Intelligent Radio Transmitters: An enabling Technology for High Performances Wireless Communication Systems

illnurturedtownvilleΚινητά – Ασύρματες Τεχνολογίες

21 Νοε 2013 (πριν από 3 χρόνια και 8 μήνες)

59 εμφανίσεις



Intelligent Radio Transmitters: An enabling Technology for High
Performances Wireless Communication Systems


Oualid Hammi and Fadhel M. Ghannouchi


Intelligent RF Radio Technology Laboratory (iRadio Lab), Department of
Electrical and Computer Engineering, Schulich School of Engineering,
University of Calgary, Calgary, AB, T2N-1N4, Canada







Corresponding author: Oualid Hammi, Ph.D. candidate
iRadio Laboratory, Dept. of Electrical and
Computer Engineering University of Calgary
Tel. (403) 210-5408
Fax. (403) 282-6855
Address : 2500 University Drive NW
ICT building, # ICT 302
Calgary, AB, T2N 1N4, Canada


Intelligent Radio Transmitters: An enabling Technology for High
Performances Wireless Communication Systems

Oualid Hammi and Fadhel M. Ghannouchi

Intelligent RF Radio Technology Laboratory (iRadio Lab), Department of
Electrical and Computer Engineering, Schulich School of Engineering,
University of Calgary, Calgary, AB, T2N-1N4, Canada


Abstract: This paper presents the critical issues in designing RF transmitters for the emerging
wireless communication applications. The power amplifier is the key element in optimizing the
transmitter performances. Moreover, the design of the RF front end calls for a mixed RF/DSP
approach to further improve the transmitter’s performances.

1. Introduction
The emerging wireless communication standards use spectrum efficient access technologies such
as Code Division Multiple Access (CDMA), Multi-channel CDMA (MC-CDMA) and
Orthogonal Frequency Division Multiplexing (OFDM) that imposes stringent requirements on the
radio transceiver. Indeed, in such communication standards, the resulting signals have high peak
to average power ratio (PAPR) which requires ultra linear transmitters. In most cases, this leads
to the design of very low-efficiency amplifiers that require large DC power modules with bulky
thermal dissipators for base stations and short battery life in mobile terminals. From the base
station prospective, these linear and low-efficient amplifiers turn out to be very costly to
manufacture and very expensive for the telecommunication operators who run the infrastructure.
In addition, the emergence of several standard calls for the design of versatile radio transmitters
that are able to be reconfigured in accordance to the communication standards. All the above
mentioned issues call for a synergetic design of the digital signal processing part of the
transmitter along with the RF front end in order to achieve higher performances.

2. Critical Aspects in Transmitters’ Design
RF power amplifiers (PAs) are one of the most critical transmitter’s subsystems in modern
wireless communication infrastructures. Indeed, both their power efficiency and linearity greatly
affect the overall efficiency and linearity of the RF transmitter. For power amplifiers, the
efficiency and linearity are two antagonist considerations which can not be achieved
simultaneously. Indeed, the PA will have good linearity but low power efficiency for low input
power levels and will become more and more non linear and power efficient as the input power
level will increase as shown on Fig. 1. This dilemma is even more critical in new communication
systems that use envelope varying signals with high peak to average power ratio to achieve higher
spectral efficiency. To improve the achievable trade off between power efficiency and linearity,
linearization schemes such as feedforward, feedback, predistortion, and dynamic biasing have
been widely used during the last years (Kenington 2002). Among these techniques, digital
predistortion appears as the most promising one since it takes advantage of the highly valuable
reconfigurability acquired by using digital signal processing. Moreover, compared to its analog
counterpart, the digital predistortion offers a better ability to fit the non linear characteristics of
the PA. However, the achieved efficiency is still around 15% (Hammi et al. 2006).


3. Intelligent Radio Transmitters Design
To further improve the performances of RF transmitters that are based on digitally
linearized single branch power amplifiers, a combined RF/DSP approach is required. For
that, the design of the RF front end and especially the power amplification stage has to be
considered closely together with the system architecture in order to ensure optimal
system-level performances. This implies that conventional transmitters’ architectures
have to be adapted or modified to be able to integrate this new type of amplifiers.
Moreover, it has been shown by the authors in (Hammi et al 2006) that multi-branch
power amplifiers have the potential to fulfill this performances boosting. A generic block
diagram of the resulting transmitter architecture is presented in Fig. 2. In such
architectures, multi-branch power amplification stages are used to linearly amplify the
signal at high power efficiency by optimizing the amplifiers characteristics in each
branch (biasing, power capabilities …). Signal processing, signal conditioning and
predistortion as well as architecture dependant signal optimization are performed up-
stream the PA stage in the digital domain at base band. This can be implemented in Filed
Programmable Gate Arrays (FPGA) and Digital Signal Processors (DSP) boards/ASICS.
First, linearity and efficiency trade-off enhancement techniques such as crest factor
reduction, linearization and pre-equalization are applied to the input digital data. Then,
envelope dependant digital signal separation is applied to the input signal to generate IF
signals that will drive the PA stage via a multi-channel IF to RF up-converter.

4. Conclusion
This paper focuses on the critical issues in designing high performances radio transmitters for
emerging wireless communication systems. The RF/DSP approach along with multi-branch based
power amplifiers is perceived as a dazzling solution to overcome the performances limitations in
today’s radio transmitters.

Acknowledgement
This work was supported by Alberta’s Informatics Circle of Research Excellence (iCORE),
Natural Sciences and Engineering Research Council of Canada (NSERC), Communications
Research Centre Canada (CRC), and TRLabs.

References
P. B. Kenington, HIGH-LINEARITY RF AMPLIFIER DESIGN . Boston, MA: Artech House, 2000.
O. Hammi, S. Boumaiza, J. Kim, S. Hong, I. Kim, B. Kim, and F. Ghannouchi, “RF Power Amplifiers for
Emerging Wireless Communications: Single Branch vs. Multi-Branch Architectures”, accepted in IEEE
2006 Canadian Conference on Electrical and Computer Engineering, Ottawa, ON, May 2006.
16
18
20
22
24
26
0
5
10
15
20
25
-30 -20 -10 0 10 20 30
Gain (dB)
Efficacité (%)
Gain (dB)
Efficacité (%)
Puissance d'entrée (dBm)
Input Power (dBm)
Efficiency (%)
Efficiency (%)
Gain (dB)
16
18
20
22
24
26
0
5
10
15
20
25
-30 -20 -10 0 10 20 30
Gain (dB)
Efficacité (%)
Gain (dB)
Efficacité (%)
Puissance d'entrée (dBm)
Input Power (dBm)
Efficiency (%)
Efficiency (%)
Gain (dB)

I
in
PBF
LO
Mixer
PBF
Mixer
DAC
DAC
PA Modeling and Monitoring
Digital Signal Processing
Digital Predistortion
Signal Conditioning
Q
in
IF-RF Up-Converter
RF
out
PA
Amplification stage
I
in
PBF
LO
Mixer
PBF
Mixer
DAC
DAC
PA Modeling and Monitoring
Digital Signal Processing
Digital Predistortion
Signal Conditioning
Q
in
IF-RF Up-Converter
RF
out
PA
Amplification stage

Fig. 1. Typical Measured gain
and efficiency of a class A/B PA.
Fig. 2. RF/DSP Generic Architecture of Intelligent Radio
Transmittersz