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ITEM: NTT DoCoMo and Huawei Technologies say they have successfully demonstrated that it’s possible to deploy LTE over the unlicensed 5 GHz spectrum band.
DoCoMo Beijing Communications Laboratories and Huawei have been experimenting with so-called Licensed-Assisted Access (LAA) – a technology that allows cellcos to use unlicensed spectrum bands for additional LTE capacity – since February this year. On Thursday, DoCoMo announced that indoor tests have showed that LAA can work in the 5 GHz band, “leading to cell capacity of approximately 1.6 times greater than that of IEEE 802.11n”.
From the press release:
... Higher-speed data communications and a higher cell capacity in dense traffic areas should be achievable by utilizing the 5GHz spectrum for LTE and LTE-Advanced on a complementary basis in coexistence with wireless LAN.
A key word there is “complementary – LTE won’t be moving into the 5 GHz band at Wi-Fi’s expense. The 3GPP, which is working on a standard for unlicensed-spectrum LTE, wants LTE to coexist with Wi-Fi, not replace it. Indeed, coexistence is a prerequisite for using a spectrum band that no one technically owns. (That’s why Wi-Fi and Bluetooth coexist in the 2.4 GHz band, for example.)
The 3GPP staged its first workshop for unlicensed-spectrum LTE this past June. According to 3GPP TSG-RAN chairman Dino Flore, who chaired that workshop, “Initial results suggest that, when augmented with the appropriate coexistence mechanisms to operate in unlicensed spectrum, e.g. Listen-Before-Talk, LTE can effectively coexist with Wi-Fi and outperform it in terms of spectral efficiency.”
The “Listen-Before-Talk” (LBT) element is crucial in part because (1) the LTE air interface has to be tweaked to enable that functionality, and (2) in many markets – like Japan – LBT is a regulatory requirement for any technology using unlicensed bands. (Interestingly, it’s not a requirement in Korea, China and the US.)
In a white paper [PDF] published in June, Qualcomm described two different co-existence mechanisms for 5 GHz LTE regardless of regulatory environment, and found that in lab simulations for both, cellcos can aggregate LTE across licensed and unlicensed spectrum and achieve “significant throughput gain” without degrading Wi-Fi performance in the same band.
It’s worth adding that 3GPP considers 5 GHz to be a starting point – the goal is to make the standard frequency-agnostic and deployable globally.
The 3GPP’s Flore also said the initial focus will likely be on “Licensed-Assisted Carrier Aggregation operation”, which means LTE cells using unlicensed spectrum will be treated as secondary cells that can opportunistically boost data rates as needed. Mobile data traffic carrying critical information or SLA-guaranteed traffic would remain on licensed spectrum bands. There’s some debate over whether secondary cells will support downlinks and uplinks or downlinks only, but initial work will likely focus on the latter.
3GPP members – including DoCoMo and Huawei – are working on a “Study Item” proposal for unlicensed-spectrum LTE that will be discussed in the regular approval process when the 3GPP meets next month.