Licensed Assisted Access: Operation Principles

Mobile wireless broadband operators rely on licensed spectrum to provide communication services with high reliability and seamless mobility over a wide coverage area. 3GPP LTE (Long Term Evolution) technology has played an important role in the provision of these services.

LTE carrier aggregation has enabled operators to utilize different pieces of licensed spectrum more efficiently to improve user experience and system capacity. However, data traffic increase in mobile networks is expected to outpace the spectrum resources for mobile wireless systems. Hence, in 2015, a new variant of LTE carrier aggregation, license assisted access (LAA), will be introduced, which enables mobile device users to be served by a combination of licensed and unlicensed spectrum bands at the same time. LAA’s incremental use of higher frequency 5 GHz spectrum on small cells, and its harmonization of licensed and unlicensed bands echo key 5G radio access focus areas, perhaps marking the beginning of the 4.5 G era.

A key feature of LAA will be fair spectrum sharing and co-existence of LAA-enabled devices with other devices using the 5 GHz band. This blog outlines the principles and practical applications of LAA solutions.

Introducing Licensed Assisted Access

Unlicensed spectrum has been made available by regulators worldwide for use by wireless communication devices. The regulations for use of the spectrum differ across regions, but they have some common elements:

• Technology neutrality: The spectrum is typically not assigned to any particular technology or service and thus allows for continuing evolution of innovative solutions.
• Spectrum sharing: Operation in unlicensed spectrum should allow sharing with other devices, even those from networks operating different technologies that may use different protocols.
• Interference mitigation: Any transmission in unlicensed spectrum is interference to everyone other than the intended receiver. Thus, an operator must accept whatever interference is received and must correct whatever interference it causes.
• Limited output power: Restricting the transmission power reduces the interference footprint and allows increased spatial reuse of the same frequency, which improves spectrum sharing.

Spectrum sharing and interference mitigation between different networks operating in unlicensed bands can be achieved in multiple ways. A large number of channels are available for use in unlicensed bands (more than 500 MHz in many regions). Selecting a channel with the least observed interference is always the most effective first step: the best interference mitigation technique is to avoid it, if possible. Smart channel selection is sufficient for isolated and sparse deployments. Several co-channel sharing and interference mitigation solutions have been utilized in widely used technologies. Bluetooth adopts a frequency hopping protocol; hence, its operation is not limited by high interference in a particular frequency, and the interference it causes to any particular frequency is corrected by hopping to a different one. Wi-Fi adopts a fast channel monitoring and response protocol; it transmits in the channel when the received power from other networks is below a certain threshold and stops the transmission after a certain duration.

Recently, utilization of unlicensed spectrum within the LTE network to meet the ever increasing traffic volume generated by a wide variety of services is being explored. In particular, the concept of licensed assisted access (LAA) is under study in the 3GPP standardization forum and by industry partners. Based on LTE carrier aggregation, the LAA framework allows additional carriers with low observed interference in the unlicensed spectrum to be used together with a licensed band carrier (see Figure 1). Essential control signaling and a basic amount of user data can be supported without interruption on the licensed band while unlicensed spectrum, when available, can be used to boost data rates. The system can be designed to use the additional radio resources from the unlicensed band for carrying just downlink (DL) traffic to the mobile unit as a DL-only LAA system or it can carry traffic in both the downlink and uplink (UL) as a DL+UL LAA system.

Figure 1: Licensed assisted access

Aggregating the unlicensed band channels in the LAA framework does not mean these carriers are always used for LAA transmissions. Intelligent scheduling algorithms in the LAA base station will consider a multitude of factors to steer traffic between the carriers. There are many situations when the unlicensed band carriers may be used sparingly. First, it is better to use the always available licensed band carrier as much as possible since it has more predictable latency characteristics. Secondly, when interference levels on the unlicensed band carriers are high and consequently feasible data rates on those carriers are reduced, it is advantageous to rely mostly on the licensed band carrier for data delivery. Thus, the LAA-capable base station will monitor unlicensed band carriers to determine the best opportunities to utilize them. The principle can be further illustrated using Figure 2. When the unlicensed band carrier is heavily utilized by another network, most of the traffic in the LAA network is served on the licensed band carrier. When the unlicensed band carrier is lightly utilized by another network, traffic can be served on both the licensed and unlicensed band carriers, giving LAA users higher throughput and lower latency.

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

In Part 2, we will further discuss different co-channel sharing solutions for LAA.