CEPT
ECC
Electronic Communications Committee
CPG-PTD(14)069
CPG-15 PTD
CPG-15 PTD #5
Rome, 13-17 January 2014
Date issued:
7 January 2014
Source:
IRT
AI 1.2 – Reduced ACLR – The side effect of high efficiency UE power
amplifiers
Subject:
Password protection required? (Y/N)
N
Summary:
Optimizing power consumption is a major consideration in designing user equipment. This
document shows that the price to be paid for improved energy efficiency is higher ACLR values,
i.e. higher out-of-band emissions.
The increases in ACLR due to partial Resource Block usage as described by Nokia in Doc
Mobile-DTT(13)19, are highly questionable once Envelope Tracking techniques are introduced.
The new amplifier design techniques are expected to make use of the maximum OOB limits
permitted within the 3GPP specification to improve the energy efficiency of the LTE user
terminals.
Proposal:
In calculations of compatibility between DTT and LTE, reduction in OOB levels due to reduced
Resource Block allocation assumptions are made regarding ACLR levels which are not
warranted.
Therefore simulations to assess such compatibility should not include reduction of OOB emission
levels due to partial resource block allocation.
Background:
Discussion on the level of unwanted emission from future IMT 700 MHz LTE UE into the adjacent
broadcasting band.
Reduced ACLR
The side effect of high efficiency UE power amplifiers
1. Introduction
Successive generations of mobile systems have each improved the data throughput,
however this has also resulted in an increase in the Peak to Average Power Ratio
(PAPR) as shown in Figure 1.
Figure 1: PAPR at different evolutionary stages [5]
Modern digital modulation techniques such as OFDM are packing more data bits into
each RF channel. This results in increasingly complex waveforms, with higher "crest
factors" - the ratio of the signal peaks to the average power level, usually expressed as
Peak to Average Power Ratio (PAPR).
The average transmitted power level generally stays the same, as this determines the
distance (range) of the RF transmission.[5]
High PAPRs pose stringent demands on the linearity of power amplifiers (PA) to avoid
significant distortion of the signal.[7]
2. Discussion
Traditional RF Power Amplifiers are supplied with a fixed DC voltage, but are only
energy efficient when they are in compression, i.e. when they are operating at the peaks
of the transmitted waveform. Most of the time signals are of a lower level and the supply
voltage is far higher than it needs to be. This leads to excess power consumption which
is dissipated as heat in the PA device; with LTE/OFDM signals up to 80% of the energy
in the PA can be wasted. [5] This has an obvious impact on the battery life.[3]
New techniques, in PA design, such as Envelope Tracking have been developed that
amongst other things provide significant power consumption savings. [2][6]
Envelope Tracking is a power supply management technique for improving the energy
efficiency of Radio Frequency Power Amplifiers. It replaces the fixed DC supply voltage
of traditional RF PA designs with a dynamic supply voltage which closely tracks the
amplitude, or "envelope", of the transmitted RF signal.[1]
2
Envelope Tracking works by dynamically adjusting the supply voltage to the PA. This
maximizes the energy efficiency of the PA by keeping it in compression over the whole
modulation cycle, instead of just at the peaks [5].
Figure 2: The Old and the New Type of Power Amplifier [5]
Keeping the PA in compression increases the energy efficiency of the PA and therefore
the whole system (i.e. LTE user equipment). However, it also means that significant
intermodulation products (out-of-band emissions) are generated at all signal levels and
these do not reduce appreciably when the number of resource blocks (RB) is reduced.
Partial RB use means that the PAPR is also reduced. In OFDM systems (i.e. also in SCFDMA systems) the PAPR is a factor of the number of carriers which in LTE are
grouped in RBs. The lower the number of RBs the lower the PAPR is [7]. The reduction
of the PAPR is used by the Envelope Tracking processing to adjust/optimize the
compression point of the PA.
Figure 3: Efficiency vs. supply voltage [4]
Figure 3 shows that for a Tx output power of 25 dBm supply voltages from 2.5 V to
4.5 V could provide that output power. Depending on what the actual signal envelope is
3
at one instance the appropriate supply voltage is chosen by the Envelope Tracking
system. A signal with a large signal envelope will require to be operate closer to 4.5 V
while a signal with a small signal envelope could be operated closer to 2.5 V. The fixed
power supply at maximum supply voltage provides only 27 % efficiency. Reducing the
supply voltage provides potentially an efficiency of up to 48 %. [4]
This is different to traditional PA design which produced lower OOB emissions at lower
output power or when only a part of RBs are allocated. In traditional amplifier design,
the OOB emissions reduced when signal levels dropped due to the fact that the signal
did not reach the compression point of the PA.
With Envelope Tracking the PA’s efficiency is optimized by ensuring that the amplifier is
always operating in compression. This means there is always a certain level of
intermodulation products and whilst the ACLR may not exceed the minimum required, it
does not improve when signal levels are reduced because this would decrease the
energy efficiency of the PA and hence the battery life of the LTE UE.
3. Conclusion
Optimizing power consumption is a major consideration in designing user equipment.
This document shows that the price to be paid for improved energy efficiency is higher
ACLR values, i.e. higher out-of-band emissions.
The increases in ACLR due to partial Resource Block usage as described by Nokia in
Doc Mobile-DTT(13)19, are highly questionable once Envelope Tracking techniques are
introduced. The new amplifier design techniques are expected to make use of the
maximum OOB limits permitted within the 3GPP specification to improve the energy
efficiency of the LTE user terminals.
However, in calculations of compatibility between DTT and LTE, reduction in OOB
levels due to reduced Resource Block allocation assumptions are made regarding
ACLR levels which are not warranted.
Therefore simulations to assess such compatibility should not include reduction of OOB
emission levels due to partial resource block allocation.
4
References:
[1] Suzuki Y., Narahashi S., Nojima, T.: Out-of-band distortion analysis of envelope tracking
technique for power amplifiers, Wireless Communication Systems, 2009. ISWCS 2009. 6th
International Symposium on
[2] TI unveils envelope tracking DC/DC converter for 3G and 4G LTE smartphones
http://newscenter.ti.com/2013-10-07-TI-unveils-envelope-tracking-DC-DC-converter-for-3G-and4G-LTE-smartphones
[3] Sahu B., Rincon-Mora G.A.: A High-Efficiency Linear RF Power Emplifier With a PowerTracking Dynamically Adaptive Buck-Boost Suplly
http://users.ece.gatech.edu/rincon-mora/publicat/journals/mtts03_pa_dyn.pdf
[4] White Paper: ET PA Characterization, 2012,
https://www.nujira.com/pages/files/Papers_and_Presentations/Technical_White_Paper__Envelope_Tracking_PA_characterization.pdf
[5] Envelope Tracking technology overview,Nujira, https://www.nujira.com/technology-pa746.php
[6] QFE1100 http://www.qualcomm.com/media/documents/files/qfe1100-the-world-s-firstenvelope-tracking-technology-for-3g-4g-lte-devices.pdf
[7] I. Orovic, N.Zaric, S. Stankovic, I. Radusinovic, Z. Veljovic: Analyisis of Power Consumption
in OFDM Systems,
http://riverpublishers.com/journal/journal_articles/download_file.php?file=RP_Journal_19044720_146.pdf
5