Unlicensed spectrum use: What’s the technology behind it?

Unlicensed LTE lets wireless carriers offload traffic to Wi-Fi frequencies, but it must coexist with Wi-Fi and not interfere. Here’s how it works.

Long-Term Evolution Unlicensed, LTE-Unlicensed, or LTE-U, is the extension of the LTE wireless standard in the unlicensed spectrum, such as the 5 GHz bands used by 802.11a and 802.11ac compliant Wi-Fi equipment. It is intended to enable wireless network operators to offload traffic and enhance service to end users instead of replying on carrier-owned Wi-Fi hotspots. LTE-U aims to bring carrier-grade quality of service (QoS) to the unlicensed spectrum. LTE-U is not a monolithic technology; there are several implementations, including License Assisted Access (LAA), enhanced LAA (eLAA), further enhanced LAA (feLAA), and MulteFire.

Compared with Wi-Fi, LTE-U is expected to bring benefits in range and link budget, spectral efficiency and capacity, configurable QoS, mobility, interoperability, high to low rate scaling, spectrum options, security, and path to 5G New Radio, called NR-U. LTE-U, and especially NR-U, have significantly different end-use targets compared with today’s 4G networks. 4G has evolved to bring increased levels of multimedia content to users; NR-U is being developed with a focus on commercial and industrial uses, including the industrial internet of things (IIoT), automation, and machine-to-machine communications. Instead of improving multimedia experiences of users, 5G NR-U is expected to be transformative in different ways, including:

• Supporting higher device densities
• Defined QoS
• Easier and more flexible deployment
• More robust security
• Higher reliability

The various LTE-U technologies are designed to leverage carrier aggregation (CA) and supplemental downlink (SDL) protocols. LTE-U is expected to improve both coverage and spectral efficiency compared with conventional Wi-Fi. The use of a licensed anchor carrier is an important factor in delivering the anticipated benefits of LTE-U in all its embodiments.

LAA, eLAA and feLAA
The process of deploying LTE-U accelerated in 2016 with the release of LAA by the Third Generation Partnership Project (3GPP). In 2020, 3GPP Release 16 became the foundation for deploying 5G NR in unlicensed spectrum (NR-U) in the unlicensed 5GHz and 6GHz bands.

The LTE-U evolution began with LAA, where LTE is operated for downlink only on an unlicensed spectrum and a carrier in the licensed spectrum. eLAA added the ability for the license-exempt operation of both downlinks and uplinks. Further enhanced LAA (feLLA) added the ability to operate a license-exempt autonomous uplink. Finally, with 3GPP Release 16, standalone operation with NR is enabled, where an NR carrier can operate independently in the unlicensed spectrum without an anchor link back to the licensed spectrum.

3GPP Release 16 was frozen in July 2020. It generally takes 14 to 18 months for products to come to market once a new 5G standard is frozen. So, products supporting NR-U’s latest release could be expected on the market beginning in the third quarter of 2021.

Coexisting with WiFi on unlicensed spectrum
Co-existence with current technologies such as Wi-Fi is an important consideration in developing the various LTE-U technologies. There are two methods commonly used to support sharing of unlicensed spectrum between WiFi and LTE-U users:

• In markets such as Europe, Japan, and India, regulations have been established for unlicensed spectrum that requires equipment to periodically check for the presence of other occupants in the channel (listen) before transmitting (talking) in millisecond scale; this is often referred to as Listen Before Talk (LBT).
• In countries such as the United States, China, and South Korea, there is no regulatory requirement for LBT for the unlicensed bands. A maximum energy detection threshold methodology is used in these countries to support robustness, fairness, scalability, and forward compatibility for cross-technology coexistence with WiFi. This methodology enables operators to deploy LTE in unlicensed bands compatible with Rel. 10/11 3GPP LTE standards.

Measurement of received energy and the detection of recognized modulated or encoded signals are commonly used by WiFi devices as part of LBT protocols. Wi-Fi devices send a “preamble” at the start of all transmissions that contain known reference signals for synchronization and the length of the pending transmission. A Wi-Fi device does not transmit if a Wi-Fi preamble is received at an energy level above a specified threshold in a 20 MHz channel or if any energy is detected during LBT above a different threshold, for example, from a non-Wi-Fi device.

Wi-Fi’s use of the preamble posed several problems for 3GPP implementations. The Wi-Fi preamble specification is based on older technologies and is not considered an efficient or compatible solution from a 3GPP perspective. And the sample rates and OFDM sub-carrier spacings are not compatible with 3GPP. As a result, 3GPP uses a maximum energy detection threshold methodology to support cross-technology coexistence.

Typical channel coexistence such as LBT and Carrier Sense Multiple Access (CSMA) used by WiFi are based on the concept of contention-based access. In these techniques, transmitters are expected to sense the channel and make sure it is clean before transmission. The goal of these algorithms is to provide coexistence across different technologies in a time-division multiplexing (TDM) fashion.

In the case of highly-dense Wi-Fi and LTE-U environments, there may be no clean channel available. When that occurs, LTE-U is designed to share a channel with a Wi-Fi device or another LET-U device by using the Carrier-Sensing Adaptive Transmission (CSAT) algorithm to employ adaptive TDM transmission to LTE-U small cells, based on long-term carrier sensing of co-channel Wi-Fi activities.

Using CSAT, LTE-U cells sense the channel for a longer duration than LBT of CSMA (from tens of msec to a few hundred msec). Based on the observed channel activities, the CSAT algorithm defines a time cycle where the LTE-U cell transmits for a fraction of the cycle and stays off the air for the remainder of the cycle. The duty cycle of the LTE-U transmissions versus staying off the air is dictated by the sensed channel activities of other technologies such as Wi-Fi.

ETSI standards for LAA require equipment to periodically check for the presence of other devices in the channel by employing LBT in the millisecond scale. The image below details the requirements defined by ETSI for LBT for frame-based equipment. The listening time is referred to as the Clear Channel Assessment (CCA) period. Before transmitting, a device has to detect the channel’s energy level at a specific time during the CCA period. If the channel’s energy level is below the defined CCA threshold, the device can transmit for the duration of the Channel Occupancy Time (COT). Once the COT has expired, the device is required to repeat the CCA process before any further transmission can occur.

Multefire
MulteFire (sometimes called MuLTEfire) is tightly aligned with 3GPP Release 13 and 3GPP Release 14 specifications for LAA and eLAA, respectively, augmenting standard LTE to operate globally unlicensed spectrum. It is designed for LTE-U deployments and can act as a neutral host. MulteFire is simple to deploy (similar to Wi-Fi) and is suited for use on any channels that need over-the-air contention for bandwidth sharing, such as the global 5 GHz unlicensed spectrum band or shared spectrum in the 3.5 GHz Citizens Broadband Radio Service (CBRS) band in the U.S.

MulteFire supports a range of LTE services, including voice over LTE (VoLTE), Video over LTE (ViLTE), mobile broadband data, user mobility, and IoT networks. A key goal of MulteFire is to deliver the performance of LTE with the simplicity of Wi-Fi. As with traditional cellular communications networks, MulteFire supports the full mobility of users as they move around in a building, with seamless handovers between small cells. In addition, MulteFire is designed to interconnect with external mobile networks to provide service continuity and seamless handoffs when users leave the area of the MulteFire network.

The recently-released MulteFire 1.1 version is a further evolution of LTE. While maintaining backward compatibility with MulteFire 1.0. MulteFire 1.1, it adds four new features to enhance MulteFire 1.0 capabilities, including:

• Grantless uplink transmission (GUL), Using GUL, the UL autonomous transmission does not rely on an SR request. Therefore, if within a predefined set of radio resources, which are configured on a per-cell basis, a UE succeeds LBT, then it can start transmitting immediately. As a result, it will naturally coexist well with Wi-Fi as the UE behavior is not different from Wi-Fi stations;
• Wide-coverage enhancement (WCE), Aims to improve the downlink (DL) link budget for LTE cellular systems operating solely on unlicensed bands system. WCE supports the improvement of the DL performance by nearly 8 dB compared to MulteFire 1.0. It is designed to support applications in which have many autonomous pieces of equipment that need to communicate with each other such as automated guided vehicles (AGVs) or autonomous robots moving around large areas;
• Autonomous user equipment (UE) mobility (AUM), is a new feature to complement the normal controlled handover procedure. When a UE is being configured for AUM mode, it is pre-configured with one or more potential target cells. Upon certain conditions being met, the UE may autonomously contact the target cell without informing the source cell. A UE can be configured on a per-cell basis by the source cell to autonomously trigger and perform handover, without receiving an explicit handover command or informing the source cell; and
• Self-organized networks (SON), encompasses solutions to self-configure and self-optimize a network. It was introduced in LTE to facilitate the deployment of a system and allow for further performance optimization. In MulteFire 1.1, SON features have been introduced focusing on the network self-configuration or the optimization of stand-alone networks operating in unlicensed spectrum and networks deployed with the neutral host network (NHN) architecture.

Summary
LTE-U aims to bring carrier-grade quality of service (QoS) to the unlicensed spectrum. LTE-U is not a monolithic technology; there are several implementations, including License Assisted Access (LAA), enhanced LAA (eLAA), further enhanced LAA (feLAA), MulteFire, and NR-U. The various LTE-U technologies are designed to leverage carrier aggregation (CA) and supplemental downlink (SDL) protocols. LTE-U is expected to improve both coverage and spectral efficiency compared with conventional Wi-Fi. The use of a licensed anchor carrier is an important factor in delivering the anticipated benefits of LTE-U.

References
3GPP technologies in unlicensed spectrum: A contributor to the common good, Ericsson
Future Is Unlicensed: Private 5G Unlicensed Network for Connecting Industries of Future, MDPI
LTE in unlicensed spectrum, Wikipedia
LTE in Unlicensed Spectrum: Harmonious Coexistence with Wi-Fi, Qualcomm
MulteFire Release 1.1 Enhancements, MulteFire.org