OFDM is a multicarrier system where data bits are encoded to multiple subcarriers and sent simultaneously in time. The result is an optimum usage of bandwidth. A set of orthogonal subcarriers together forms an OFDM symbol. To avoid ISI due to multipath, successive OFDM symbols are separated by guard band. This makes the OFDM system resistant to multipath effects.
VLSI implementation of OFDM modem
Aseem Pandey, Shyam Ratan Agrawalla & Shrikant Manivannan  Wipro Technologies
VLSI
implementation of OFDM modem OFDM is a multicarrier system where data bits are encoded to multiple subcarriers and sent simultaneously in time. The result is an optimum usage of bandwidth. A set of orthogonal subcarriers together forms an OFDM symbol. To avoid ISI due to multipath, successive OFDM symbols are separated by guard band. This makes the OFDM system resistant to multipath effects. 
Abstract
OFDM is a multicarrier system where data bits are encoded to multiple subcarriers and sent simultaneously in time. The result is an optimum usage of bandwidth. A set of orthogonal subcarriers together forms an OFDM symbol. To avoid ISI due to multipath, successive OFDM symbols are separated by guard band. This makes the OFDM system resistant to multipath effects.
1. Introduction
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 nonlinearities.
The frequencies used in OFDM system are orthogonal. Neighboring frequencies with overlapping spectrum can therefore be used. This property is shown in the figure where f1, f2 and f3 orthogonal. This results in efficient usage of BW. The OFDM is therefore able to provide higher data rate for the same BW.
OFDM is fast gaining popularity in broadband standards and highspeed wireless
LAN.
2. OFDM Transceiver
Each subcarrier in an OFDM system is modulated in amplitude and phase by the data bits. Depending on the kind of modulation technique used one or more bits are used to modulate each subcarrier. Modulation techniques typically used are BPSK, QPSK, 16QAM, 64QAM etc. The process of combining different subcarriers to form a composite timedomain signal is achieved using Fast Fourier transform. Different coding schemes like block coding, convolutional coding or both are used to achieve better performance in low SNR conditions. Interleaving is done which involves assigning adjacent data bits to nonadjacent bits to avoid burst errors under highly selective fading.
Block diagram of an OFDM transceiver is shown below.
Figure 1: Block diagram of the 802.11a OFDM transceiver
3. Different implementation techniques
Figure 1 shows an OFDM transciever. Following choices are available for implementing an OFDM system.
 DSP based implementation
 DSP based implementation with hardware accelerators
 VLSI implementation The pros and cons of each approach are explained in the following sections.
3.1 DSP based implementation
High performance Digital Signal Processors are widely available in the market today. The compute intensive and time critical functions that were traditionally implemented in hardware are nowadays being implemented in software running on these processors. Implementing the entire OFDM transceiver in software on DSPs is thus an option to be considered for some applications.
It has the following advantages:
 Reduced development time and quick prototyping. Quick time to market.
 Flexibility. It can quickly adapt to changing or different standards as it needs only a software change.
 Ideal for multimode Basebands where multiple standards are supported by the same device
DSP based implementation has the following disadvantages:
 Not very optimum in terms of area and power consumption
 High MIPS requirement.
The approximate MIPS requirement for different blocks in OFDM is given below
Module 
MIPS 
Viterbi decoder 
4000 
FFT 
500 
NCO 
120 
Interleaver 
150 
Channel compensation 
100 
Scrambler & others 
50 
The total MIPS requirement is 4500+. Such high CPU power is not available even with the fastest DSPs in the market today. One way out is parallel processing with multiple DSPs as shown in figure Figure 2
Figure 2: DSP solution
3.2 DSP with hardware accelerators
To overcome the MIPS limitation and yet retain the flexibility of software implementation, some blocks can be implemented in H/W. Figure 3 shows an implementation which can reduce the MIPS requirement by around 4000 MIPS.
Figure 3: DSP + H/W accelerators
3.3 VLSI implementation
Figure 4: VLSI Implementation
In the approach shown in Figure 4 the entire functionality is implemented in hardware. Following are the advantages of this approach:
 Lower gate count compared to DSP+RAM+ROM, hence lower cost
 Low power consumption
Due to the advantages mentioned above a VLSI based approach was considered for implementation of an 802.11a Baseband. Following sections describe the VLSI based implementation in details.
4. Design Methodology
The design approach for the OFDM modem is slightly different than a typical ASIC flow. Early in the development cycle, different communication and signal processing algorithms are evaluated for their performance under different conditions like noise, multipath channel and radio nonlinearity. Since most of these algorithms are coded in "C" or tools like Matlab, it is important to have a verification mechanism which ensures that the hardware implementation (RTL) is same as the "C" implementation of the algorithm. The flow is shown in the Figure 5.
Figure 5: Design flow for Baseband development 5 Architecture definition
Following points need to be considered in the architecture definition phase.
5.1 Specifications of the OFDM transceiver
 Data rates to be supported
 Range and multipath tolerance
 Indoor/Outdoor applications
 Multimode: 802.11a only or 802.11a+HiperLAN/2
5.2 Design tradeoffs
 Area  Smaller the die size lesser the chip cost
 Power  Low power crucial for battery operated mobile devices
 Ease of implementation  Easy to debug and maintain
 Customizability  Should be customizable to future standards with variations in OFDM parameters
6. Algorithm survey & simulation
The simulation at algorithmic level is to determine performance of algorithms for various nonlinearities and imperfections. The algorithms are tweaked and fine tuned to get the required performance. The following algorithms/parameters are verified
 Channel estimation and compensation for different channel models (Rayleigh, Rician, JTC, Two ray) for different delay spreads
 Correlator performance for different delay spreads and different SNR (AWGN model)
 Frequency estimation algorithm for different SNR and frequency offsets
 Compensation for Phase noise and error in Frequency offset estimation
 System tolerance for I/Q phase and amplitude imbalance
 FFT simulation to determine the optimum fixedpoint widths
 Wave shaping filter to get the desired spectrum mask
 Viterbi BER performance for different SNR and traceback length
 Determine clipping levels for efficient PA use
 Effect of ADC/DAC width on the EVM and optimum ADC/DAC width
 Receive AGC
6.1 Fixed point simulation
One of the decisions to be taken early in the design cycle is the format or representation of data. Floating point implementation results in higher hardware costs and additional circuits related with normalizing of numbers. Floating point representation is useful when dealing with data of different ranges. But this however is not true as the Baseband circuits have a fair idea of the range of values they will work on. So a fixedpoint representation will be more efficient. Further in fixed point a choice can be made between signed and 2's complement representation.
The width of representation need not be constant throughout the Baseband and it depends on the accuracy needed at different points in transmit or receive path. A small change in the number of bits in the representation could result in a significant change in the size of arithmetic circuits especially multipliers.
Module 
Width 
Gate count 
Complex Multiplier 
12 
6K 
16 
10K 

FFT (Radix4 with 3 complex multipliers) 
12 
24 K (excluding RAM) 
16 
36 K (excluding RAM) 
Shown below is the loss of SNR because of decrease in the width of representation.
Module 
Width 
SNR dB (Signal to Quantization noise ratio) 
ADC 
8 
48 
12 
72 
Simulations for different bitwidths tell us which is the optimum bitwidth that maintains the required level of accuracy. Significant area and power savings could be made if accurate estimation of fixedpoint widths is made. Simulations are performed to determine the required precision.
6.2 Simulation setup
The algorithms could be simulated in a variety of tools/languages like SPW, MATLAB, "C" or a mix of these. SPW has an exhaustive floating point and fixedpoint library. SPW also provides feature to plugin RTL modules and do a cosimulation of SPW system and Verilog. This helps in verifying the RTL implementation of algorithms against the SPW/C implementation.
7. Hardware design
7.1 Interface definition
Baseband interfaces with two external modules: MAC and Radio.
7.1.1 Interface to MAC
Baseband should support the following for MAC
 Should support transfer of data at different rates
 Transmit and receive control
 RSSI/CCA indication
 Register programming for power and frequency control
Following options are available for MAC interface:
 Serial data interface  Clock provided along with data. Clock speed changes for different data rates
 Varying data width, single speed clock  The number of data lines vary according to the data rate. The clock remains same for all rates.
 Single clock, Parallel data with ready indication  Clock speed and data width is same for all data rates. Ready signal used to indicate valid data
 Interfaces like SPI/Microwire/JTAG could be used for register programming
7.1.2 Radio
Two kinds of radio interfaces are described below
 I/Q interface
On the transmit side, the complex Baseband signal is sent to the radio unit that first does a Quadrature modulation followed by upconversion at 5 GHz. On the receive side, following the downconversion to IF, Quadrature demodulation is done and complex I/Q signal is sent to Baseband. Shown below is the interface.
Figure 6: I/Q interface to Baseband
 IF interface
The Baseband does the Quadrature modulation and demodulation digitally.
Figure 7: IF interface to Baseband
I/Q interface 
IF interface 
I/Q Phase/Amplitude imbalance is an issue as the modulation/demodulation is done in analog 
No phase imbalance as Quadrature components are produced digitally 
Two ADC/DAC channels required for I/Q 
Single ADC/DAC channel required 
Sampling frequency is lower (>BW) 
Higher sampling frequency needed (> 2BW) 
DCoffset introduced by I/Q ADC has to be compensated 
DCoffset introduced at the receiver ADC is not a problem as there is a mixing stage inside 
7.2 Clocking strategy
The 802.11a supports different data rates from 6 Mbps to 54 Mbps. The clock scheme chosen for the Baseband should be able to support all rates and also result in low power consumption. We know from our Basic ASIC design guidelines that most circuits should run at the lowest clock.
Two options are shown below:
 Above scheme requires different clock sources or a very high clock rate from which all these clocks could be generated.
 The modules must work for the highest frequency of 54 MHz.
 Shown in the previous figure is a simpler clocking scheme with only one clock speed for all data rates
 Varying duty cycles for different data rates is provided by the data enable signal
 All the circuits in the transmit and receive chain work on parallel data (4 bits)
 Overhead is the Data enable logic in all the modules
7.3 Design of crucial blocks
7.3.1 FFT
Requirement: 64 point FFT computation in 4 us as the 802.11a OFDM symbol including the guard interval is 4 us wide.
Figure 8: 64 point Radix4 FF T data flow diagram
7.3.1.1 Different architectures
 Radix4 SinglePath delay commutator
 Radix4 MultiPath delay commutator
 Radix4 SinglePath delay feedback
 Pipelined or nonpipelined
7.3.1.2 FFT storage
 Using single RAM  As only one RAM is available, large delays occur because of read and write cycles and therefore faster clock required to meet FFT time requirement of 4 us
 Using multiple storage, Data load/store happen in parallel, FFT Radix4 utilization is improved and FFT computation time is less
Figure 9: Different storage schemes for FFT
7.3.1.3 Twiddle factor complex multiplication
Comparison shown for two options
CORDIC 
4 Real Multipliers 
High latency or Pipeline delay 
Single clock multiplication can be achieved 
No twiddle ROM required 
ROM required to store the twiddle factors 
7.3.1.4 Butterfly construction
Since multipliers are the biggest block in Radix4 butterfly, designer may choose to have 1, 2 or 3 complex multiplier instances based on clock, timing and latency requirements. Shown below are both the kinds
Figure 10: Butterfly operation
Figure 11: Lower latency with three parallel multipliers
Figure 12: Single complex multiplier, higher latency (or higher clock required for same latency)
7.3.2 Viterbi
The ½ , length 7, convolutionally encoded stream is decoded using a Viterbi decoder.
Figure 13: Viterbi Construction
7.3.2.1 BMU
Branch metrics computation unit calculates the hamming distances for the incoming pair of codes from four possible codes
7.3.2.2 ACS
Add, compare and select unit is used to update the path metric for all the 64 states and select the predecessor. For each of the 64 states, it adds current path metric and branch metric for both the predecessor states and selects the lower of the two as the new path metric and the predecessor information is passed on to the SMU unit. The width of the Path metric register and the ACS adders and subtractor will change based on whether a softdecision or a harddecision viterbi is ued. It also depends on the maximum metrics accumulated by metrics registers before a normalization is done.
7.3.2.3 SMU
Survivor metrics unit can be implemented by registerexchange or traceback memory method.
Registerexchange 
Traceback memory 
Data bits for all possible paths in the trellis are stored in Flipflops 
Decision bits are stored in traceback RAM 
Low latency = Traceback length 
High latency = 4 x Traceback length 
High gate count =~ 60 K for traceback length of 64 
Low gate count =~25K + 256x64 RAM required 
High power consumption because of operation of all Flipflops 
Low power consumption 
7.3.3 NCO
NCO (Numerically controlled oscillator) is used for frequency offset correction. NCO generates sine and cosine waves that are mixed with the incoming Baseband signal to correct the frequency error. Various design parameters to be decided in NCO are given below
Figure 14: NCO
 Width of phaseaccumulator. Will decide on the accuracy or "ppm" of generated waveform
 Width of Sine and cosine outputs. Decides Quantization error. But this also decides the size of ROM used to keep the sine and cosine tables
 By using the fact the cos (q) = sin (90  q), a single LUT can be used to generate both sine and cosine values
 The need for Sine/Cosine ROM can be eliminated by using a CORDIC rotator (if the pipeline delay that the CORDIC introduces can be tolerated).
7.3.4 Arctan
The tan1 circuit is used during the estimation of the frequency error caused by local frequency PPM errors. This could be implemented as a simple LUT, which contains the Arctan values for different angles or it can be implemented by using a CORDIC circuit in vectoring mode. CORDIC is an abbreviation for Coordinate rotation digital computer. It involves performing the following equations iteratively. Let us say the complex vector is x0 + jy0 and our objective is to find z = tan1(y0/x0), it can be achieved by doing the following.
xi+1 = xi  yi*di*2i
yi+1 = yi + xi*di*2i
zi+1 = zi  di*tan1(2i).
Where
di = +1 if yi < 0, 1 otherwise
i is the iteration number and decides the accuracy of the result. As can be seen, the CORDIC circuit is simple to construct and involves only shifts, additions and subtractions.
Using LUT 
CORDIC 
Huge RAM needed for high accuracy 
Low gate count 
Low latency 
High latency because of iterative method 
CORDIC circuit is preferred as it results in a low gate count implementation.
7.4 Optimize usage of hardware resources by reusing different blocks
Hardware resources can be reused considering the fact that 802.11a system is a halfduplex system. The following blocks are reused
 FFT/IFFT
 Interleaver/Deinterleaver
 Scrambler/Descrambler
 Intermediate data buffers
Since Adders and Multipliers are costly resources, special attention should be given to reuse them. An example shown below where an Adder/Multiplier pool is created and different blocks are connected to this.
Figure 15: Sharing of H/W resources
7.5 Optimize the widely used circuits
Identify the blocks that are used at several places (several instances of the same unit) and optimize them. Optimization can be done for power and area. Some of the circuits that can be optimized are:
7.5.1 Multipliers
They are the most widely used circuits. Synthesis tools usually provide highly optimized circuits for multipliers and adders. In case optimized multipliers are not available, multipliers could be designed using different techniques like booth (Non) recoded Wallace.
7.5.2 ACS unit
There are 64 instantiations of ACS unit in the Viterbi decoder. Optimization of ACS unit results in significant savings. Custom cell design (using foundry information) for adders and comparators could be considered.
8. Debug support
 To enable debugging the hardware a serial port or a parallel port interface could be provided
 The port could be used to control the core, issue transmit and receive commands, analyzing the receive data for errors, monitoring BER etc
 Test mode support can be provided in the core to facilitate selective testing of the modules inside
9. RTL Simulations
RTL simulations are done to achieve the following objectives:
 Functional verification for all transmit and receive Baseband functions for different data rates is done
 Necessary models are written to introduce noise and channel effects. Verilog PLI interface can be used to plugin "C" models if they are available
 It is verified that different algorithmic blocks are implemented correctly in RTL, the same set of vectors used in algorithm simulations are applied to the RTL system and the outputs are compared. If simulations for algorithms are done in a tool like SPW, then this can be easily be done by importing the RTL blocks in SPW system
Figure 16: Simulation setup in SPW environment
Figure 17: Simulation setup in Verilog environment
After algorithm verification, the verilog RTL code is typically tested on a prototype board using FPGAs before fabricating the ASIC. The details of these activities are outside the scope of this paper.
10. Conclusion
In this paper, design approach for an OFDM Modem was presented. Different algorithms implemented in OFDM modem are identified.
Implementation alternatives for different components of OFDM modem were discussed. It was found during the algorithm design that many blocks need complex multipliers and adders and therefore special attention needs to be given to optimize these circuits and maximize reusability. The need for verifying the algorithms in the same environment or the same set of test vectors with which the Fixedpoint "C" implementation of algorithms are run is highlighted.
11. Acknowledgements
The authors wish to gratefully acknowledge the guidance and direction provided by Madhav Rao throughout the design of the OFDM Modem. We thank Vivek Wandile for his suggestions during the project and especially during definition of the development plan and methodology. Many thanks to Uday Ramachandran, Dilip Thakur and A. Vasudevan for providing us the required resources.
We thank Binny John and Uma Vaithy for getting us all the needed literature, especially the IEEE papers.
12. References
1. ISO/IEC 880211 ANSI/IEEE Std 802.111999, Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications, IEEE, 20th August 1999
2. IEEE Std 802.11a1999(Supplement to IEEE Std 802.111999), Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications, IEEE, September 1999
3. Digital signal Processing, J.G.Proakis, D.G Manolakis, Third Edition
4. Digital communications, Simon Haykin, John Wiley and sons
5. Very Fast Fourier Transform Algorithms Hardware for Implementation, Alvin M. Despain, IEEE transactions on computers, Vol. c28 No 5, May 1979
6. Robust Frequency and Timing Synchronization for OFDM, Timothy M. Schmidl and Donald C. Cox, IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. 45, NO. 12, DECEMBER 1997
7. A New Approach for Evaluating Clipping Distortion in Multicarrier Systems, Ahmad R.S. Bahai, Manoneet Singh, Andrea J. Goldsmith, and Burton R. Saltzberg, IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, VOL. 20, NO. 5, MAY 2002
8. "OFDM for multimedia wireless communications" by Van Nee, Richard and Ramjee Prasad
9. Performance Analysis of Viterbi Decoding for 64DAPSK and 64QAM Modulated OFDM Signals, Thomas May, Hermann Rohling, and Volker Engels, IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. 46, NO. 2, FEBRUARY 1998
10. An Equalization Technique for Orthogonal FrequencyDivision Multiplexing Systems in TimeVariant Multipath Channels, Won Gi Jeon, Kyung Hi Chang and Yong Soo Cho, IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. 47, NO. 1, JANUARY 1999
11. Optimum Nyquist Windowing for Improved OFDM Receivers, Stefan H. MullerWeinfurtner and Johannes B. Huber, Proc. of the IEEE Global Telecommunications Conference GLOBECOM 2000, San Francisco, CA, USA, pp. 711715, Nov. 2000
13. Acronyms and definitions
AGC Automatic gain control
AWGN Additive white gaussian noise
BER Bit error rate
BPSK Binary phase shift keying
BW Bandwidth
EVM Error vector magnitude
FFT Fast Fourier transform
IF Intermediate frequency
ISI Inter symbol interference
PA Power amplifier
OFDM Orthogonal frequency division multiplexing
QPSK Quadrature phase shift keying
QAM Quadrature amplitude modulation
SPW Signal processing Worksystem from Cadence
SNR Signal to noise ratio
14. About the Authors
Aseem Pandey is a senior engineer with the VLSI and Systems design division. He is currently working in a group that develops physical layers of different wireless standards.
Shyam Ratan Agrawalla is a senior engineer with the VLSI and Systems design division. He is working on the 802.11a OFDM modem development.
Shrikant Manivannan is the technical lead for the 802.11a OFDM modem program at Wipro technologies. His focus since joining Wipro has been the design of Baseband for different Wireless Standards
The content & opinions in this article are the authorâ€™s and do not necessarily represent the views of HomeToys
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