The Evolution from LTE in Unlicensed Spectrum (LTE-U) to Licensed Assisted Access (LAA)

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Andreas Roessler, Rohde & Schwarz
Andreas Roessler, Rohde & Schwarz

Facing the capacity crunch

Due to the ongoing transition from voice-centric mobile phones to smartphones and tablets, which has been greatly accelerated by the launch of LTE networks starting in2009, mobile broadband data consumption has increased exponentially over the past five years. Global mobile data traffic grew 69 % in 2014 alone and reached 2.5 exabytes per month, up from 1.5 exabytes per month in 2013.In 2015, additional growth of 59 % is forecast, reaching 4.2 exabytes per month. Video streaming is the dominant traffic type and accounts for more than 55 % of all mobile data traffic in 2014.Such ongoing exponential growth represents quite a challenge for mobile network operators worldwide.

To deliver excellent user experience while offering high data rates on an average basis to every subscriber, service providers must efficiently use the spectrum that is available to them.LTE is the technology of choice; however, spectrum is not an infinite resource. Up until today, service providers have invested billions of dollars on a global scale over the past years to increase their spectrum holdings and thus their system capacity. However, there is only a limited amount of frequencies available that local regulators can auction off to these service providers, thereby leading to tough competition as well as bidding wars in extreme cases to acquire additional licenses.

Due to this shortage, alternatives are required. One very promising alternative is to take advantage of unlicensed spectrum, such as the industrial, scientific and medical (ISM) frequency bands, and especially the underutilized 5 GHz frequency band. Opportunistic usage of the spectrum, while deploying LTE component carriers in this frequency band, allows network operators to increase their system capacity while adding additional spectrum resources at literally no cost. This alternative is known across the industry as “LTE in unlicensed spectrum”, or LTE-U for short. The feature has gained significant momentum, especially in the United States.

Accordingly, the 3GPP, the standardization body behind LTE, took on the challenge of enhancing the technology while adding the required functionality to support LTE-U. The feature is currently being standardized as “Licensed Assisted Access using LTE” (LAA) .Of course, there is no free lunch and everything has its price. Coexistence and fair sharing of the spectrum resource between LTE-U operators, but more importantly with existing technologies such as WiFi, are important prerequisites holding the key to the success of LTE-U/LAA.

Figure 1  5 GHz spectrum allocation.
Figure 1  5 GHz spectrum allocation

In the following article, we will explore the basic principles behind these two buzzwords in the wireless industry, examine the challenges and present ways to ensure coexistence and fair sharing using appropriate test equipment and measurement techniques.

5 GHz spectrum regulation – similar, but different

First, let’s take a closer look at the frequency resource. The 5 GHz spectrum is regulated in a similar manner throughout the world, but additional rules do apply in the different regions. Frequency regulation from a global perspective is administered by the International Telecommunication Union(ITU)on a regional basis. There are three regions defined.ITU region 1 is primarily Europe, ITU Region 2 is America, including the United States, Canada and Brazil for example, and ITU Region 3 is Asia with China, Japan and South Korea. Countries belonging to any of these regions typically adhere to the overall concept for that region; however, there can be additional regulations in place locally that apply to certain parts of the spectrum. In this article, we consider the regulatory aspects for the 5 GHz spectrum in the United States (US) since there is a very strong interest in LTE-U and LAA that is driven by local Tier1 operators.

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Figure 1 shows the situation in the US. The spectrum between 5150 MHz and 5925 MHz is divided into four domains that are marked UNII-1 to UNII-4, where UNII stands for Unlicensed National Information Infrastructure. For the four domains, different regulatory rules apply, for example in terms of the allowed maximum conducted output power, the peak power spectral density (PSD) and out-of-band (OoB) emissions By way of example, Table 1 shows the requirements set by the Federal Communications Commission (FCC), which is the regulatory authority in the US.

As can be gathered from Table 1, UNII-2 devices need to support transmit power control (TPC) as well as dynamic frequency selection (DFS; see also Figure 1).In contrast, UNII-1 and UNII-3 do not require this additional mechanism to ensure coexistence with other systems, thereby making the lower and upper portion of the spectrum the first targeted frequencies that will be used by LTE-U and LAA. Consequently, two new frequency bands were defined by the 3GPP as Band 252 and Band 255 which correspond to UNII-1 and UNII-3, respectively. Note that the channel raster definition for these two bands follows the WiFi channel assignment to avoid in-channel interference. With the band definition, the 3GPP is acknowledging the initial work that has been done within an industry alliance, called the LTE-U Forum, which was forged to accelerate the time to market for LTE-U. The founding members of the LTE-U Forum were Verizon Wireless, Qualcomm, Ericsson, Alcatel-Lucent and Samsung. These key players in the wireless industry have agreed on coexistence aspects to allow fair sharing of the spectrum resource with other LTE-U operators and technologies such as WiFi. Furthermore, they agreed on a set of specifications to define the minimum requirements for handsets and base stations supporting LTE-U. For example, the minimum requirements for an eNB (LTE base station) are based on the 3GPP’s Technical Specification (TS) 36.104.This document takes the limits and tolerances that are provided within TS 36.104 for RF measurements such as adjacent channel leakage power ratio (ACLR) or spectrum emission mask (SEM) and adapts them for base stations that support LTE-U in terms of the regulatory aspects. Rohde & Schwarz was one of the first suppliers of test and measurement solutions to integrate these new, additional limit definitions into its test instruments such as the R&S FSW signal and spectrum analyzer. The R&S FSW is the instrument of choice for performing RF measurements on infrastructure elements such as base stations and components such as power amplifiers. Figure 2 shows an adapted spectrum emission mask (SEM) measurement on an LTE-U capable base station operating in frequency band 255 (UNII-3).

Figure 2: Spectrum emission mask (SEM) measurement using the R&S®FSW signal and spectrum analyzer in line with the LTE-U eNB minimum requirements specification
Figure 2: Spectrum emission mask (SEM) measurement using the R&S®FSW signal and spectrum analyzer in line with the LTE-U eNB minimum requirements specification

One aspect we should mention here to distinguish the work done by the LTE-U Forum from that of the 3GPP is the fact that from a forum perspective, LTE-U is defined to use bands 252 and 255 as a supplemental downlink only. The licensed assisted access (LAA) work item also defines the anchor carrier (primary component carrier, PCC) for the communications link to reside in a licensed frequency band but does not exclude the use of the 5 GHz spectrum for uplink carrier aggregation at a later stage as well. However, for the moment the 3GPP is also considering the (secondary) component carrier, placed in the 5 GHz bands, solely as a transmission resource.

How to ensure coexistence for LTE-U/LAA?

The LTE-U Forum members have agreed on a two-step approach to ensure coexistence with other technologies and LTE-U operators along with fair sharing of the spectrum resource with existing technologies. First, this is based on smart channel selection during the initial boot-up phase, which is then continued dynamically during operation. In other words, an LTE-U capable base station (similar to a WiFi access point) periodically monitors the frequency band and selects its channel based on channel quality measurements and input parameters such as traffic load. A “channel penalty” function has been proposed that has multiple input parameters with variable weighting factors. Based on the measurements and weighting factors, a penalty for each potential channel is determined. The channel with the lowest penalty is selected and a 20 MHz LTE component carrier is transmitted on this frequency channel. A terminal that supports LTE-U is informed about the exact carrier frequency via the defined signaling methods for carrier aggregation and can thus access that component carrier. As of now, aggregation of up to three component carriers is foreseen. One is in a licensed frequency band that could have any bandwidth depending on the spectrum holding of the respective operator. In addition, there can be up to two 20 MHz component carriers in the unlicensed frequency band. Such carriers are always 20 MHz wide (and no more and no less due to the WiFi channel definition in which the minimum bandwidth is 20 MHz).In total, there could be an aggregated transmission bandwidth of up to 60 MHz, including 2×2 MIMO operation per component carrier and a maximum peak data rate of 450 Mbps.

After initial channel selection, the LTE-U capable base station must use carrier sensitive adaptive transmission (CSAT) to ensure fair sharing with other LTE-U operators using the spectrum and WiFi. The basic principle behind CSAT is to define a cycle with a duration of some milliseconds that is divided into an ON period and an OFF period. The length of the cycle and thus the duration of the ON and OFF period are dynamically adaptable based on the traffic situation. Figure 3 shows an example .If there is a heavy load on the selected channel, e.g. many WiFi access points and other LTE-U base stations are active, then the CSAT cycle might be long, up to 150 ms, whereas the ON period is short, e.g. 20 ms only. If the channel is not heavily occupied, a shorter CSAT cycle might be appropriate with a longer ON period and therefore a shorter OFF period. Note that the values given in Figure 3 were suggestions by LTE-U Forum members which were presented at a related workshop in May 2015.

Figure 3  LTE-U base stations use CSAT to ensure fair spectrum sharing with other LTE-U operators and Wi-Fi.
Figure 3  LTE-U base stations use CSAT to ensure fair spectrum sharing with other LTE-U operators and Wi-Fi.

As we can gather from Figure 3, during the ON period of the CSAT cycle a few sub frames are periodically punctured and configured as almost blank sub frames (ABS).The actual quantity depends on the duration of the ON period and thus on the traffic load. The puncturing of subframes is intended to ensure that latency-sensitive applications that run over WiFi such as voiceover WiFi (VoWiFi) can still function once LTE-U is operational.

Testing the performance and coexistence of LTE-U handsets

Besides testing the RF conformance of LTE-U capable base stations while measuring the transmission power, SEM and ACLR, it is important to test handsets in terms of their performance and coexistence. Rohde&Schwarz was the first to demonstrate LTE-U performance at Mobile World Congress 2015 in February 2015 in Barcelona, Spain. The R&S CMW flexx setup, consisting of two R&S CMW500 wideband radio communications testers, was used to emulate and aggregate three LTE component carriers with a bandwidth of 20 MHz each and 2×2 MIMO on top. Two of the component carriers were placed in a licensed frequency band and the other component carrier was placed in the UNII-3 domain of the 5 GHz spectrum. Aggregating these three carriers allows a maximum data rate of 450 Mbps. The demonstration involved a maximum throughput test to verify that the device under test is capable of handling this high data rate [4].

From a coexistence point of view, it is important to verify that the device is able to support CSAT. Rohde & Schwarz has developed a test case package for LTE-U/LAA based on the R&S CMW500 wideband radio communications tester that allows the user to verify support for CSAT, including reconfiguration of the CSAT cycle during operation.

Outlook

As we have already highlighted, the 3GPP is standardizing the LTE-U functionality known as licensed assisted access (LAA).Part of the standardization work involves integration of” listen before talk” (LBT) functionality which is required in Europe and Japan to use the 5 GHz spectrum. A device that wants to utilize the spectrum must always sense the channel first before starting to transmit. At this time, the standardized LBT functionality is based solely on energy detection.

Summary and conclusion

LTE-U is currently a hot topic in the wireless industry. The feature provides an attractive alternative for network operators who want additional spectrum so they can increase their system capacity. Fair sharing of the resource among operators, and also with existing technologies such as WiFi,is the key to the success of LTE-U.Rohde & Schwarz provides turnkey test solutions to enable infrastructure vendors and handset manufacturers to test their products in terms of the support for LTE-U.

 

 

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