IEEE Communications Magazine - June 2017 - page 172

IEEE Communications Magazine • June 2017
170
0163-6804/17/$25.00 © 2017 IEEE
A
bstract
In order to provide higher data rates and to
improve radio spectrum utilization, 3GPP has
introduced the concept of CA in its Release 10
and onward commonly known as LTE-Advanced.
The CA technology, particularly when applied in
a noncontiguous manner, poses serious design
and implementation challenges for radio trans-
ceivers, mainly due to the allowed flexibility in the
transmitted signal characteristics and the nonlin-
ear RF components in the TX and RX chains. As
a consequence, substantial nonlinear distortion
may occur that not only degrades the transmit-
ted signal quality but can also affect the concur-
rent operation of the coexisting receiver when
operating in the FDD mode. In this article, the key
technical design challenges in terms of linearity
requirements for LTE-Advanced mobile terminals
are reviewed, and the corresponding self-interfer-
ence problem related to the potential desensitiza-
tion of the device’s own receiver is highlighted.
Then technical solutions to mitigate the self-inter-
ference at the RX band due to a nonlinear PA in
the transmitter chain are reviewed, with specific
emphasis on digital self-interference cancellation
methods. As demonstrated through simulation
and actual RF measurement examples, the can-
cellation solutions can substantially mitigate the
RX desensitization problem, thus relaxing the RF
isolation requirements between the TX and RX
chains. Such cancellation methods are one poten-
tial enabling technique toward the full exploita-
tion of the fragmented RF spectrum and the CA
technology in future LTE-Advanced and beyond
mobile networks.
I
ntroduction
Carrier aggregation (CA) is one of the key fea-
tures of the Third Generation Partnership Proj-
ect (3GPP) Long Term Evolution (LTE)-Advanced
networks to meet or even surpass the peak data
rate targets for the International Mobile Tele-
communications-Advanced (IMT-Advanced), or
so-called fourth generation (4G) mobile systems.
CA enables the aggregation of multiple LTE com-
ponent carriers (CCs), which can have any band-
width defined within the LTE specifications, while
ensuring backward compatibility with legacy LTE
systems. It allows operators to flexibly aggregate
the scattered spectral resources that lie in the
same LTE band (intraband CA) or in different LTE
bands (interband CA). Moreover, the aggregated
carriers can have different bandwidths and may
also be noncontiguously located even in the intra-
band case [1–3].
The flexibility of CA technology has several
implications on the design and implementation
of radio transceivers, in particular related to trans-
ceiver linearity requirements. While contiguous
intraband CA and Release 8 single-carrier signals
are still largely similar from the emissions perspec-
tive, the adoption of noncontiguous CA imposes
significantly more stringent linearity requirements
on the power amplifier (PA). This is because
when excited with a noncongtinuous CA signal,
the PA nonlinearities produce unwanted emis-
sions that can interfere not only with the adjacent
channels but also with more distant portions of
the spectrum.
The levels of unwanted emissions are generally
controlled and regulated by regional and internation-
al standardization bodies in order to protect other
devices and radio systems. In this context, the fre-
quency-division duplexing (FDD) mode of operation
is generally more challenging, because transmitter
emissions may leak into the RX chain, causing “own”
receiver (self-)desensitization [4–6, 8].
In general, a practical approach to meet the
emission requirements as well as to relax the
RX self-desensitization problem is to reduce the
transmit power level, such that the PA is oper-
ating in a more linear region. In the 3GPP LTE/
LTE-Advanced user equipment (UE) context, this
is called maximum power reduction (MPR) [2].
However, reduced transmit power directly trans-
lates into reduced coverage and power efficien-
cy. One specific solution for mitigating the RX
desensitization problem is to improve the stop-
band attenuation of the duplexer filters; how-
ever, this will increase the passband insertion
loss and duplexer cost. Therefore, it is imperative
to explore solutions for meeting the required
isolation between the TX and RX, while at the
same time minimizing the insertion loss in order
to improve the power efficiency and receiver
sensitivity.
This article reviews the linearity requirements
of LTE-Advanced mobile transmitters adopting
CA waveforms, and the emissions and distortion
resulting from nonlinear TX and RX components.
A summary of uplink CA technology in different
Linearity Challenges of
LTE-Advanced Mobile Transmitters:
Requirements and Potential Solutions
Adnan Kiayani, Vesa Lehtinen, Lauri Anttila, Toni Lähteensuo, and Mikko Valkama
R
adio
C
ommunications
The authors review the
key technical design
challenges in terms of
linearity requirements
for LTE-Advanced mobile
terminals, and they
highlight the correspond-
ing self-interference
problem related to the
potential desensitization
of the device’s own
receiver. They review
technical solutions to
mitigate self-interference
at the RX band due to
a nonlinear PA in the
transmitter chain, with
specific emphasis on
digital self-interference
cancellation methods.
Adnan Kiayani, Vesa Lehtinen, Lauri Anttila, and Mikko Valkama are with Tampere University of Technology;. Toni Lähteensuo is with Nokia Networks.
Digital Object Identifier:
10.1109/MCOM.2017.1600651
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