Efficient DPD Coefficient Extraction For Compensating Antenna Crosstalk And Mismatch Effects In Advanced Antenna System

Abstract

The demand for high data rates in a wireless communication system has experienced a
remarkable growth over the last years. This demand is continually increasing with the
growing number of wirelessly connected devices. Thus, there is a continuing hunt for
increasing the data transmission rate in wireless communication systems.
To increase the data rate, the modern wireless communication system cannot only
rely on bandwidth, as usable frequency spectrum is getting crowded. However, advanced
antenna system, i.e Multiple input multiple outputs (MIMO), Multi-user MIMO, massive
MIMO, lead to significant advancement in the modern wireless system by increasing data
rate, spectrum efficiency, channel capacity, etc. These advanced antenna systems rely
on the multi-antenna transmitter, using radio frequency (RF) power amplifiers (PAs) to
excite an antenna array. These RF PAs are one of most power consuming devices in
a wireless transmitter. Thus, the Energy efficiency of PA is a major concern. Higher
efficiency can be achieved by increasing the input power to the PA. However, signals, using modern modulation schemes, like Orthogonal Frequency Division Multiplexing
(OFDM) and Wideband Code Division Multiple Access (W-CDMA), have non-constant
envelope and high Peak to average power ratio (PAPR). These signals create the conflict
between power efficiency and frequency spectrum efficiency (linearity). Linearization
techniques such as Digital pre-distortion (DPD) is most widely used to linearize the PA so
that it can be driven in high power mode for maximizing energy efficiency.
However, the coupling between array elements modulates the load on the output of
each PA, creating post-PA crosstalk. Thus, Power Amplifier behavior changes which
affect the DPD linearization of the PAs since DPD technique is performed without considering
these coupling effects. Thus, some degradation of the system performance in
terms of linearity is expected. To address this problem, PA linearization techniques have
been explored in this thesis. This thesis work is related to the field of research towards a
study of post-PA crosstalk effects and exploring a solution to compensate for both nonlinearity
and crosstalk at the output of the power amplifier.
To emulate multi-antenna transmitter system, a uniform linear array with four transmit
antennas and four PAs is built in this thesis. Digital beamforming technique is applied
to the antenna array to mimic advanced antenna system. The coupling between array elements
is extracted using S-parameters of the array elements. There is no significant impact
found on PA output signal Adjacent channel power while post-PA crosstalk is present
but without DPD. However, the impact of post-PA crosstalk found on DPD linearization
performance.
A simulation model is developed using static behavioral PA models in crosstalk environment
and extracted S-parameter from the array antenna. The simulation using conventional
DPD in crosstalk environment shows around 30dB degradation of linearization
performance in the high coupling scenario.
An algorithm called Crosstalk Digital Pre-distortion (CDPD) is developed and proposed
in this thesis to compensate crosstalk effects on DPD. The proposed solution is tested in a
simulation environment. The simulation results show that the proposed method improves
the DPD linearization performance about 25dB under the very high coupling scenario.
The new algorithm also improves the performance in extracting the model parameters for
digital pre-distortion in a multi-antenna system.

Popular Abstract

The modern wireless communication system is growing fast with the advent of 5G and IoT
development. This has pushed the data rate limits higher than ever before, further pushing
the need for higher bandwidth requirements. Multiple Input Multi-Output (MIMO) technology
which uses LTE (4G communication) signals evolved to address the issue. But for
MIMO systems, with a large array of antennas closely packed in a small area, emitting
signals of the same frequency, at the same time and using antenna processing technique
such as beamforming, a new problem of Radio Frequency (RF) system, called crosstalk,
becomes significant.
Some of the signals transmitted by one antenna can be received by other transmitting
antennas and this can affect the signal emitted by the first antenna. This is referred
to as coupling between antennas. Due to the close proximity to each other coupling between
antennas become dominant. This can result in post-PA crosstalk effects. Indeed the
coupling between two antennas may reach high coupling under certain conditions.
Crosstalk may also lead to signal distortion and nonlinearity. This crosstalk effect has
to be addressed for next generation of wireless systems and for cellular communications
in the mmWave 5G system. The motivation is to improve Power Amplifier performance
for the development of efficient and low-cost small cell radio base station that can be
used in a future 5G system. This means, there is a necessity for a robust algorithm to
compensate for these effects in the 5G system for better system performance.
This thesis studies the distortions at the power amplifier output in MIMO transmitters
considering the post-PA crosstalk effects and proposes a Digital Pre-Distortion based
solution to compensate for the aforementioned crosstalk effects.

• author: Gaur, Himanshu and Shahin, Md Zahidul Islam
• supervisor: Johan Wernehag; Liang Liu
• organization: Department of Electrical and Information Technology
• alternative title: Efficient DPD Coefficient Extraction For Compensating Antenna Crosstalk Effects In Advanced Antenna System
• course: EITM02 20181
• year: 2018
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering
• report number: LU/LTH-EIT 2018-630
• language: English
• id: 8945324
• date added to LUP: 2018-06-11 07:38:33
• date last changed: 2018-06-11 07:38:33