5G New Radio – Unlicensed Frequency Communication using LBT

Physical channels inheriting the choices of duplex mode, waveform, carrier bandwidth, subcarrier spacing, frame structure, and physical layer design made as part of the NR and avoiding unnecessary divergence with decisions made in the NR.

1.      Unlicensed bands both below and above 6GHz, up to 52.6GHz

2.      Unlicensed bands above 52.6GHz to the extent that waveform design principles remain unchanged with respect to below 52.6GHz bands.

In the high-frequency unlicensed spectrum, some regulations are mandatory to compel, including limitations on transmit power, power spectrum density, and channel occupation bandwidth. Besides, in some regions, Listen-Before-Talk (LBT), LAA technology is also an option used to provide friendly coexistence with other RATs.

Licensed Assisted Access (LAA) Part of LTE Advanced Pro. It uses carrier aggregation in the down link to combine LTE in unlicensed spectrum (5 GHz) with LTE in the licensed band.

The LBT provides lower the interference and reduces the collision probability with other terminals contending the same channel. The LBT is well designed in LTE LAA to support coexisting in a friendly way with other LAA nodes and other RATs, such as Wi-Fi. Although LBT is not a mandatory requirement to implement in high-frequency unlicensed spectrum LBT still provides a promising opportunity to enhance the transmission reliability and lower the successful transmission latency due to the collision.

In this article, we are focusing on LBT in high-frequency unlicensed spectrum in NR and Channel condition difference for different beams when designing the channel access mechanism in NR unlicensed spectrum

In high-frequency unlicensed spectrum (e.g. 60 GHz), 802.11ad and the developing 802.11ay are currently used in these frequency bands. If we consider performing LBT on such a high-frequency spectrum, one possible impact on conventional channel access mechanism used in LTE LAA may result from the beamforming technology. Beamforming technology is expected to effectively alleviate the impact of the high propagation power loss due to a high-frequency band and to enlarge the cell coverage. In addition to the benefits brought by high beamforming gain, the narrower beam compared to omnidirectional transmission also takes advantage of interference avoidance. However, the high directional beam also increases the difficulty on hearing the transmission from other terminals, which may be hard to avoid the collision. In our opinion, NR not only needs to handle the coexistence with WiFi in the high-frequency unlicensed spectrum but also coexist well with other NR LAA nodes.

Different from LTE, NR takes advantage of beamforming technology and TRP-based topology to expand cell coverage and also enhance edge user performance. With these features, LBT in NR may be different in corresponding design compared with LTE. Owing to the beamformed transmission and TRP-based topology, we can expect the channel condition experienced would be quiet distinct within a gNB coverage, even within a TRP coverage. In order for transmitting data and signals to UEs in a certain range, TRP needs to transmit different beams toward different directions, which can be within the same or different time units. Hence, for one TRP, it may happen that some beams experience an unoccupied channel and the other beams encounter a channel with higher interference from other coexisting nodes. In addition, in the view of the whole gNB cell, different TRPs may serve UEs located in different areas. One example is that one TRP serve UEs around a wide driving road or track and another one TRP serves UE in a business building with lots of coexisting nodes crowded. The channel condition and corresponding channel usage may be quite different for these two TRPs. As a result, unlike the situation in LTE, there are a variety of channel environments in a gNB cell.

We can find some modifications are needed for channel access mechanism in legacy. In LTE, the access mechanism is applicable to the whole cell and also the subsequent channel usage, if a successful LBT. However, in NR, the channel condition difference between the beams intra-TRP and inter-TRP within the same cell should be considered when designing the access procedure. Otherwise, one gNB is unable to use the unlicensed spectrum due to just a busy channel happening in one location in the cell. In conclusion, in order to improve the spectrum usage efficiency, the revision on LBT mechanism and channel usage procedure is necessary.

References:-

Extending LTE to unlicensed spectrum globally – LAA, Qualcomm

R1-1703535, "Evaluation assumptions for Phase 2 NR MIMO link level calibration" ZTE

R4-151542, “LAA adjacent channel coexistence with Wi-Fi,” Qualcomm.

R4-151641, “Preliminary simulation results for coexistence of LAA and Wi-Fi,” Huawei.

R4-151537, “LAA Adjacent channel coexistence simulation results,” Nokia Networks.

R4-152178, “LAA adjacent channel coexistence with Wi-Fi,” Intel Corporation.

R4-152126, “Adjacent coexistence simulation results for LAA system,” LG

R4-151912, “Initial simulation results for LAA coexistence studies,” ZTE.

RP-170828, "New SID on NR-based Access to Unlicensed Spectrum", Qualcomm,

RP-170848, "New Work Item on Enhancements to LTE operation in unlicensed spectrum", Qualcomm

3GPP TR 36.889, 3GPP TS 22.368, 3GPP TS 36.300