
Research Article


VLSI Design of Pipelined R2MDC FFT for MIMO OFDM Transceivers


N. Kirubanandasarathy
and
K. Karthikeyan



ABSTRACT

In this study, an areaefficient low power FFT (Fast Fourier
Transform) processor is proposed for MIMOOFDM (Multi Input Multi OutputOrthogonal
Frequency Division Multiplexing) that consists of a modified architecture of
radix2 algorithm which is described as Radix2 Multipath Delay Commutation
(R2MDC). Orthogonal frequencydivision multiplexing is a popular method for
highdatarate wireless transmission. OFDM may be combined with multiple antennas
at both the access point and mobile terminal to increase diversity gain and/or
Enhance system capacity on a timevarying multi path fading channel, resulting
in a multipleinput multipleoutput OFDM system. This study described the VLSI
design of R2MDC FFT for high throughput MIMO OFDM transceivers targeted to future
wireless LAN systems. The proposed system is pipelined Radix 2 multipath delay
commutation FFT has been designed for MIMO OFDM. The MIMO OFDM transceivers
have been designed according to the proposed OFDM parameters. A lowpower efficient
and fullpipelined architecture enables the realtime operations of MIMO OFDM
transceivers. The FPGA board has been developed to verify their circuit behavior
and implementation of MIMO OFDM Transceivers.





Received: October 16, 2012;
Accepted: December 22, 2012;
Published: February 01, 2013


INTRODUCTION
MIMOOFDM is the efficient solution for transmitting and receiving the data
over the long distance. The subcarrier frequency has been chosen in our proposed
OFDM transceivers so that crosstalk between the subchannels are eliminated,
hence the inter carrier guard bands are not required. Jongren
et al. (2002) was also used such type of guard band for eliminating
the crosstalk between channels. This greatly simplifies the design of both
the transmitter and the receiver; unlike conventional FDM, a separate filter
for each subchannel is not required. The orthogonally allows for efficient
modulator and demodulator implementation using the FFT algorithm. OFDM Transceivers
is popular for wideband communications today by way of lowcost MIMO OFDM Transceivers
requires very accurate frequency synchronization between the receiver and they
have their reduced the complexity. This matter has also discussed clearly by
Bolcskei et al. (2002). In this study, a Pipelined
FFT processor is proposed for MIMOOFDM. The proposed FFT Processor is based
on radix2 multipath delay commutation. The Radix2 algorithm with MDC Architecture
is to support 4channel 8, 16, 32, 64, 128, 512, 1024 and 2048point FFT operations.
We compare this proposed architecture with existing 8point radix 2 and radix
4 FFT and also give the design and implementation results of the proposed FFF
processor. The FPGA design and implementation has been studied by Coulton
and Carline (2004) and Dick and Harris (2003).
ABOUT MIMO OFDM
The general transceiver structure of MIMO OFDM is presented in Fig.
1. The system consists of N transmitter antennas and M receiver antennas.
According to Alamouti (1998) and Kirubanandasarathy
et al. (2010), the cyclic prefix is assumed to be a longer than the
channel delay spread. The OFDM signal for each antenna is obtained by using
IFFT and can be detected by Fast Fourier Transform (FFT).
OFDM is a multicarrier system where data bits are encoded to multiple subcarriers.
Unlike single carrier systems, all the frequencies are sent simultaneously in
time. OFDM offers several advantages over single carrier system like better
multipath effect immunity, simpler channel equalization and relaxed timing acquisition
constraints. But it is more susceptible to local frequency offset and radio
frontend nonlinearity. The above discussion is fully based on Blum
et al. (2001); they have also analyzed the above said matter. The
frequencies used in OFDM system are orthogonal. Neighboring frequencies with
overlapping spectrum can therefore be used. This property is shown in the Fig.
2, where A, B, C, D and E orthogonal. This results in efficient usage of
BW. The OFDM is therefore able to provide higher data rate for the same BW.

Fig. 1: 
Architecture for MIMOOFDM 
Terry and Heiskala (2002) also gave detailed discussion
about OFDM Wireless LANs.
PROPOSED PIPELINED ARCHITECTURE FOR MIMOOFDM The radix2 multipath delay commutation (R2MDC) is one of the commutated architectures
of radix2 FFT algorithm which is used to commutate the values as fast as possible
in order to process the values and to commutate the FFT inputs, the architecture
shown in the Fig. 3 is consists of different blocks which
must be used in the R2MDC. Kirubanandasarathy et al.
(2010) was investigated Radix2 pipelined streaming FFT block, which is
used in the baseline MIMOOFDM system. But we use radix2 multipath delay commutation
in the proposed system.
One of the most straightforward approaches for pipelined implementation of
radix2 FFT algorithm is Radix2 Multipath Delay Commutator (R2MDC) architecture.
Figure 4 shows the radix2 multipath delay commutation architecture
with butterfly II structure. It is the simplest way to rearrange data for the
FFT/IFFT algorithm. Becker (2002) and Han
et al. (2005) says that the input data sequence are broken into two
parallel data stream flowing forward, with correct distance between data elements
entering the butterfly scheduled by proper delays. The 8point FFT in R2MDC
architecture is shown in Fig. 3. At each stage of this architecture
half of the data flow is delayed via the memory (Register) and processed with
the second half data stream.
The A input comes from the previous component Twiddle Factor Multipliers (TFM). The B output is fed to the next component, normally BFII. In first cycles, multiplexors direct the input data to the feedback registers until they are filled (position “0”). On next cycles, the multiplexors select the output of the adders/sub tractors (position “1”), the butterfly computes a 2point DFT with incoming data and the data stored in the feedback registers. The detailed structure of BFI is shown in Fig. 5a.
The B input comes from the previous component, BFI. The Z output fed to the
next component, normally TFM. In first cycles, multiplexors direct the input
data to the feedback registers until they are filled (position “0”).
On next cycles, the multiplexors select the output of the adders/sub tractors
(position “1”), the butterfly computes a 2point DFT with incoming
data and the data stored in the feedback registers. The multiplication by j
involves realimaginary swapping and sign inversion. The realimaginary swapping
is handled by the multiplexors MUX in efficiently and the sign inversion is
handled by switching the addingsubtracting operations by mean of MUX.

Fig. 3: 
Proposed FFT Architecture block 

Fig. 4: 
Radix2 multipath delay commutation architecture 

Fig. 5(ab): 
(a) BFI structure and (b) BF II structure 
When there is a need for multiplication by j, all multiplexors switches to
position “1”, the realimaginary data are swapped and the addingsubtracting
operations are switched. The detailed structure of BFI and BFII are shown in
Fig. 5a and b. The adders and sub tractors
in BFI and BFII are fullypipelined and followed by divideby2 and rounding.
The divideby2 is used. The algorithm used here is to commutate the radix2
algorithm in the IFFT architecture and to replace by R2MDC architecture in order
to get a low area than the existing system. Kirubanandasarathy
and Karthikeyan (2012) have discussed Radix2 pipelined streaming FFT block
versus a Radix4 FFT block without multi delay commutation.
RESULTS
The results consists of three different types of architectures that can be
implemented in the Xilinx Virtex4 Xc4vlx2512ff668 FPGA. We have designed all
coding using Hardware Description Language (HDL). To get power, and area report,
we use Xilinx ISE Design Suite 10.1 as synthesis tool and ModelSim 6.3 c for
simulation. The comparison of Radix2 FFT and Radix4 with Proposed R2MDC FFT
is shown in the Fig. 6.

Fig. 6: 
Comparison results of proposed R2MDC IFFT architecture with
existing radix2 and radix4 architecture 
The Proposed FFT gives better result than Radix2 FFT and Radix4 FFT in terms
of area and power consumption as shown in the Fig. 6. The
FPGA board has developed to verify their circuit behavior and implementation
of MIMO OFDM Transceivers.
CONCLUSION The study concludes that the proposed R2MDC architecture is taken a low area and less power than the existing radix2 and radix 4 algorithm architecture; the results are concluded that the proposed architecture is shows that it can be used in for low power applications such as MIMOOFDM transceivers.

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