Japan where the regulator specifies the spectrum access mechanisms unlike the FCC. Nevertheless, I do think it is a precursor to another implementation that I think can make a big difference in the future.

5 GHz Band allocation (Source: Qualcomm)

This other implementation is to perform carrier aggregation in shared spectrum such as 3.5 GHz. This past week, FCC chairman Tom Wheeler outlined his vision for how dynamic spectrum access will be defined and suggested it will include three-tiered access prioritization for federal and non-federal incumbent users, priority licensees and the general authorized access users (general public). Moreover, the band plan will not be divided, thus keeping a wide accessible allocation which provides much flexibility. This will unleash a 100 MHz of additional spectrum. It is expected to be cleaner than the 5.8 GHz band as use of the spectrum by incumbents can be geographically sparse and intermittent in time. It also offers better propagation characteristics than the 5.8 GHz band with the potential to deploy on macro cells. Provided this is possible, it presents a good opportunity for operators to tap into spectrum on as-need basis. In fact, this would have the potential to radically change the spectrum landscape with prospects for real-time spectrum and capacity trading much as is the case in electric utility.

Looking at the LTE feature roadmap, I think we are close to ‘hitting the wall’: carrier aggregation is relatively the simplest of LTE-Advanced features to implement and the least expensive in proportions to its benefits. It solves the problem of fragmented spectrum while it increases the user’s data rate. But what comes after CA successively adds less capacity for higher cost. We are at a stage where the ratio of incremental cost to benefit will start getting lower and lower. Spectrum has to date been one of the pillars for expanding network capacity.  Implementing carrier aggregation in shared spectrum along similar lines to LTE-U is a strong candidate to be the ‘next thing’ – if we can get the rules for dynamic spectrum access in place quickly. I think we can.