The buzz surrounding the unlicensed spectrum to offload data-intensive services is growing stronger. The roll picked up speed last September when the coexistence standard was released, and recent announcements by carriers that they plan to launch LTE-U services this spring has placed an emphasis on pushing products to market for the unlicensed bands. This has placed as much stress on engineers and the product verification process as there will be on the actual devices that switch from licensed to unlicensed cells.
The business case for the unlicensed band usage is sound and well-known. A finite number of radio frequencies are supporting a rapidly expanding range of spectrum uses: Broadcasting, government/civil (defense, public safety), industrial, scientific, medical, and of course commercial services. Given the diversity of technologies, coexistence is critical—both in the technical sense of inhibiting issues such as interference and saturation, as well as in the civil sense of ensuring fair sharing and equitable access through ethical spectrum management.
Among the challenges associated with LTE-U are:
- Coexisting with technologies that might use the same 5 GHz spectrum, most notably WiFi.
- Coexisting with other operators in the same unlicensed band given that different operators may access the same portion of the unlicensed band (unlike with licensed spectrum).
- Building a solution that can work globally.
- Not interfering with critical ISM uses, like defense, radar, and medical applications.
Engineers designing chipsets, mobile devices, modems, and other products for the unlicensed ecosystem must account for three major LTE flavors:
LTE-U – This technology (Figure 1) uses the unlicensed spectrum for a secondary downlink (SDL) carrier, using it in conjunction with the primary licensed carrier, similar to carrier aggregation (CA). The LTE-U mechanism begins with a sweep of the available unlicensed channels to locate a clear channel to avoid interference. If a clean channel isn’t available, the LTE transmission shares a channel with another user via a technology such as WiFi or separate unlicensed LTE transmitter from a different operator. To fair share the channel with other users, carrier-sensing adaptive transmission (CSAT) is adopted.
Figure 1: Unlicensed CA in SDL and TDD via the licensed carrier.
License Assisted Access (LAA) – Much like LTE-U, LAA uses a licensed anchor/carrier in conjunction with unlicensed supplemental downlink carrier(s). The big difference is LAA uses the Listen Before Talk (LBT) mechanism to enable the coexistence of LTE with other unlicensed technologies. LBT also makes LAA available for worldwide use.
LTE WiFi Aggregation (LWA) – This mechanism leverages the best of LTE and WiFi, by using the existing WiFi MAC (link layer) and PHY (physical layer), tunneling LTE data through them and aggregating it so that it does not create an additional, separate LTE MAC layer.
Engineers designing devices that will make coexistence possible must ensure that the products meet certain basic requirements. Among the key parameters that must be verified via testing are:
Transmit Power – Unlicensed spectrum users must comply to power requirements, limiting the downlink transmit power to between 100 mW and 1W. Transmitter Power Control (TPC) is also mandated in certain UNII-2A and UNII-2C bands to reduce the transmit power to the minimum capable of maintaining link transmission quality to reduce interference and extend battery life.
Radar Avoidance Using Dynamic Frequency Selection (DFS) – In some geographical regions, radar systems may be operating. Regulations require that if in this scenario, unlicensed devices must move to a different channel when they detect radar signals.
LBT – Some geographical regions specify that unlicensed devices must use an LBT mechanism to avoid collision with other users’ technologies. Many standard bodies and industry organizations are addressing this, including 3GPP, IEEE, WFA, LTE-U Forum, Evolve, and CableLabs.
Testing in an Unlicensed World
While there are some similarities in the verification process for products used in unlicensed environments, engineers face some new challenges. For example, the unlicensed secondary networks have broader bandwidths requiring higher orders of modulation. Additional antennas will also be designed into both modems and mobile devices to accommodate the new network configuration.
Incorporating the various secondary cells adds stress to modems used in an unlicensed environment. The digital signal processor (DSP) has to be enhanced, which means additional and highly stringent performance tests must be conducted. More antennas will be designed into the front end as well, meaning the number of measurements to confirm modem performance meets specification will increase.
Device verification has similar hurdles. Protocol tests to ensure the ability of mobile devices to efficiently conduct seamless handoffs from the primary licensed network to the numerous unlicensed secondary networks need to be performed.
From a carrier perspective, the fact that the unlicensed bands are at a higher frequency is not the most important variable. Operators will need to ensure that the primary LTE network will be able to recognize and report on the available unlicensed secondary networks. The result is Carrier Acceptance Tests (CATs) will be developed that will add to the verification process for device engineers.
Finding the Proper Solution
Just as chipsets, devices, and networks will have alterations to accommodate the unlicensed spectrum, how engineers select test solutions needs to undergo a minor adjustment. A base station simulator, which is a standard test tool in R&D and manufacturing, must have advanced capabilities in terms of testing multi-technology devices. Expanded report generation tools for authentication are also a necessity. Given the evolving standards and test requirements, it will be equally important that signal analyzers are compatible with software that allows engineers to easily create and execute customized test cases.
For physical layer RF tests, communications analyzers will need to be able to conduct Over-the-Air (OTA) tests. These solutions use actual radio waves so engineers can confirm UE TRX performance is not adversely affected by factors such as the antenna form and characteristics.
The integration of the unlicensed spectrum for commercial services is now upon us. For this bridge technology to be successful and meet the needs of carriers and consumer alike, device and network performance must meet a higher standard. All these factors make selecting the proper test solutions critical.
