HOW TO OPTIMIZE AN LTE AMPLIFIER PERFORMANCE USING

AWR’S VISUAL SYSTEM SIMULATOR

Long Term Evolution (LTE) is rapidly being deployed by major US carriers and will

serve most, if not all, top-tier markets some time during 2012. LTE is often called

a fourth-generation (4G) standard, and provides signifi cantly increased peak data

rates, with the potential for 100 Mbps downstream and 30 Mbps upstream,

reduced latency, scalable bandwidth capacity, and backwards compatibility with

existing Global System for Mobile Communications (GSM) and Universal Mobile

Telecommunications System (UMTS) technology.

For LTE to work with existing UMTS networks, the enhancements must employ

complex modulation schemes, multiple-input and multiple-output (MIMO), and other

features that make it a challenge for subsystem and system designers. Fortunately,

software tools such as AWR’s Visual System Simulator (VSS) make evaluating the

performance of these components signifi cantly easier and more accurate.

The challenges for designers in producing the best performance in systems

destined for LTE service are similar to those for other standards such as

Worldwide Interoperability for Microwave Access (WiMAX) and include high

peak-to-average ratios (PARs) and tight requirements for adjacent channel leakage

ratio (ACLR) and error vector magnitude (EVM). Like all of the communications

standards incorporated within VSS, the LTE library includes all requirements for

Layer 1 (physical layer) outlined in the LTE standard from the 3rd Generation

Partnership Project (3GPP).

The following example illustrates how to use VSS for:

• Performance optimization of an LTE system power amplifi er,

• Ensuring the amplifi er’s EVM and ACLR performance meet the 3GPP

specifi cation, and

• Simplifying the system designer’s process/work fl ow for achieving optimum

amplifi er performance.

ACLR AND EVM

In today’s digital communications systems, fi lters are used to reduce ACLR.

However, trade-offs are invariably required to ensure that EVM performance is not

sacrifi ced. A fi lter that reduces out-of-band distortion (and thus reduces ACLR)

may cause in-band distortion (EVM) and vice-versa – a fi lter that reduces in-band

distortion (and thus reduces EVM) can increase out-of-band distortion (ACLR).

A test bench created in VSS illustrates how the software can smooth out this

trade-off process for a power amplifi er within an LTE system (Figure 1). The

modulation scheme selected is 64 quadrature amplitude modulation (QAM)

and EVM is measured over one LTE frame. The goal is to keep EVM below

8% while also meeting ACLR requirements. The 3GPP standard requires

ACLR to be under -44.2 dB and is measured at different channel offsets and

transmission bandwidths – ACLR is measured using a root square cosine fi lter

with full transmission bandwidth, or root-raised cosine (RRC) fi lters (=0.22)

with 3.84 Mcps chip rate.

Visual SystemSimulator™

Application Note

AWR “How To” -Optimization of an LTE Amplifi er

1. The LTE signal fi rst passes through the amplifi er design in order to obtain a benchmark of 64QAM signal performance

without fi lters. Here VSS is using a reference LTE receiver for demodulating the transmitted signal that passes through

the power amplifi er.

2. Three fi lters are then identifi ed and selected: two seventh-order elliptical fi lters with 20 MHz and 30 MHz bandwidths

and a fourth-order elliptical fi lter with 20 MHz bandwidth (Figure 2).

3. With just the power amplifi er (PA) in the lineup, the VSS simulation results in EVM performance within specifi cation, but

ACLR did not meet the requirement. Trialing the various fi lters results in the following:

• Seventh-order fi lter after the PA: ACLR is acceptable but EVM is not, illustrating the confl ict mentioned earlier.

• Seventh-order 30 MHz bandwidth fi lter after the PA: ACLR misses the specifi cation but EVM is acceptable.

• Fourth-order 20 MHz fi lter after the PA: Allows both requirements to be met.

A second test bench is then created in VSS in order to analyze EVM for subcarriers at the center of the orthogonal

frequency-division multiplexing (OFDM) symbol and for subcarriers at the edges of the OFDM symbol (Figure 3).

1. EVM is measured over 20 resource blocks (240 subcarriers) at each edge of the OFDM symbol as well as in the center

(VSS software can measure EVM over the specifi ed subcarriers or individual subcarriers). The goal is to determine EVM

at narrow portions of the fi lter response.

2. Filter options explored:

• Seventh-order fi lter: EVM performance proves acceptable in the center but degrades at the edges and is 9.4% overall.

• Both seventh-order and fourth-order fi lters can produce acceptable EVM performance in the center and edges of

the symbol, as well as the overall symbol.

Figure 2. Three elliptical fi lters and their responses that were chosen for this discussion.

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Figure 3. Second test bench, for evaluating EVM subcarriers.

Visual SystemSimulator

Application Note

Figure 1. First VSS test bench, for evaluating ACLR and EVM.

Figure 2. Three elliptical fi lters and their responses

OPTIMIZING AMPLIFIER EFFICIENCY

LTE signals have high PARs and to maintain linearity, amplifi er manufacturers back off the drive level to the fi nal amplifi er

to ensure it operates without saturating under any PAR conditions that can possibly be encountered in service. Crest

factor reduction decreases a signal’s PAR through use of clipping so the amplifi er can be operated closer to the point of

a 1 dB gain compression (P1dB), increasing effi ciency. Two types of clipping are common and include scalar clipping and

magnitude vector clipping. The scalar technique clips the I and Q signals separately so that their maximum amplitude is A:

AQIABS <),(

Magnitude vector clipping attempts to keep the magnitude of the IQ waveform below a particular value and passes a

percentage of the original signal through the power amplifi er rather than all of it in order to reduce the PAR. The complex

baseband signal, x =I + jQ, is clipped so its maximum absolute value is A. This can be expressed as:

A magnitude clipping circuit can be created within VSS as shown in Figure 4. The IQ signal is split into its phase and

magnitude components and the signal’s magnitude is measured. VSS allows the user to constrain the clipped magnitude to

a specifi c level (in this case 55%), which ensures that the portion of the signal with magnitude less than the clipping level

passes through the fi lter undistorted.

The LTE downlink source generator within VSS creates a signal that is passed through the subcircuit while EVM and PAR are

measured. The results of this example (Figure 5) show that clipping reduced the PAR by 2.36 dB with a slight degradation of

EVM and a slight increase of the signal bandwidth at levels below –45 dBm.

SUMMARY

As this article demonstrates, AWR’s VSS software can readily optimize the performance on an LTE amplifi er -- based

upon PAR, ACLR, EVM or any number of performance metrics. VSS software can also be used effectively to evaluate the

“in situ” performance of other devices in the system, not just for LTE, but for all other current cellular standards as well.

More information about VSS software can be found at www.awrcorp.com.

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Figure 5. VSS simulation results of crest factor reduction.

Visual SystemSimulator

Application Note

AWR, 1960 East Grand Avenue, Suite 430, El Segundo, CA 90245, USATel: +1 (310) 726-3000 Fax: +1 (310) 726-3005 www.awrcorp.com

© 2012 AWR Corporation. All rights reserved. AWR is a registered trademark and the AWR logo and Visual System Simulator are trademarks of AWR Corporation. Other product and company names listed are trademarks or trade names of their respective companies.

Figure 4. A magnitude clipping circuit within VSS.

AN-VSS-LTE-2012.06.18