Comparative Study of OFDM Implementation Platforms

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Comparative Study of OFDMImplementation Platforms

Examiner

Prof. H. F. Ragae

Faculty of EngineeringElectronics Engineering & Electrical Comm. Department

Technical Report

Ahmed Saeed

November 20, 2010


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Contents

Introduction. History and development of OFDM. OFDM Transceiver. COFDM Transceiver.

Advantages of OFDM System. Disadvantages of OFDM System. OFDM System Design Requirements.

OFDM System Design Parameters. OFDM System Implementation.

FPGA ASIC DSP Hybrid platform

Conclusion

ReferencesComparative Study of OFDM Implementation Platforms 2

2


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Introduction

Wireless communication has gained a momentum in thelast decade of 2th century by 2G.

The primary services of 2G were all voice transmission.

The explosive demand for communication systems of alltypes of services - voice, data and video justifies the need for a communications system that utilizes the

available bandwidth efficiently and provides high data rates.

3G wireless systems, it is expected that higher mobilitywith reasonable data rate [1].

Comparative Study of OFDM Implementation Platforms 3


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Introduction

As the first 4G, WiMAX has the potential to do forbroadband what the cell phone did for voice.

It combines the performance of Wi-Fi with the QoS of a cellulartechnology.

LTE technologies designed to increase the capacity andspeed of mobile telephone networks [2].

OFDM & MIMO have emerged as the technologies ofchoice to satisfy this growth, for WiMAX & LTE as wellas Wi-Fi [2].



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Introduction

OFDM is a FDM scheme utilized as a digital multi-carriermodulation method.

A large number of closely-spaced orthogonal sub-carriers are used

to carry data.

The data is divided into several parallel data streams or channels,one for each sub-carrier.


Analogy Courtesy:www.complextoreal.com


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Introduction

Typically the subcarriers are selected such that they are allorthogonal to one another. To eliminates the interference [3,4, 7].

The operation of converting the data from serial to parallel

then modulate each subcarrier by this data is similar to IFFT. pipelined streaming algorithm has a minimum transform time

among all the FFT libraries [19].

Both FFT & IFFT will produce the same result. BUT, using

IFFT is more satisfying because we are producing a timedomain signal.

The data rate on each of the subchannels is much less than thetotal data rate.



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History and development of OFDM

OFDM has a long history of existence [1].

It is reported that OFDM based systems were in existenceduring the Second World War.

OFDM had been used by US military in several high frequencymilitary systems such as KINEPLEX, ANDEFT andKATHRYN.

In December 1966, Robert W. Chang outlined a theoreticalway to transmit simultaneous data stream trough bandlimited channel without ISI and ICI. Subsequently, heobtained the first US patent on OFDM in 1970.



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History and development of OFDM

A major breakthrough in the history of OFDM came in 1971 Weinstein and Ebert used DFT to perform baseband modulation

and demodulation.

This eliminated the need for bank of subcarrier oscillators,

more useful and efficient implementation of the system.

By the time, inclusion of FFT in OFDM system made it animportant part of telecommunications landscape.

DAB was the first commercial use of OFDM technology.

At the dawn of the 20th century, several WLAN standardsadopted OFDM on their physical layers.



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OFDM Transceiver


As mentioned before (as, for example, in [7]) the IFFT of magnitudeNrealizes an OFDM signal,

each symbol is transmitted on one of theNorthogonalfrequencies.

The input data stream is modulated by a QAM modulator,resulting in a complex symbol stream X[0],X[1], . . .,X[N 1].

This symbol stream is passed through a serial-to-parallelconverter.

Every symbol transmitted over one of the subcarriers [7].

In order to generate s(t), these frequency components areconverted into time samples by performing an IFFT .

The IFFT yields:


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OFDM Transceiver


Basic block diagram of an OFDM System [7].


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OFDM Transceiver


Then, cyclic prefix is then added to the OFDM symbol, and the resulting time samples are ordered by the parallel-to-serial

converter and passed through a D/A converter,

resulting in the baseband OFDM signal , which is then up convertedto frequency fo [7].

Experiences show that basic OFDM system is not able to obtain a BERof 105 or 106without channel coding.

Thus, all OFDM systems now-a-days are converted to Coded-OFDM(COFDM).

The benefits of COFDM [5].

the channel coding brings the robustness to burst error.

the interleaver ensures that adjacent outputs from channel encoderare placed far apart in frequency domain.


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COFDM Transceiver


Block diagram of COFDM System [6].


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Advantages of OFDM


The OFDM System has the following Advantages [1]:

Combating ISI and Reducing ICI CP helps to maintain orthogonality between the sub carriers.

Spectral Efficiency as result of orthogonality, the subcarriers can be overlapped in

frequency domain.

The beauty of OFDM lies in its simplicity.

In a relatively slow time-varying channel, it is possible to significantlyenhance the capacityby using adaptive modulation.

OFDM is more resistant to frequency selective fading thansingle carrier systems.


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Disadvantages of OFDM


The OFDM System has the following Disadvantages [1]:

Strict Synchronization Requirement

OFDM is highly sensitive to time and frequency synchronizationerrors, especially at frequency synchronization errors, everything

can go wrong.

Peak-to-Average Power Ratio(PAPR)

PAPR is proportional to the number of subcarriers.

Large PAPR of a system makes the implementation of DAC andADC to be extremely difficult.

The design of RF amplifier also becomes increasingly difficult as thePAPR increases.


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OFDM System Design Requirements


OFDM systems depend on four system requirements [1]:

1. available bandwidth

play a significant role in determining number of subcarriers.

2. bit rate,

It depends on the application the system made for.

3. delay spread, and

It determines the length of CP and so the energy loss.

4. the Doppler value.

It should be taken into account when designing a mobilestation.


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OFDM System Design Parameters


The design parameters are derived according to the systemrequirements [6]:

1. Number of subcarriers: Increasing number of subcarriers will reduce the data rate via each subcarrier, which will make sure

that the relative amount of dispersion in time caused by multipath delay will be decreased.

But when there are large numbers of subcarriers, the synchronization at the receiver side will beextremely difficult as well as PAPR.

2. Guard time (CP interval) and symbol duration: As a thumb rule, the CP interval must be two to four times larger than the Root-Mean-Square (RMS)delay spread.

Symbol duration should be much larger than the guard time to minimize the loss of SNR, but withinreasonable amount [12].

3. Subcarrier spacing: Subcarrier spacing must be kept at a level so that synchronization is achievable.

This parameter will largely depend on available bandwidth and the required number of subchannels.

4. Modulation type per subcarrier: This is trivial, because different modulation scheme will give different performance. Adaptive

modulation and bit loading may be needed depending on the performance requirement.

5. FEC coding: A suitable FEC coding will make sure that the channel is robust to all the random errors.


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OFDM System Implementation


In the past, communications systems have typically been implementedusing discrete analog components.

complicated to build and difficult to tune.

if any aspect of the n system needed to be changed, physical needed tobe replaced with new components.

DSP techniques extended the capabilities of radio systems

Initially, the ASICs were used for this task.

it incur large NRE expenses.

cannot be modified.

More recent trends have been to implement the DSP functions in a DSP

or a FPGA [13]. Both DSPs and FPGAs are programmed using abstract

programming languages without replacing or fabricating newdevices.

Most recent FPGAs contains a dedicated DSP blocks [18].

This programmability results in much lower NRE than ASICs.


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Comparison of implementation platforms [10]


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The implementation platforms must satisfy two requirements [11]: satisfy the tremendous data rate.

flexibility.

For the SoC to adapt to different operating conditions andstandards, there need to be real time conversion of mode, like in

[18], in a wireless communication protocol, but also a conversionbetween different protocols.

May be a more power efficient system with moderate data rate is theneed of the day than a system that consumes a lot of energy to give very

high data rates.

The field is totally open and one may go in any direction just carryingonly one objective to develop better and usable communication system[1].


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OFDM System Implementation-FPGA


Recent FPGAs are made of millions of logic gates with specialHW banks.

The recent FPGAs are complete and concise in supportingOFDM operation.

Moreover, there are user-friendly tools available that can beused to program the FPGAs in very short time [1]. So, it is an immense requirement to design an OFDM testbed in

FPGA.

A lot of work done to implement the OFDM system usingFPGA. Some of it used FPGA just as a validating and prototyping stage for

an ASIC.


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Qingbo Wang et. al. [12] presented an FPGA-based multi-modereconfigurable OFDM wireless communication system.

The proposed design enables the system to switch between differentcommunication modes, e.g. SISO-OFDM with QPSK or QAM-16 asmodulation schemes.

They used WARP boards as implementation platform. The FPGA board is the mother-board with a Xilinx Virtex II Pro VP70 and

extension slots.

It is used to implement all the PHY layer logic, basic MAC layer protocol, anduser level MAC protocols.

Radio and Clock boards are add-ons, which are plugged into the extensionslots on the FPGA board.

The radio board implements the analog functionalities, including D/A, A/D,and RF transceiver.

The Clock board is used to synchronize the clocks between different boards.


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they developed a C program on PPC to extend the basic MAClayer protocol.

The MAC layer software controlled the underlying hardware.

System BlockDiagram of [12]


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Experimental Setup used two PCs. They each were connected to an FPGA board through its

Ethernet port.

On one of the PCs, media player waited for incoming videopackets.

On the other PC, was configured to send video clips to thewaiting machine.

Experimentalsetup of [12]


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ExperimentalBoards

FPGA Utilization


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OFDM System Implementation-ASIC


ASIC is an integrated circuit customized for a particular use.

Little is known about suitable VLSI architectures for MIMO-OFDM systems and the corresponding silicon complexity [13].

The first commercial MIMO-OFDMA chip set was developed byIospan Wireless, Inc. in 2002 for a proprietary fixed wirelesssystem.

Several companies have announced MIMO-OFDM chip sets for theupcoming IEEE 802.11n WiFi standard.

Airgo Networks, Inc. offered a prestandard chip set earlier in 2005.

In the mobile WiMAX area (IEEE 802.16e), BeceemCommunications, Inc. has developed MIMO-OFDMA chip sets.


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In [14], D. Perels. et al. presented an ASICimplementation of the baseband signal processing unit ofa 4 4 MIMO OFDM transceiver.

The basic system architecture is based on the SISO IEEE802.11a Physical Medium Dependent layer.

In the transmitter, the high-rate data stream is first split intofour parallel lower rate streams, which are OFDM modulated

and sent through the MIMO channel.

The received signals, consisting of a superposition of alltransmitted signals, are then OFDM demodulated, spatiallyseparated and demultiplexed into a single high-rate stream.


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The design does not include forward error correctioncoding.

Moreover, one I/FFT hardware block is shared by the

four transmit (receive) antennas.


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The area requirement of the SISO system is 1.9mm2 and 12.8mm2for the MIMO-OFDM system.

Most of the SISO components have to be replicated in the MIMOcase which results in a fourfold chip area increase.

The area occupied by the I/FFT processor increases only by 50% dueto an increase in memory requirements in the MIMO case.

Layout of the MIMO-OFDM baseband ASICmanufactured in 0.25 m 1P/5M 2.5V CMOStechnology.


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OFDM System Implementation-DSP


DSP is a special-purpose CPU that provides ultra-fast instructionsequences, such as shift and add, and multiply and add, which arecommonly used in math-intensive signal processing applications.

DSPs are classified by their dynamic range, and the type of arithmetic itperforms (fixed or floating point).

In [15], H. Yan et. al. presented an implementation the acousticOFDM transmitter and receiver on a TMS320C6713 DSP board.

The TMS320C6713 DSP board contains C6713, the recent version ofthe TMS320 family of DSPs provided by Texas Instrument.

It uses VelociTI, a high-performance architecture. The C6713 core has eight independent functional units: 2 fixed-point ALUs, 4

floating-/fixed-point ALUs, and 2 multipliers. So it can execute up to eight 32-bit instructions per cycle. There are two general-purpose register files, A and B,which have 32 32-bit registers in total.


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The key components of the acoustic modem is the audio signalinput/output module.

The C6713 development board has a built-in module for samplingand generating audio signals.

The DSP based prototype for acoustic OFDMmodem.


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The OFDM modem prototype works well in the air tasting and theresults are similar to those based on Matlab implementations.

The initial implementation is to translate the Matlab programs in toC programs that can run on the DSP board.

This initial version takes on the order of minutes for the receiverprocessing. To identify the time-consuming components, theyprofiled the execution of the program.


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They proposed for future work the following options:

Single-precision (instead of double-precision) floating point operations mayspeed up the process (i.e. decreasing the number of bits from 64- to 32-bits).

The hybrid DSP/FPGA-based solution, which moves time-consuming tasks

into FPGA.

Pie-chart of the total execution time.


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OFDM System Implementation-Hybrid


The growing requirements in the marketplace for designflexibility however, are driving the need for hybridASIC/FPGA devices [16] .

DSP designers using both ASIC and FPGA within the same

design can optimize a system for performance.

Different processing elements are used for differentpurposes: The general purpose processors are fully programmable to perform

different computational tasks, but they are not energy efficient. The dedicated ASICs are optimized for power and cost. However, they

can not be reconfigured to adapt to new applications. FPGAs which are reconfigurable by nature, are good at performing bit-

level operations but not that efficient for word level DSP operations. NoC is used to connect these tiles.


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OFDM system of [17].


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Q Zhang et. al. [17] used MPSoC to implement flexibleOFDM baseband for Cognitive Radio.

An example of a heterogeneous MPSoC.


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The parameterizable OFDM part can be reconfigured to adapt todifferent OFDM systems.

The TTL reconfiguration interface is proposed to describe thedynamic behavior of applications and facilitate the implementationon an MPSoC.

An example of a heterogeneous MPSoC.


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Conclusion


The main goal was to present the implementation of any OFDMsystem, so we concentrated on several important platforms likeFPGA, ASIC, DSP, and hybrid platform.

One of the primary aims was to identify possible future scopes for

research in implementation of the OFDM system design.

The field is totally open and one may go in any direction justcarrying only one objective to develop better and usablecommunication system


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References


[1] M. Rahman , S. Das , F. Fitzek, OFDM based WLAN systems, TechnicalReport, Aalborg University, Denmark, Feb. 2005.

[2] WiMAX Forum. "Deployment of Mobile WiMAX Networks by Operators withExisting 2G & 3G Networks" 2007. online:

http://www.wimaxforum.org/technology/downloads/deployment_of_mobile_wimax.pdf

[3] J. G. Andrews, A. Ghosh, and R. Muhamed, Fundamentals of WiMAX:Understanding Broadband Wireless Networking, Prentice Hall PTR, March2007.

[4] L. Nuaymi, WiMAX: Technology for Broadband Wireless Access.WileyBlackwell, Jan. 2007.

[5] C.R. Nassar et al., Multi-carrier Technologies for Wireless Communication.Kluwer Academic Publishers, 2002.

[6] R. Van Nee and R. Prasad, OFDM for Wireless Multimedia Communications,Artech House Publishers, Jan. 2000.

[7] A. Goldsmith, Wireless Communications. Cambridge University Press, August2005.
http://www.wimaxforum.org/technology/downloads/deployment_of_mobile_wimax.pdfhttp://www.wimaxforum.org/technology/downloads/deployment_of_mobile_wimax.pdfhttp://www.wimaxforum.org/technology/downloads/deployment_of_mobile_wimax.pdfhttp://www.wimaxforum.org/technology/downloads/deployment_of_mobile_wimax.pdfhttp://www.wimaxforum.org/technology/downloads/deployment_of_mobile_wimax.pdfhttp://www.wimaxforum.org/technology/downloads/deployment_of_mobile_wimax.pdfhttp://www.wimaxforum.org/technology/downloads/deployment_of_mobile_wimax.pdfhttp://www.wimaxforum.org/technology/downloads/deployment_of_mobile_wimax.pdfhttp://www.wimaxforum.org/technology/downloads/deployment_of_mobile_wimax.pdfhttp://www.wimaxforum.org/technology/downloads/deployment_of_mobile_wimax.pdfhttp://www.wimaxforum.org/technology/downloads/deployment_of_mobile_wimax.pdfhttp://www.wimaxforum.org/technology/downloads/deployment_of_mobile_wimax.pdf


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References


[8] T. Pollet et al., BERSensitivity of OFDM Systems to Carrier Frequency Offsetand Wiener Phase Noise, IEEE Trans. Comm., vol. 43, no. 2-3-4, pp. 191193,February-April 1995.

[9] D. Aspel, "Adaptive Multilevel Quadrature Amplitude Radio Implementation inProgrammable Logic," master's thesis, Dept. of Electrical Engineering,University of Saskatchewan Saskatoon, Saskatchewan, 2004.

[10] N. VOROS and K. MASSELOS, System Level Design of Reconfigurable System-On-Chip. Springer, 2005.

[11] J. Park, H. Jung, and V. Prasanna, Efficient fpga based implementations ofmimo-ofda physical layer, in ERSA, 2006.

[12] Q. Wang, L. Zhuo, V. Prasanna, "MULTI-MODE RECONFIGURABLE OFDMCOMMUNICATION SYSTEM ON FPGA", Military and AerospaceProgrammable Logic Devices Conference (MAPLD), Sep. 2008.

[13] H. Bolcskei, Mimo-ofdm wireless systems: Basics, perspectives andchallenges, IEEE Wireless Communications, vol. 13, no. 4, pp. 3137, Aug.2006.


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References


[14] D. Perels et al., ASIC Implementation of a MIMO OFDM Transceiver for 192 Mb/s

WLANs, Euro. Solid-State Circuits Conf., Sept. 2005, pp. 21518.

[15] H. Yan, S. Zhou, Z. Shi, and B. Li, A DSP implementation of OFDM acousticmodem, in Proc. of the ACM International Workshop on Under Water Networks(WUWNet), Montreal, Quebec, Canada, September 14, 2007.

[16] Zuchowski, P.S.; Reynolds, C.B.; Grupp, R.J.; Davis, S.G.; Cremen, B.; Troxel, B.; , "Ahybrid ASIC and FPGA architecture," Computer Aided Design, 2002. ICCAD 2002.IEEE/ACM International Conference on , vol., no., pp. 187- 194, 10-14 Nov. 2002.

[17] Qiwei Zhang, Andre B.J. Kokkeler and Gerard J.M. Smit Adaptive OFDM SystemDesign For Cognitive Radio, In: 11th International OFDM Workshop 2006, Aug.Hamburg, Germany, 2006.

[18] A. Saeed et al. "Design and implementation of an Adaptive modulation system,"master's thesis, Dept. of Electronics and comm. Engineering, Helwan University,Egypt, 2010.

[19] Ahmed Saeed et. al., Efficient FPGA Implementation ofFFT/IFFT. Internationaljournal of circuits, systems and signal processing, 3(3):103110, August 2009.


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Comparative Study of OFDM Implementation Platforms