This chapter covers the material on project background, project objectives, project scope and project outline. Introduction on this chapter covers about the OFDM, WiMAX standard, OFDM implementation method description. The problem statement of this project also is carried out through this chapter.
In the high growth of digital communication, the increase of the need for high-speed data transmission has caused the mobile telecommunication industry faces the problem that provide the better technology which able to support various services. Many systems have been proposed but OFDM system has gained the most attention among these systems. OFDM is a promising technique in the current broadband wireless communication system for the high data rate transmission and it has the ability to against frequency selective fading. Although OFDM was first proposed and developed in mid-1960s, but it has been recognized as a better solution for high speed data communication in recent years.
Since OFDM is carried out in the digital domain, there were several applications for this system. One of the applications of the OFDM is wireless LANs based on IEEE 802.11 or Wi-Fi standard. Wi-Fi typically provides local network access for around a few hundred feet with speeds of up to 54 Mbps. On the other hand, another application of the OFDM is WiMAX based on IEEE 802. 16. WiMAX antenna is expected to have a range of up to 40 miles with speeds of 70 Mbps or more.
Wi-Fi does not guarantee any Quality of Service (QoS) but WiMAX will provide several level of QoS. Besides that, WiMAX can bring the underlying Internet connection needed to service local Wi-Fi networks. Wi-Fi does not provide ubiquitous broadband while WiMAX does. Due to the fact that WiMAX is one the latest technology for the wireless broadband networking system, WiMAX standard had been chosen as the preferences for this system design.
Several methods to implement the OFDM system include the ASIC (Application Specific Integrated Circuit), general purpose Microprocessor or Microcontroller, and FPGA (Field-Programmable Gate Array). ASIC is the fastest, smallest, and lowest power way to implement OFDM but it involved the inflexibility design process. In contrast, the general purpose Microprocessor or Microcontroller is highly programmable and flexible in term of changing the OFDM design but it needs memory to support the operation and it is the slowest in producing the output compare to other hardware.
On the contrary, FPGA combines the speed, power, and density attributes of an ASIC with the programmability of a general purpose processor. It gives advantages to the OFDM system. Therefore, FPGA is programmable and the designer has full control over the actual design implementation without the need (and delay) for any physical IC fabrication facility.
The best OFDM implementation would be FPGA since it provide the flexibility for program design and having a low cost hardware component if compared to others hardware.
Project Objective
The aim for this project is to design an OFDM transmitter that includes IFFT (Inverse Fast Fourier Transform), modulator mapping, serial to parallel, parallel to serial converter and CP (Cyclic Prefix) insertion by using hardware programming language (VHDL). Besides that, the OFDM transmitter will be done to comply with the WiMAX standard, IEEE 802.16.
The OFDM transmitter will be implemented on the FPGA board by using the Altera DE2 Board. The completion of the implementation on the FPGA will require the VHDL programming to be used for all the modules of the OFDM transmitter. In order to complete the OFDM transceiver design system, the OFDM transmitters and receiver will be implemented on two different FPGA boards with a connection wire between the boards.
Project Scope
FFT function will be focused on the design of the processing. The design also includes demodulation mapping, serial to parallel, parallel to serial and CP removal. The preferences of the WiMAX standard 802.16 are applied on each processing block. All design need to be verified separately to ensure that there is no error in the VHDL programming before it is simulated.
After all the designs had been verified to be free from errors, the second scope of this project will proceed with the implement of the design onto the FPGA board. The implementation on the FPGA board includes hardware programming and software programming.
Subsequently, the verification of the result will be done on each processing block that had been designed. In order to have the accurate result on each processing block, we need to provide the input on each block and compare the output with the function of that processing block. Besides that, the results that will be performed on the FPGA board need to be compared with software computation result by using appropriate software.
Furthermore, comparison of the output of PFDM receiver and input of the OFDM transmitter will be carried out. A result that shows the output of OFDM receiver is the same as the input of the OFDM transmitter will be produced.
Project Outline
This project is divided into six chapters, which consists of introduction, literature review, methodology, hardware design, software design, result, analysis and discussion, and conclusion.
Chapter 1 is the introduction part where it discusses the basic idea of this project that included the project background, project objective, and project scope.
Chapter 2 shows the literature review of the OFDM system and WiMAX, it includes the comparison of WiMAX and Wi-Fi, basic principles of OFDM system, advantages and disadvantages of OFDM system, as well as the application of the OFDM.
Chapter 3 is the methodology of this project. The project workflow consists of design process, implementation, test and analysis. The tools and design languages consists of altera quartus II, C/C++, VHDL and altera DE2 board.
Chapter 2
LITERATURE REVIEW
WiMAX Technology
WiMAX or Worldwide Interoperability of Microwave Access is a telecommunication trade name related to the IEEE 802.16 standard. It is ratified by WiMAX Forum, a private organization formed in June 2001 and independent of IEEE in supporting certification of compliance to IEEE 802.16 Standard.
IEEE 802.16 Standard
802.16 is a group of broadband wireless communication standards for metropolitan area networks. 802.16-2001 published in December 2001 is the first standard of this group which specified point-to-multipoint system operating in the physical layer of 10-66 GHz. Newer revision, 802.16-2004 added the spectrum in 2-11 GHz range. The current version of IEEE 802.16 Standard is IEEE 802.16-2004, as amended by IEEE 802.16e and 802.16f, introduced support for the network connection of mobile devices.
Comparison of WiMAX with Wi-Fi
Both WiMAX and Wi-Fi are wireless connectivity, thus this cause some confusion frequently. Wi-Fi is a trademark of Wi-Fi Alliance that related to IEEE 802.11 Standard. The IEEE 802.11 is wireless LAN standards that enable mobile devices connect to internet when the wireless network is connected to the internet. The current version of the standard is IEEE 802.11-2007.
WiMAX is a long range (in kilometres) system where its MAC is full QoS and connection-oriented while Wi-Fi is a shorter range system (in meters) where its MAC use CSMA (Carrier Sense Multiple Access), which is connectionless. WiMAX end users must be connected via a base station which is centralized controlled while Wi-Fi enables peer-to-peer networking between end user devices without an access point.
Characteristic of OFDM
Orthogonal Frequency Division Multiplexing (OFDM) is a multi-carrier transmission technique, which divides the available spectrum into many orthogonal sub-carriers. Each sub-carrier is individually modulated by using conventional modulation scheme like phase shift keying (PSK) or quadrature amplitude modulation (QAM) in a small bandwidth range. In OFDM, the signal is split into independent channels at first, modulated and then being multiplexed to form OFDM carriers.
Orthogonality
The main concept of OFDM that differentiate it among others is the orthogonality of the sub-carriers. The orthogonality allows simultaneous transmission on a lot of sub-carries in a tight frequency space without interference from each other. Figure 2.1(a) below shows the individual sub-carrier spectrum which will be multiplexed orthogonally with other sub-carriers to form an OFDM symbol in Figure 2.1(b).
(a) (b)
Figure 2.1 Spectra of (a) an OFDM sub-carrier and (b) an OFDM signal
Figure 2 shows that there is zero interference or no crosstalk at centre frequency for each overlapping sub-carriers. This is possible as there is zero integral from the view of mathematical practice:
Hence, the sub-carriers can be orthogonally overlapped without the worry of interference, and can be packed as tight as theoretically possible.
Generation of OFDM Signal
In the transmitter, the data is first digitally modulated by using conventional modulation scheme such as PSK and QAM. Then all individual modulated carriers shall be sum up or multiplexed to form an OFDM signal, which is done by a block called the IFFT (Inverse Fast Fourier Transform).
The IFFT is a mathematical concept that will convert the signal from frequency domain where it is expressed as the phase or amplitude of particular frequency into time domain, as shown in Figure 2.2 below.
The frequency of the sub-carriers is chosen such that the original data signal can be gained back without loss or distortion at the receiver side. Hence, both transmitter and receiver must be perfectly synchronized in term of frequency and time-scale transmission.
Figure 2.2 Frequency domain signal comes out of an IFFT as time domain signal
Guard Period and Cyclic Prefix
OFDM is said to be a perfect method for multi-carriers transmission system as it reduce the crucial problems of other technique. Multipath delay spread is one of them. The lower rate parallel transmissions of OFDM subcarriers that pack them in a tight frequency reduce the relative delay spread.
Besides, the inter-symbol interference (ISI) is also eliminated by introducing a guard period between transmitted symbols. The guard period chosen is larger than the expected delay spread. The guard period is important to avoid the noise caused by previous symbol into the front of next symbol. However, it is not a good practice to leave blank for the guard period, as the problem of inter-carrier interference (ICI) would arise. Hence, the cyclic prefix is used.
The OFDM is cyclically extended into the guard time, by copy the end of symbol to the front, as shown in Figure 2.3 below. After transmitted, this copied part will be removed, to obtain the original periodic signal before pass through the FFT core in the receiver side.
Although the OFDM has many sub-carriers, we no need to add cyclic prefix for each carriers. In practice, the OFDM signal is a continuous collection of periodic signal. We can add cyclic prefix just once to OFDM signal, where around 10% to 25% of the symbol time.
Figure 2.3 Illustration of adding the cyclic prefix to the front of symbol
However, the additions of cyclic prefix which reduce the interference impose an increase in bandwidth.
Advantages of OFDM
OFDM is widely used in recent digital communication such as the digital video and audio broadcasting, wireless networking and broadband internet access. This happens because OFDM offers many unique benefits among other modulation technique applied in wireless system. Next are some notable benefits of OFDM.
Bandwidth Efficiency
As the technology develops, the demand for transferring more data over a connectivity process is arises. Hence, the idea of multipath transmission is implemented. However, this multipath carriers system imposes the issues of bandwidth efficiency.
In OFDM, the frequency band is split into lower rate parallel sub-carriers that are orthogonal to each other, such that they can be separated out without interference with adjacent carriers at receiver side. In this orthogonality process, all sub-carriers can be arranged as closer as possible for more efficient use of the spectrum than simple frequency division multiplexing (FDM). As illustrated in Figure 2.4, OFDM save up to 50% of the total bandwidth compare to the wasted spacing in FDM.
FDM
OFDM
Figure 2.4 OFDM versus FDM in bandwidth efficiency
Ideally Zero ISI
The limitation of high bit rate transmission is the effect of inter-symbol interference. As the transfer speed is increased with the technology improvement, the data transmission time is reduced. However, this will cause multipath delay spread that in turn cause the happen of ISI. Luckily, this is not a problem for OFDM technique.
As fore mentioned, OFDM is a collection of low-frequency parallel sub-carriers that is orthogonal to each other. This implies a long symbol duration that leads to minimum ISI. Moreover, OFDM is mathematically zero ISI due to the orthogonality and the adding of cyclic prefix.
Minimize the Effect of Frequency Selective Fading
By dividing the carrier signal into narrowband parallel sub-carriers, OFDM is more resistant to frequency selective fading that single carrier system. Channel equalization become simpler than single carrier system too.
The Problem of OFDM
Although the OFDM offers many benefits, it shows some weaknesses that should be concern for improvement or implementation.
Peak-to-Mean Power Ratio
The OFDM signal has a drawback where the amplitude varies in a large dynamic range. It is crucial to maintain the amplitude as they describe the information carried. Therefore, it requires a power amplifier with high peak-to-mean power ration to avoid distorting the peak. The result is a linear amplifier with a high bias current resulting in poor power efficiency.
Sensitive to Frequency Synchronization Errors
The OFDM is very sensitive to the carrier frequency offset and drift between the transmitter and the receiver. In the mobile connectivity, the relative movement between transmitter and receiver will suffer the effect of Doppler shift. Moreover, the time and frequency offset of carriers can never be perfectly synchronized for an air transfer. These synchronization errors distort the orthogonality of OFDM sub-carriers and thus cause the inter-carrier interference (ICI) to happen.
To overcome this performance drawback, a known pilot tone is embedded in the transmitted OFDM signal for tracking of the start of signal at receiver or an tracking algorithm is attached in the cyclic extension.
Chapter 3
METHODOLOGY
This chapter covers the project workflow, tools and design languages used in this project. Methodology on this chapter covers the method that used on this project include the process of design and implementation. Besides that, all the tools and design languages been used throughout this project also cover in this chapter.
Project Workflow
The workflow of this project divided into three main stages which are design process, implementation and test and analysis. The project starts with the design process, then follow by implementation. Lastly, test and analysis process will be performed.
Design Process
This project starts with the design process which consists of IFFT, modulation mapping, serial to parallel, parallel to serial and CP insertion. The VHDL programming languages is used in these processing block design. Besides that, the preferences of the WiMAX standard 802.16 will be apply on each processing block.
After that, the design verification will be performed to confirm the design is free from error. The functional and timing simulations are the two different type of design verification that to be performed in this project after the design was done. The functional simulation is the simulation that shown the hardware function clearly. But, the functional simulation doesn't show in this design because we didn't concentrate on the software part. On the other hand, the timing simulation is the simulation that shown the timing function for the designed hardware.
Implementation (Software and Hardware Programming)
After the design process had done, we will proceed to the next stage which is the implementation stage. On this stage, the design will be implemented on the FPGA board by using the Altera Quartus II to program the FPGA board.
This is the hardware programming that used the VHDL programming to implement on the FPGA board. On the other hand, the software programming also was carry out by creating the test vector program in C.
Test and Analysis
On the next stage which is the final stage, the design proceeds to the test and analysis. When perform the test and analysis process, the result that will be performed on the FPGA board need to be compared with software computation result by using appropriate software. The comparison is to ensure that the design module is work correctly as on other software too. If the comparison result doesn't match or equal, the design was confirm to have error or the design was wrong. The design should be check again or else it needs to be performed again from the early stage, design process.
Tools and Design Languages
There were several tools and software that we used for this project design which including the software used, programming languages, hardware tools. In this part, it had included Altera Quartus II, C/C++, VHDL, and Altera DE2 board.
Altera Quartus II
Altera Quartus II is a software tool used for analysis and synthesis of HDL designs, which enables the developer to compile the design, perform the timing analysis and configure the target device with the programmer.
The Quartus II software is a fully integrated, architecture-independent package for designing logic with Altera® programmable logic devices, including ACEX® 1K, APEX™ 20K, APEX 20KC, APEX 20KE, APEX™ II, ARM® based Excalibur™, Cyclone™, FLEX® 6000, FLEX 10K®, FLEX 10KA, FLEX 10KE, MAX® 3000A, MAX 7000AE, MAX 7000B, MAX 7000S, Mercury™, Stratix, and Stratix™ GX devices.
The Quartus II software provide a full spectrum of logic design capabilities such as design entry using schematics, block diagrams, AHDL, VHDL, and Verilog HDL, functional and timing simulation, timing analysis, combined compilation and software projects, device programming and verification and etc. The Quartus II software also able to reads standard EDIF netlist files, VHDL netlist files, and Verilog HDL netlist files, and generates VHDL and Verilog HDL netlist files.
In this project design, the main software used is Altera Quartus II which used as the hardware design entry compilation and simulation tool for the FPGA implementation. Altera Quartus II is the main interval for us to implement the design from the VHDL programming code onto the FPGA board.
C/C++
C/C++ programming language is the languages which used to perform the test and analysis stages. By using appropriate software like Matlab, the C/C++ programming language used to check the design can be work correctly on the other software too.
VHDL
VHDL stand for VHSIC (very-high-speed integrated circuits) hardware description language. VHDL is a hardware description language used in electronic design automation to describe digital and mixed-signal systems such as field-programmable gate arrays and integrated circuits.
VHDL can be refer as the combination of many languages which includes sequential, concurrent, netlist, timing specification, waveform generation languages. Therefore the VHDL language that had constructed that enables to express the concurrent or sequential behaviour of a digital system with or without timing. It allowed modelling of the system as an interconnection to the components. Test waveforms can be generated by using the same constructs. All the constructs above can be combined into a single model. Therefore, it can provide a comprehensive description of the system.
In this project design, VHDL is the main hardware programming language that used to design the OFDM system. All the processing block of the design will be written in the VHDL programming code.
Altera DE2 Board
In this project design, we used the Altera DE2 board as our FPGA board for us to implement the design. On the Alter DE2 board, there were several components and interfaces that can be seen on the board such as Altera Cyclone II 2C35 FPGA with 35000 Les, Altera Serial Configuration devices (EPCS16) for Cyclone II 2C35, USB Blaster built in on board for programming and user API controlling, 1Mbyte Flash Memory (upgradeable to 4Mbyte), 8Mbyte (1M x 4 x 16) SDRAM, 9 Green User LEDs and etc.
Cyclone II FPGAs extend the low-cost FPGA density range to 68,416 logic elements (LEs) and provide up to 622 usable I/O pins and up to 1.1 Megabits of embedded memory. Cyclone II devices can support complex digital systems on a single chip at a cost that rivals that of ASICs. Cyclone II offers 60% higher performance and half the power consumption of competing 90-nm FPGAs. Therefore it is a low-cost, high-performance embedded process system.
