A Paper on

VLSI IMPLEMENTATION
OF
OFDM

Presented By

M.Sai Krishna		                             G.Mohan Rao

	  

DEPARTMENT OF ELECTRONICS & COMMUNICATION ENGINEERING
CHIRALA ENGINEERING COLLEGE
CHIRALA - 523 157                                                       

INDEX:

•	Abstract
•	Introduction
•	OFDM transceiver
•	VLSI implementation
•	Design methodology
•	Algorithm survey & simulations
•	Hardware design 
•	Interfacing 
•	Clocking strategy
•	Conclusion
•	References

Abstract:
	OFDM is a multi-carrier system where data bits are encoded to multiple sub-carriers and sent simultaneously in time. The result is an optimum usage of bandwidth. A set of orthogonal sub-carriers together forms an OFDM symbol. To avoid ISI due to multi-path, successive OFDM symbols are separated by guard band. This makes the OFDM system resistant to multi-path effects. Although OFDM in theory has been in existence for a long time, recent developments in DSP and VLSI technologies have made it a feasible option.  This paper describes the VLSI implementation of OFDM in details. Specifically the 802.11a OFDM system has been considered in this paper. However, the same considerations would be helpful in implementing any OFDM system in VLSI.OFDM is fast gaining popularity in broadband standards and high-speed wireless LAN.

1. Introduction:
	OFDM is a multi-carrier system where data bits are encoded to multiple sub-carriers. Unlike single carrier systems, all the frequencies are sent simultaneously in time. OFDM offers several advantages over single carrier system like better multi-path effect immunity, simpler channel equalization and relaxed timing acquisition constraints. But it is more susceptible to local frequency offset and radio front-end non-linearity. 
	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.

2. OFDM Transceiver
	Each sub-carrier 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 sub-carrier. Modulation techniques typically used are BPSK, QPSK, 16QAM, 64QAM etc. The process of combining different sub-carriers to form a composite time-domain signal is achieved using Fast Fourier transform. Different coding schemes like block coding, convolution coding or both is used to achieve better performance in low SNR conditions. Interleaving is done which involves assigning adjacent data bits to non-adjacent bits to avoid burst errors under highly selective fading.
	                                                  Block diagram of an OFDM transceiver

3. VLSI implementation

 VLSI Implementation
In the approach shown in Figure 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 Base band. Following sections describe the VLSI based implementation in details.
4. Design Methodology 
	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 non-linearity. Since most of these algorithms are coded in "C" or tools like Mat lab, 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.

                              Design flow for Base band development 5 Architecture definition
5.1 Specifications of the OFDM transceiver
•	Data rates to be supported 
•	Range and multipath tolerance 
•	Indoor/Outdoor applications 
•	Multi-mode: 802.11a only or 802.11a+HiperLAN/2 

5.2 Design trade-offs
•	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 non-linearity’s 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 
•	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 fixed-point widths 
•	Wave shaping filter to get the desired spectrum mask 
•	Viterbi BER performance for different SNR and trace back 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 Base band circuits have a fair idea of the range of values they will work on. So a fixed-point 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.
Shown below is the loss of SNR because of decrease in the width of representation. 
Module	Width	SNR in db	
ADC	8	48	
	12	72	

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 fixed-point library. SPW also provides feature to plug-in RTL modules and do a co-simulation 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
Base band interfaces with two external modules: MAC and Radio.
7.1.1 Interface to MAC
Base band 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/Micro-wire/JTAG could be used for register programming 
7.1.2 Interface to Radio:
	Two kinds of radio interfaces are described below
•	I/Q interface 
	On the transmit side, the complex Base band signal is sent to the radio unit that first does a Quadrature modulation followed by up-conversion at 5 GHz. On the receive side, following the down-conversion to IF, Quadrature demodulation is done and complex I/Q signal is sent to Base band. Shown below is the interface.

Figure: I/Q interface to Base band
•	IF interface 
The Base band does the Quadrature modulation and demodulation digitally.

Figure: IF interface to Base band
                                                                                                                                                                            
     7.2 Clocking strategy                                                                                                                                                         
	The 802.11a supports different data rates from 6 Mbps to 54 Mbps. The clock scheme chosen for the Base band 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 Optimize usage of hardware resources by reusing different blocks
	Hardware resources can be reused considering the fact that 802.11a system is a half-duplex system. The following blocks are re-used 
•	FFT/IFFT 
•	Interleaver/De-interleaver 
•	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 
7.4 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:
7.4.1 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.
7.4.2 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. 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 Fixed-point "C" implementation of algorithms are run is highlighted.
9. References
1. ISO/IEC 8802-11 ANSI/IEEE Std 802.11-1999, Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications, IEEE, 20th August 1999
2. IEEE Std 802.11a-1999(Supplement to IEEE Std 802.11-1999), 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 "OFDM for multimedia wireless communications" by Van Nee, Richard and Ramjee Prasad
6.n Equalization Technique for Orthogonal Frequency-Division Multiplexing Systems in Time-Variant Multipath Channels, Won Gi Jeon, Kyung Hi Chang and Yong Soo Cho, IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. 47, NO. 1, JANUARY 1999