Licensed Assisted Access: Practical Coexistence Solutions

Channel scanning and selection to operate a wireless network in frequencies with low received interference is always the first and most effective step for spectrum sharing and interference mitigation. Smart channel selection is thus sufficient in most scenarios (e.g., isolated and sparse deployments). In this second blog post on LAA, we discuss co-channel operation measures when different systems need to operate on the same frequencies.

Airport arrival

A Multitude of Co-channel Sharing Solutions

The range of channel monitoring and traffic steering solutions for the license assisted access (LAA) system can be roughly classified based on the time granularity they use to make such measurements and steering decisions. Solutions with a larger granularity are feasible using current LTE specifications and are being explored by industry partners. Solutions with finer granularity require changes to LTE specifications and are under study in 3GPP. Ericsson is an active participant in these studies and in the development of solutions that improve channel measurement and traffic steering. We have also built and demonstrated prototype and test-bed systems for both approaches with industry partners.

For the first type of solutions where the LAA system operates channel monitoring and traffic steering with a larger time granularity, the base station ceases all radio transmissions on the unlicensed channel when no traffic needs to be sent on it. The extended silent period is used by the base station to monitor received power levels and activity burst patterns in the unlicensed spectrum. These measurements allow the LAA base station to update certain long term metrics representing radio conditions on the unlicensed spectrum. These metrics are used by the LAA base station scheduler to determine when to start data transmission on the unlicensed channel and for how long, as we showed in Figure 2 of yesterday's post. With good algorithms to adapt the OFF duration and the ON duration, system simulation studies have shown this type of LAA solution can achieve fair spectrum sharing with other wireless technologies.

Our wired demonstration used an Ericsson LAA base station and a Qualcomm mobile unit to test a DL-only LAA system based on this type of solution. We showed that the Wi-Fi link maintained an average throughput of 60 Mbps when LAA operation was started on the same unlicensed channel as compared to the 115 Mbps average throughput the Wi-Fi link achieved when there was no co-channel operation, as seen in Figure 1. At the same time, the LAA link achieved 65 Mbps throughput on the unlicensed band carrier. Therefore, the total traffic throughput carried by the unlicensed band channel was increased from 115 Mbps to 125 Mbps and the algorithm in the test system showed that this first type of LAA system can coexist well with Wi-Fi on the same unlicensed carrier. On the licensed band carrier, LTE maintained 150 Mbps throughput leading to an aggregated throughput of 215 Mbps as shown in Figure 1.

Figure 1: Coexistence test results for first type of LAA solutions

A second type of solution is currently being studied in 3GPP. Here, the LAA system operates channel monitoring and traffic steering with a finer time granularity. Similar to the first solution, when the LAA base station does not send traffic on the unlicensed channel, the base station ceases all radio transmissions and monitors the received power levels and activity bursts in the unlicensed spectrum. However, for the second type of solution, shorter term channel measurements are computed and the calculated metrics are used to make traffic steering decisions with a finer time granularity. This allows LAA traffic to opportunistically access the unlicensed band channel more quickly as illustrated in Figure 2. Being able to utilize the additional radio resource on a finer granularity generally allows the LAA system to serve traffic on the unlicensed band with reduced latency.

Figure 2: Channel monitoring and traffic steering with a finer time granularity

The exact details of such fast channel monitoring and response protocols are being studied in 3GPP with multiple proposals from companies in both the LTE and Wi-Fi industries. Given the complexity of the issue and the required changes to existing LTE specifications, coexistence studies are being carried out in multiple phases in 3GPP. Coexistence performance evaluations for a DL-only LAA network coexisting with a Wi-Fi network having only DL data traffic (but with Wi-Fi acknowledgments in the UL) are currently ongoing. Additional coexistence performance evaluations for a DL-only LAA network coexisting with a Wi-Fi network having both DL and UL data traffic, as well as a DL+UL LAA network coexisting with a Wi-Fi network with both DL and UL data traffic will be conducted in the coming months as part of the 3GPP study.

We have built a flexible LAA system test-bed that can provide further verification and insight into different coexistence algorithms for this second type of LAA solutions. As a first step, the coexistence algorithm described in R1-150584 has been implemented for a DL-only LAA system and was recently shown in an over-the-air demonstration (Figure 3) to coexist with a Wi-Fi network carrying only DL traffic. In the tests, FTP traffic is generated for both links and the LAA algorithm targets a channel occupancy time of 4 ms. When two Wi-Fi APs share the spectrum, each Wi-Fi link has a throughput of around 50 Mbps. When one of the Wi-Fi systems is replaced with an LAA system, the LAA and Wi-Fi systems achieve throughputs of around 55 Mbps (Figure 4). This result shows the feasibility of coexistence between a DL-only LAA system with a Wi-Fi system also serving only DL traffic. Aggregating 150 Mbps from the licensed band carrier, LTE achieved an aggregated throughput of above 200 Mbps, as seen in Figure 4. Further tests for other scenarios will be carried out in the future.

Figure 3: Setup for over-the-air coexistence test 

Figure 4: Coexistence test results for second type of LAA solutions

In summary, there is a large amount of unlicensed spectrum (more than 500 MHz in many regions) available for sharing. Smart channel selection is always the first and most effective step for spectrum sharing. The LAA framework, with its continued access to the licensed carrier, allows an LTE network with wide coverage and seamless mobility to provide a high quality of service and end-user experience, while accessing unlicensed spectrum in an adaptive manner. Such opportunistic access to unlicensed spectrum is based on channel monitoring and traffic steering principles which enable coexistence with other networks operating in unlicensed spectrum. There are a multitude of solutions based on these principles that allow both LAA networks and Wi-Fi networks to share unlicensed spectrum while achieving good coexistence. We’ll continue to investigate, verify and enhance this technology.

Thomas Cheng, Sorour Falahati and Daniel Larsson, Ericsson Research

If you missed part one, here is the link to Licensed Assisted Access: Operation Principles.

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