fpga design and synthesis of reconfigurable ofdm transceivers for sdr

7/27/2019 FPGA Design and Synthesis of Reconfigurable OFDM Transceivers for SDR

1/5

International Conference on Computing and Control Engineering (ICCCE 2012), 12 & 13 April, 2012

ISBN 978-1-4675-2248-9 2012 Published by Coimbatore Institute of Information Technology

Abstract

This paper presents a software-defined radio (SDR)

system with reconfigurable architecture for wireless communications.

The baseband software implementation by using a low-power fixed-

point digital signal processor (DSP) is applied to demonstrate the

concept of SDRs for different standards, and different operational

modes.For simplicity, two operational modes, quadrature amplitude

modulation (QAM) and quadrature phase shift keying (QPSK) of

OFDM baseband transceivers are implemented. The interoperability

and adaptability among these operational modes of this OFDM

System is discussed. Both modes employ radix-2 decimation-in-

time fast Fourier transform (FFT) algorithms. The architecture

presented is synthesised using a hardware description language

(verilog HDL) code. The outcome of the design is a portable,

scalable, quickly adaptable and reconfigurable system which supplies

a high quality signal.

KeywordsSoftware-Defined Radio, Orthogonal Frequency

Division Multiplexing, FFT, IFFT, Verilog HDL

I. INTRODUCTIONOW a days, digital processors (DSPs) based on

reconfigurable logic devices (FPGAs and CPLDs) are

taking relevance in the digital communications world.

The software defined radio was created to obtain both

permanent communications inside different bands of the radio

and microwave spectrum with a single device and adaptability

as opposed to new innovations of components and equipment.Basically, one is to transfer to software many of the functions

that have been taking place in hardware[1].

As semiconductor devices are shrinking, the rate of new

Services introduced will soon exceed the rate of

miniaturization in electronic packaging. What is needed is a

flexible, universal radio platform for receive and transmit,

which can be programmed to steer to any band, tune to a

channel of any bandwidth, and receive any modulationall

within reasonable physical constraints, including size, weight,

power consumption, and more important, cost. B.Kelly

proposed Software Defined Radio for OFDM protocols in

2009[3].

A more research had been taken place and still newmethodologies and techniques have been coming in the area of

SDR-OFDM.H.G.Yeh and V.R.Ramirez demonstrated M-ary

PSK and QAM OFDM System in a TMS320VC5416

DigitalSignal Processor[4] and FFT[5][6].This paper proposes

a novel concept on reconfigurable OFDM transceiver

S. Prabu, Research Scholar, St.peters University, Chennai, India. E-Mail:

[email protected]

Dr.E. Logashanmugam, Research Scholar, Satyabama University, E-Mail:

[email protected]

implementation with hardware description language such as

verilog HDL.

Rest of the paper is organized as follows. Section II

provides basic idea about Architecture of Software Defined

Radio. Section III provides the idea of interoperability and

adaptability of Software Defined Radio. Section IV provides

the details about the proposed Reconfigurable OFDM models.

Section V provides experimental results. Section VI provides

the application with image data and section VII concludes the

work.

II. ARCHITECTURE OF SOFTWARE DEFINED RADIOA.Reconfigurable ArchitectureFig. 1 depicts a SDR transmitter and receiver with a

Reconfigurable architecture. At the transmitter, all baseband

Operations are software-based processing modules as depicted

in Fig. 1(a). The DSP software performs source encoding,

channel coding, and data stream multiplexing and modulation

as needed. Different modulation requirements are

implemented with different software modules, including

preamble sequences and hand-shaking protocols for

transmitting. Similarly, at the receiver, all reversed baseband

operations, such as demultiplexing, de-modulation, channel

decoding, source decoding are performed by software-based

processing units as depicted in the Fig. 1(b). Note that receiver

software modules also perform the signal detection andsynchronization operations in the beginning stage in order to

determine the required operational modes and standards of the

incoming waveforms as part of the receiving tasks.

B.Flexibility, Upgrade, and Backward CompatibilityThe flexibility of DSP-based solutions in the SDR systems

is due to the programmability. It provides many benefits. For

example, algorithms can continually be updated and improved

as computational methods advance. For standards-based

modules, such as IEEE 802.11, a programmable

implementation allows modules to remain compliant as the

standards upgraded. A software upgrade is usually all that is

needed while the backward compatibility still promised in theExisting Module.

Transmitter:

Fig. 1(a): Software reconfigurable transmitter block

FPGA Design and Synthesis of Reconfigurable

OFDM Transceivers for SDRS. Prabu and Dr.E. Logashanmugam

N


2/5



Receiver:

Fig. 1(b): Software reconfigurable receiver block

III. INTEROPERABILITY AND ADAPTABILITYWhile programmable devices serve as the heart of the

hardware platform within SDR systems, the software-based

transceiver enjoys its flexibility and adaptability for many

applications. For example, certified software components

compliant with 802.11 Wi-Fi standards are guaranteed to be

interoperable with other modules. Note that the

802.11standard is part of a family of IEEE standards devotedto characterize local and wide area networks. These standards

deal with the physical layer and data link layers defined by the

international standards organization open systems

interconnect(ISO-OSI). Hence, the interoperability among

different operational modes in wireless networks is achieved

via software modules with relatively low cost on development

Since the wireless channel varies with time, link adaptation is

recommended in order to support reliable communications and

maximize the throughput. For example, IEEE 802.16

and802.11n standards define a large set of modulation and

coding schemes to facilitate this goal. Moreover, adaptive

modulation schemes can be used to minimize the required

antenna sizes of the link, while still being able to transporthigh capacities. This may reduce antenna front end equipment

costs. More notably, this will also reduce continuing rise rent

costs, which are a significant portion of operating costs for

many commercial

Service providers. On the other hand, in the new

deployment With cognitive radio techniques, adaptive

modulation may be used to optimize spectrum usage, and

minimize annual frequency lease costs. As a result, efficient

and practical link adaptive techniques are needed for wireless

networks. The software modulation and demodulation

modules of DSP based architecture can be efficiently updated

and switched to meet this new design requirement. For this

reason, we focus on the software models in the following

Sections. Specifically, the M-ary phase shift keying (PSK),

and fast Fourier transform (FFT) modules of orthogonal

frequency division multiplexing (OFDM) systems [4-5] are

discussed. TheM-ary QAM (16-QAM and 64-QAM) case has

been reported in [6] and will not be presented in this paper.

The channel or the medium through which the communication

is processed is software programmed. We define the

boundary/scope of a signal detected at the receiver.

IV. QPSKAND QAMOFDMMODELSThe general analytic expression forM-ary PSK waveform

is:

Where:

The parameter E is symbol energy, is symbol timeduration. The quadrature PSK (QPSK) modulation, M=4, and

the modulation data signal shifts the phase of the

waveform The QPSK bandwidth efficiency is 2

bits/Hz.Like many digital modulation schemes, the

constellation diagram is a useful representation. In QAM, the

constellation points are usually arranged in a square grid with

equal vertical and horizontal spacing, although other

configurations are possible (e.g. Cross-QAM). Since in digital

telecommunications the data are usually binary, the number of

points in the grid is usually a power of 2 (2, 4, 8 ...). Since

QAM is usually square, some of these are rare. The most

common forms are 16-QAM, 64-QAM and 256-QAM. By

moving to a higher-order constellation, it is possible to

transmit more bits per symbol. However, if the mean energy

of the constellation is to remain the same (by way of making a

fair comparison), the points must be closer together and are

thus more susceptible to noise and other corruption. This

results in a higher bit error rate and so higher-order QAM can

deliver more data less reliably than lower-order QAM, for

constant mean constellation energy.

A.Baseband OFDM TransmitterThe OFDM transmitter can be implemented by using a

regular IFFT, but without dividing the outputs by N as

follows:

Where is the predefined data symbol from bit stream

and , n= 0,1,.....N-1.Represents the corresponding

orthogonal frequencies of theNsub-carriers. Fig. 2.a shows a

simplified OFDM transmitter block diagram. Note that the S/P

is the serial-to-parallel converter and P/S is the parallel-to

serial converter. All baseband operations are software-based

processing modules. After P/S, the digital signal stream is then

passed through the software programmed channel and

transmitted wirelessly.

B.Baseband OFDM ReceiverThe simplified receiver architecture is depicted in Fig. 2.b.

At the receiver, the received signal is down converted.

Assuming that the synchronization process has performed, the

digital sampled signal is passed through S/P, FFT

processing, P/S,and demodulation operation. The final

detected signal of the mthOFDM symbol in additive white

Gaussian noise (AWGN)channel is represented as follows.


3/5



Where

Fig.2 a. Block Diagram of Simplified OFDM Transmitter

Fig. 2. b.: Block Diagram of Simplified OFDM Receiver

C.Algorithms:i. QPSK Modulated SignalQPSK waveform is another form of angle modulation

where four output phases are possible for a single carrier

frequency. With the four different output phase possibilities,

there must also be four corresponding input conditions (00, 01,

11, 10), which enjoy for the Gray code QPSK system to

transmit twice as many data bits as the BPSK system with the

same transmission bandwidth. Two serial bits b0b1 form a

QPSK symbol. The b0 bit is used to encode the in-phase axis

I and b1 bit is used to encode the quadrature axis Q.

QPSK signal constellation with Gray coding is illustrated in

Fig. 3.

Fig. 3. QPSK Signal Constellation with Gray Coding

ii. 64-QAM modulated signal

Fig. 4: Illustration of 64-QAM modulated signal

The mapper converts input data into complex valued

constellation points, according to a given constellation. Which

constellation is to choose depends on the channel quality.

Three examples of how the constellations can be chosen

are:

1. Only one constellation is included, which is often thecase in low-end transmitters. How to choose the

included constellation is a design decision, depending

on the delay and multipath propagation situation.2. More than one constellation is included, but only one

constellation is used per OFDM frame, which is the

case in the Hiperlan/2 and IEEE 802.11a standards.

The choice of constellation, can be based on

measurements of the BER.

3. More than one constellation is included, where eachsubcarrier can use a different constellation. This is

called bit loading. Bit-loading algorithms base the

choice of constellation on the frequency response in

each subcarrier. A subcarrier with high SNR will get a

larger constellation and vice versa. Thus a flexible

transmitter must provide the user with the possibility

to use one of several constellations for each

subcarrier.

iii. IFFT and FFTAs shown in Figs 2(a) and 2(b), the IFFT and FFT [5-6] are

the most time consuming part of the base-band OFDM

processing for transmitter and receiver, respectively. Note that

the IFFT operation can be performed using the FFT operation

depicted in Fig. 5. By swapping the real and imaginary parts

of the input sequence and swapping the real and imaginary

parts of the output sequence, the FFT function is employed for

the IFFT computation. Hence, if the OFDM transceiver is

operated in time division multiplexing (TDM) mode, there isno additional hardware or software required for using the

OFDM transmitter and receiver separately. In other words,

one DSP should be able to handle both IFFT and FFT

operations if its throughput is fast enough. Due to the

simplicity, the radix-2, decimation-in-time FFT algorithm is

chosen, implemented, and used for both IFFT and FFT

operation at the transmitter and receiver, respectively. The

butterfly is the smallest computational unit and implemented

by Verilog HDL code.

Fig. 5: The FFT operation of IFFT

iv. Software ChannelThe channel used in this paper is a software defined AWGN

noise which will be introduced into the transmitted signal by

MATLAB programming and the same noise will be reduced


4/5



by the different constant constellation points depending on the

Modulating signal mapper values which are in complex valued

form.

V. EXPERIMETAL RESULTSTABLE 1

OFDMLENGTH VS PROGRAMME MEMORY FORTWIDDLE FACTORS.

N-Point FFT/IFFT

Memory Size

(16-bit word)

16 17

64 129

256 769

1024 4,097

Fig.6: simulation output result of 64-QAM at receiver

Fig. 7: simulation output result of 16-QPSK at the receiver

Fig. 8: simulation result of Transmitted 64-bit QAM signal

Fig. 9: Simulation result of 16-bit QPSK Transmitted signal

VI. APPLICATIONS TO IMAGE DATAA 256X 256 Bit image file was used to test the respective

N=64 OFDM FFT which depict the effects of AWGN on the

image quality using QPSK and QAM schemes discussed.

Total number of OFDM frames needed is 8100 for QPSK-

OFDM; As Ec/No (individual OFDM carrier energy to one-sided spectral density of additive white Gaussian noise ratio)

at the receiver increases, the image degradation improves. The

execution time can be further reduced by several approaches,

such as increased clock rate, multiple faster DSP engines,

higher level signalling modulations (16-QAM, 64-QAM),

longer length (256- or 1024-point) FFT, etc.

Fig. 10: Input image at the transmitter part for 64-bit QAM

Fig. 11: Received image at the output of receiver for 64-bit QAM

VII.CONCLUSIONThis paper presents a SDR system using OFDM transceiver.

Both the interoperability and adaptability among QPSK and

64-QAM operational modes of the OFDM systems is

discussed. Software defined antennas are implemented by

using this approach through MATLAB programming.

Adaptive modulation can be applied to this system which

minimizes the antenna sizes in physical world, while still

being able to provide high data rate. The software modulation

and demodulation modules of a DSP-based architecture can be


5/5



updated or reconfigured to meet these design requirements as

discussed in this paper.

Additionally, differences and similarities in data carrying

capabilities between QPSK- and 64-QAM of OFDM systems

and the associated clock cycles required to demodulate data

using FFT programming methods are provided. Higher

execution speed is achieved by using this method. The

tradeoff of this optimization is a larger program memory

requirement of verilog code. Programming of FFT code andthe QPSK-OFDM combination come with a resulting cost

associated with an increase in clock cycles, which must be

taken into consideration.

REFERENCES

[1] Rivet et al., A disruptive architecture dedicated to software-definedradio, IEEE Trans. Circuits Syst II: Express Briefs, vol. 55, no.4, April2008, pp. 344-348.

[2] SDR Forum, www.sdrforum.org.[3] B. Kelley, Software Defined Radio for Broadband OFDMProtocols,

Proc. IEEE Intern. Conf. Systems, Man, Cybernetics, SanAntoniou, TX,

Oct. 2009, pp. 2309-2314.

[4] H. G. Yeh and C. C. Wang, New Parallel Algorithm For MitigatingTheFrequency Offset of OFDM Systems, Proc. IEEE VTC Fall 2004,

pp.2087-2091, Sept, L.A.

[5] H. G. Yeh and V. R. Ramirez, Implementation and Performance of aM-ary PSK and QAM OFDM System in a TMS320VC5416 DigitalSignal

Processor, Proc. 2nd Intern. Conf. on Digital Communications,Santa

Clara, CA, July, 2007.[6] W. H. Chang and T. Nguyen, An OFDM -specified lossless

FFTarchitecture, IEEE Trans. Circuits Syst II: Express Briefs, vol. 53,

no.6, pp. 1235-1243, June 2006.
http://www.sdrforum.org/http://www.sdrforum.org/

fpga design and synthesis of reconfigurable ofdm transceivers for sdr

Documents