Keysight Technologies
Measuring ACLR Performance
in LTE Transmitters
Application Note
抑制血亲Introduction
As wireless service providers push for more bandwidth to deliver IP services to more users, LTE has
胶南开发区emerged as a next-generation cellular technology with great potential to enhance current deployments of 3GPP networks and to enable signiicant new service opportunities. However, LTE’s complex, evolved archi-tecture introduces new challenges in designing and testing network and user equipment. The commercial success of LTE will depend in part on the ability of all devices to work as speciied. One of the particular challenges at the air interface will be power management during signal transmission. In a digital communication system such as LTE, the power that leaks from a transmitted signal into adjacent channels can interfere with transmissions in the neighboring channels and impair system performance. The adjacent channel leakage-power ratio (ACLR) test veriies that system transmitters are performing within speciied limits. This critical yet complex transmitter test can be made quickly and accurately using modern signal analyzers such as the Keysight Technologies, Inc. X-Series (PXA/MXA/EXA) signal analyzers with LTE measurement software and signal generators such as the Keysight MXG signal generator with LTE
signal creation software.
Challenges of LTE transmitter design
LTE product development is underway, and RF engineers are tackling the many design and measurement challenges this complex technology presents. LTE requires support for six channel bandwidths (from 1.4 to 20 MHz), different transmission schemes for the downlink and the uplink (OFDMA and SC-FDMA), two transmission modes (FDD and TDD), and multiple antenna techniques (MIMO spatial multiplexing, diversity, beamsteering). As a result of LTE’s lexible transmission schemes, the physical channel coniguration has a large impact on RF performance—much greater than in current CDMA-based systems. With performance targets set exceptionally high for LTE, engineers have to make careful design tradeoffs to cover each criti-cal part of the radio transmitter chain.
One important aspect of transmitter design is the need to minimize unwanted emissions. Because LTE will be deployed in the same frequency bands as W-CDMA and other legacy cellular technologies, the 3GPP speciications regulate emissions to minimize interference and ensure compatibility between the different radio systems. The primary concern is control of spurious emissio
ns, which can occur at any frequency. In this respect LTE is similar to other radio systems. However, new challenges arise at the band edges, where the transmitted signal must comply with rigorous power leakage requirements. With LTE supporting chan-nel bandwidths up to 20 MHz, and with many bands too narrow to support more than a few channels, a large proportion of the LTE channels will be at the edge of the band.
Controlling transmitter performance at the edge of the band requires a design with iltering to attenuate out-of-band emissions without affecting in-channel performance. Factors such as cost, power eficiencies, physical size, and location in the transmitter block diagram are also important considerations. Ultimately the LTE transmitter must meet all speciied limits for unwanted emissions, including limits on the amount of power that leaks into adjacent channels, as deined by ACLR.
ACLR test requirements ACLR is a key transmitter characteristic included in the LTE RF transmitter
conformance tests (Table 1). These tests verify that minimum requirements are
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being met in the base station (eNB) and user equipment (UE). Most of the LTE
conformance tests for out-of-band emissions are similar in scope and purpose
to those for W-CDMA and should look familiar. However, while W-CDMA speci-
fies a root-raised cosine (RRC) filter for making transmitter measurements, no
equivalent filter is defined for LTE. Thus different filter implementations can
be used for LTE transmitter testing to optimize either in-channel performance,
resulting in improved error vector magnitude, or out-of-channel performance,
resulting in better adjacent channel power characteristics.
Table 1. Conformance tests for RF transmitters (from 3GPP TS 36.141 [1] and 36.521-1 [2]) Array Array
Given the extensive number of complex transmitter configurations that can
be used to test transmitter performance, LTE specifies a series of downlink
signal configurations known as E-UTRA test models (E-TM) for testing the
eNB. These test models are grouped into three classes: E-TM1, E-TM2, and
E-TM3. The first and third classes are further subdivided into E-TM1.1, E-TM1.2,
E-TM3.1, E-TM3.2, and E-TM3.3 (Table 2). Note that the “E” in E-UTRA stands for
“enhanced” and designates LTE UMTS terrestrial radio access, whereas UTRA
without the “E” refers to W-CDMA.
ACLR test requirements (continued)For UE testing, transmitter tests are carried out using the reference measurement channels (RMC) specified for eNB receiver testing. The ACLR requirement for the UE is not as stringent as it is for the eNB, so our focus will be on the latter.
Table 2. E-UTRA test models (from 3GPP TS 36.141 [1])
The 3GPP specifications for LTE define ACLR as the ratio of the filtered mean power centered on the assigned channel frequency to the filtered mean power centered on an adjacent channel frequency. Minimum ACLR conformance requirements for the eNB are given for two scenarios: for adjacent E-UTRA (LTE) channel carriers of the same bandwidth, E-UTRA
ACLR1
, and for the UTRA (W-CDMA) adjacent and alternate channel carriers, UTRA
ACLR1
and UTRA
ACLR2
, respectively.
Different limits and measurement filters are specified for E-UTRA and UTRA adja-cent channels, and are provided for both paired spectrum (FDD) operation and unpaired spectrum (TDD) operation. The E-UTRA channels are measured using a square measurement filter, while UTRA channels are measured using an RRC filter with a roll-off factor of = 0.22 and a bandwidth equal to the chip rate, which
is 3.84 MHz in the example of paired spectrum operation shown in Figure 1.
ms-dosFigure 1. Measurement filters for
wdb620
ACLR measurements ACLR limits defined
for adjacent E-UTRA carriers ACLR limits defined
for adjacent
UTRA carriers
ACLR test requirements (continued)ACLR test requirements for the eNB including paired and unpaired spectrum operation are summarized in Table 3. As of the September 2009 specification release, the ACLR test cases for the UE were not fully complete. However, the UE test procedure is essentially the same as that used for the base station.
Table 3. ACLR base station conformance test requirements (from 3GPP TS 36.141 6.2 [1]). Note that the specification defines the minimum requirement plus the test tolerance (TT)). * Relative limits are 44.2 dB = 45 dB min requirement + 0.8 dB TT. Both Absolute and Relative limits
are provided. Whichever is less stringent is to be used for the conformance requirement. Sophisticated signal evaluation tools are available for making complex LTE measure-ments quickly and accurately. Power measurements including ACLR generally are made using a spectrum or signal analyzer, and the required test signals are built using a signal generator. In the following examples, Keysight’s PC-based Signal Studio application connected to an MXG signal generator is used to build the standards-compliant E-TM signal required for transmitter testing. The output signal
is connected to the RF input of an Keysight X-Series signal analyzer running N9080A LTE measure-ment application, which is used for signal analysis. This equipment setup follows the simple block diagram provided in the 3GPP LTE specifications (Figure 2). Although the measurement process described here is for FDD operation, the process for TDD operation is similar.
According to the specifications, the carrier frequency must be set within a frequency band supported by the base station under test, and ACLR must be measured for frequency offsets on both sides of the channel frequency, as specified for paired or unpaired spectrum operation (T able 3). The test is performed first using a transmitted signal of type E-TM1.1, in which all of the PDSCH resource blocks have the same power, and then again using E-TM1.2, in which power boosting and deboosting are used. The E-TM1.2 configuration is useful because it simulates multiple users whose devices are operating at different power levels. This scenario results in a higher crest factor, which makes it more difficult to amplify the signal without creating additional,
unwanted spectral content—i.e., ACLR.
Spectrum Bands Adjacent Carrier Limits (Min req + TT) *
Paired
Spectrum
Category A
E-UTRA 44.2 dB or -13 dBm/MHz
UTRA 44.2 dB or -13 dBm/MHz Paired
Spectrum
Category B
E-UTRA 44.2 dB or -15 dBm/MHz
UTRA 44.2 dB or -15 dBm/MHz
Unpaired
氯甲苯
spectrum
Category A
E-UTRA (LTE) 44.2 dB or -13 dBm/MHz
1.28 Mcps UTRA 44.2 dB or -13 dBm/MHz
3.84 Mcps UTRA 4
4.2 dB or -13 dBm/MHz
7.82 Mcps UTRA 44.2 dB or -13 dBm/MHz
Unpaired
spectrum
Category B
E-UTRA (LTE) 44.2 dB or -15 dBm/MHz
1.28 Mcps UTRA 44.2 dB or -15 dBm/MHz
3.84 Mcps UTRA 4
4.2 dB or -15 dBm/MHz
7.82 Mcps UTRA 44.2 dB or -15 dBm/MHz
Setting up the ACLR test
Figure 2. Measurement equipment setup (3GPP TS 36.141 [1] Annex I, Figure I.1-1)
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