VLSI Design and Implementation of Reconfigurable

IEEE INTERNATIONAL CONFERENCE ON DEVICES, CIRCUITS & SYSTEMS (ICDCS), MARCH 2012

VLSI Design and Implementation of Reconfigurable OFDM Transceivers for Software Defined Radio P. Vamshi Krishna*, S. Prabu, E. Logashanmugam ECE Department, Hindustan University1,2, Satyabama University3 Chennai, India. [email protected], [email protected], [email protected]

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-intime fast Fourier transform (FFT) algorithms. The architecture presented is implemented using a hardware description language (verilog HDL) code. The outcome of

the implementation is a portable, scalable, quickly adaptable and reconfigurable system which supplies a high quality signal. Keywords- software-defined radio, orthogonal frequency division multiplexing, FFT, IFFT, verilog HDL

I.

INTRODUCTION

Now 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 modulation—all 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 new methodologies 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

Digital Object Identifier 10.1109/ICDCSyst2012.6188725

Digital Signal Processor [4] and FFT [5] [6]. This paper proposes a novel concept on reconfigurable OFDM transceiver 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 RADIO A. Reconfigurable Architecture Fig. 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, demodulation, 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 and synchronization 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 Compatibility The 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 the Existing Module.

IEEE INTERNATIONAL CONFERENCE ON DEVICES, CIRCUITS & SYSTEMS (ICDCS), MARCH 2012

Transmitter: Software Reconfigurable Portion Other sources

Info Source

Source Encoder

Channel Encoder

Multiplexer TDM, FDM, OFDM

Fig. 1(a): Software reconfigurable transmitter block Receiver: Software reconfigurable portion Other Destinations

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. The M-ary QAM (16QAM 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. QPSK and QAM OFDM MODELS The general analytic expression for M-ary PSK waveform is: 𝑠𝑖 𝑡 = 𝐴. cos(𝑤𝑐 𝑡 + ∅𝑖 (𝑡))

𝑖 = 0,1,2, … … . 𝑀 − 1 (1) 𝑚

Output Device

Source Decoder

Channel Decoder

De-multiplexer TDM, FDM, OFDM

Fig. 1(b): Software reconfigurable receiver block III. INTEROPERABILITY, AND ADAPTABILITY While programmable devices serve as the heart of the hardware platform within SDR systems, the softwarebased 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 802.11 standard is part of a family of IEEE standards devoted to 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 and 802.11n 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 transport high 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

Digital Object Identifier 10.1109/ICDCSyst2012.6188725

Where: 𝐴 = 2𝐸/𝑇𝑠 , ∅𝑖 = 2. 𝜋. 𝑀 , 𝑚 = 0,1,2, … … . . 𝑀 − 1 (2) The parameter E is symbol energy, 𝑇𝑠 is symbol time duration. The quadrature PSK (QPSK) modulation, M=4, and the modulation data signal shifts the phase of the waveform 𝑠𝑖 𝑡 . 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 256QAM. 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 Transmitter The OFDM transmitter can be implemented by using a regular IFFT, but without dividing the outputs by N as follows: 𝑁−1

𝑥𝑘 =

𝑑𝑛 . 𝑒

𝑗 2𝜋𝑛𝑘 𝑁

𝑘 = 0,1,2, … 𝑁 − 1

3

𝑛 =0

Where 𝑑𝑛 is the predefined data symbol from bit stream 𝑏𝑚 and 𝑒 𝑗 2𝜋𝑛𝑘 /𝑁 , n= 0,1,.....N-1. Represents the corresponding orthogonal frequencies of the N sub-carriers. Fig. 2.a shows a simplified OFDM transmitter block diagram. Note that the

IEEE INTERNATIONAL CONFERENCE ON DEVICES, CIRCUITS & SYSTEMS (ICDCS), MARCH 2012

S/P is the serial-to-parallel converter and P/S is the parallelto serial converter. All baseband operations are softwarebased processing modules. After P/S, the digital signal stream is then passed through the software programmed channel and transmitted wirelessly. B. Baseband OFDM Receiver The 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 𝑟𝑘 is passed through S/P, FFT processing, P/S, and demodulation operation. The final detected signal 𝑑𝑛 of the mth OFDM symbol in additive white Gaussian noise (AWGN) channel is represented as follows. 𝑑𝑛 =

1 𝑁

𝑁−1

𝑗 2𝜋𝑛𝑘 𝑁

𝑟𝑘 𝑒 −

Fig. 3. QPSK Signal Constellation with Gray Coding

B. 64-QAM modulated signal

, 𝑛 = 0,1, … . 𝑁 − 1 (4)

𝐾=0

Where 𝑟𝑘 = 𝑟𝑘 ,𝑚 = 𝑥𝑘 + 𝑤𝑘

𝑏𝑚

Modulation (QPSK, 64 QAM, )

𝑑𝑛

S/P

N-Point IFFT

P/S

Fig.2 a. Block Diagram of Simplified OFDM Transmitter

𝑏𝑚

𝑑𝑛 DeModulation

P/S

N-Point FFT

S/P

Fig. 4: Illustration of 64-QAM modulated signal Fig. 2. b.: Block Diagram of Simplified OFDM Reciever C. Algorithms: A. QPSK Modulated Signal QPSK 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.

Digital Object Identifier 10.1109/ICDCSyst2012.6188725

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 the case 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 each subcarrier 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.

IEEE INTERNATIONAL CONFERENCE ON DEVICES, CIRCUITS & SYSTEMS (ICDCS), MARCH 2012

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. C. IFFT and FFT As 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 is no 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, decimationin-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.6: simulation output result of 64-QAM at receiver

Fig. 5: The FFT operation of IFFT D. Software Channel The 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 by the different constant constellation points depending on the Modulating signal mapper values which are in complex valued form.

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

V. EXPERIMETAL RESULTS N-Point FFT/IFFT

Memory Size (16-bit word)

16 64 256 1024

17 129 769 4,097

Table 1: OFDM Length vs Programme Memory for Twiddle Factors.

Digital Object Identifier 10.1109/ICDCSyst2012.6188725

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

IEEE INTERNATIONAL CONFERENCE ON DEVICES, CIRCUITS & SYSTEMS (ICDCS), MARCH 2012

Fig. 9: Simulation result of 16-bit QPSK Transmitted signal VI. APPLICATIONS TO IMAGE DATA A 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 signaling modulations (16-QAM, 64-QAM), longer length (256- or 1024-point) FFT, etc.

VII. CONCLUSIONS This 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 DSPbased architecture can be 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 trade off of this optimization is a larger program memory requirement of verilog code. Programming of FFT code and the 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-defined radio,” IEEE Trans. Circuits Syst II: Express Briefs, vol. 55, no. 4, April 2008, pp. 344-348. [2] SDR Forum, www.sdrforum.org. [3] B. Kelley, “Software Defined Radio for Broadband OFDM Protocols,” Proc. IEEE Intern. Conf. Systems, Man, Cybernetics, San Antoniou, TX, Oct. 2009, pp. 2309-2314. [4] H. G. Yeh and C. C. Wang, “New Parallel Algorithm For Mitigating The Frequency Offset of OFDM Systems,” Proc. IEEE VTC Fall 2004, pp. 2087-2091, Sept, L.A.

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

[5] H. G. Yeh and V. R. Ramirez, “Implementation and Performance of a M-ary PSK and QAM OFDM System in a TMS320VC5416 Digital Signal Processor,” Proc. 2nd Intern. Conf. on Digital Communications, Santa Clara, CA, July, 2007. [6] W. H. Chang and T. Nguyen, “An OFDM-specified lossless FFT architecture,” IEEE Trans. Circuits Syst II: Express Briefs, vol. 53, no. 6, pp. 1235-1243, June 2006.

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

Digital Object Identifier 10.1109/ICDCSyst2012.6188725