DESIGN OF QAM MODULATOR AND GENERATION OF QAM SEQUENCE FOR ISI FREE COMMUNICATION Chethan B 1, Ravisimha B N 2, Dr. M Z Kurian 3

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International Journal of Computer Engineering and Applications, Volume VI, Issue I, April 14 www.ijcea.

1 International Journal of Computer Engineering and Applications, Volume VI, Issue I, April 14 ISSN DESIGN OF QAM MODULATOR AND GENERATION OF QAM SEQUENCE FOR ISI FREE COMMUNICATION Chethan B 1, Ravisimha B N 2, Dr. M Z Kurian th Semester, M.Tech(Digital Electronics), SSIT, Tumkur, Karnataka (chethan.raj.84@gmail.com) 2 Assistant Professor, Dept of ECE, SSIT, Tumkur, Karnataka 3 H.O.D, Dept of ECE, SSIT, Tumkur, Karnataka ABSTRACT: The Communication field has achieved the vast development these days. The digital communication is a fast developing domain that is achieving the new aspects every now and then. Digital communication involves an important process called the Modulation. Modulation in digital communication is called Keying. There are many different modulation techniques such as Phase Shift Keying (PSK), Differential Phase Shift Keying (DPSK), Quadrature Phase Shift Keying (QPSK), Quadrature Amplitude Modulation (QAM), Orthogonal Frequency Division Multiplexing (OFDM) etc. In this paper QAM modulation technique is discussed Digital Modulation in digital communication is known for their advantages in the cost, bandwidth, efficiency, requirement of memory for storage etc. In this paper, the design QAM modulator and the generation of QAM sequences are discussed. Keywords: Digital Communication, Digital Modulation, Keying Techniques, QPSK, QAM, QAM Modulator, QAM sequence. [1] INTRODUCTION The analog communication system is the basic communication technology from which further technologies are developed. The analog communication requires more hardware and its maintenance cost is also high [1, 2]. The Digital communication has its advantages over the analog communication in many different prospects such as bandwidth efficiency, amount of data to be exchanged, ease of Modulation, accuracy of the system etc [3]. The digital modulation process first involves the basic step of process called the Sampling where the digital signal is sampled at a fixed amount of time to get the different samples. The next step is called the Quantization process where the sampled signals are averaged to particular amplitude to form a step waveform. This Quantized Signal is now the actual analog signal in the digital form. Again the levels of the quantized signals are in binary format i.e. in 1 s and 0 s which makes it easy to represent in the memory of a system [4, 5]. After the Sampling and Quantization part comes the Modulation part. In digital communication the modulation process is called the Keying, since the carrier wave is keyed according to the message signal. Based on the parameters which are modulated according to Chethan B, Ravisimha B N and Dr. M Z Kurian 53

2 DESIGN OF QAM MODULATOR AND GENERATION OF QAM SEQUENCE FOR ISI FREE COMMUNICATION. the message signal, there are different keying techniques and the other digital modulation techniques are the combination of two or more keying techniques and other modulation techniques. The basic three Keying techniques are Amplitude Shift Keying (ASK), Frequency Shift Keying (FSK) and the Phase Shift Keying (PSK) and the other modulation techniques are just the combination of these three and other modulation techniques. Other digital modulation techniques are Differential Phase Shift Keying (DPSK), Quadrature Phase Shift Keying (QPSK), Minimum Shift Keying (MSK), Gaussian Minimum Shift Keying (GMSK), Quadrature Amplitude Modulation (QAM) or Phase Amplitude Modulation (PAM), Orthogonal Frequency Division Multiplexing (OFDM) etc [4, 5, 6]. Among these different digital modulation techniques QPSK and QAM are widely used techniques, but QAM has more advantages over the QPSK technique. Hence QAM is used in Wired and Wireless communication, Mobile communications, Radio and TV broadcasting and other such applications. [2] COMPARISON OF QPSK AND QAM The Quadrature Phase Shift Keying and the Quadrature Amplitude Modulation both are important forms of Phase shift Keying but the QAM has more advantages over QPSK in terms of bandwidth consumption, higher data carrying capacity, efficiency and storage requirements [4, 5, 6]. QAM is the combination of both the PSK and ASK. The amplitude and the phase of the carrier wave are varied in accordance to the incoming digital message wave. The QAM is displayed on a constellation diagram. The constellation diagram is a plot that displays the amplitude and phase of a particular symbol on an X-Y plane. The distance from the Origin to a particular symbol point is the amplitude of that symbol and the angle from X-axis is the phase angle of that symbol [5, 6, 7]. The main differences between QPSK and QAM are as illustrated in the [Table 1] below. QPSK It is another form of PSK where there is 90 0 phase shift among the symbols The amplitude remains constant and only phase changes It can carry only two bits per symbol per clock pulse. They are called dibits It may be extended to carry three bits It has a variant called 8-QPSK QAM It is the combination of both ASK and PSK. Both the amplitude and the phase changes It can carry 4 bits and above per clock pulse to represent a symbol It may be extended to carry 5, 6,7 and 8 bits It has variants like 32- QAM,64- QAM,128-QAM, 256-QAM 54

3 International Journal of Computer Engineering and Applications, Volume VI, Issue I, April 14 ISSN The constellation diagram is either square or circular It occupies the whole bandwidth to carry only two bits per clock pulse The constellation diagram is either square or rectangular It carries 4 bits and more bits per clock pulse in the same bandwidth and hence it is bandwidth efficient Table 1: Comparison between QPSK and QAM. [3] SYSTEM IMPLEMENTATION The generation of 16-QAM modulated waveform using Verilog involves the following steps. 1. The serial incoming wave is converted to a 4-bit binary number by serial to parallel conversion method. 2. The four bit number is mapped on to the constellation diagram by mapping them to their location in the constellation diagram. 3. Then the position of the symbol is verified with the MATLAB constellation diagram. 3.1 Serial to parallel conversion The [Figure- 1] illustrates the flowchart of the first step. For a QAM modulator the input will be the serial data bit stream, this bit stream is converted into parallel data in order to transmit the data over the channel. Here since 16-QAM is used i.e. 16=24, which means that the serial data is converted to 4-bit parallel data that can be transmitted using single clock cycle. The reset is initially set to initialize, and count is initialized to 00. Then at the rising edge of the clock the serial Initialize input is Serial shifted Input, to clock left by one position and the count is incremented. After 4 counts, when count and reaches Reset 11 then it is reset to 00 and the 4-bit parallel data are stored in parallel data register. The data is presented as output only after 4 cycles So the count flag is checked for 00 before presenting the parallel data as output else previously stored parallel data as presented If as output. Yes Count=00 Reset=1 No If rising edge of Clock Count= Count+1 & Shift the serial input by 1 position After 4 shifts Previous Parallel Data or 0000 No If Count=00 Yes Parallel Data Chethan B, Ravisimha B N and Dr. M Z Kurian 55 Output

4 DESIGN OF QAM MODULATOR AND GENERATION OF QAM SEQUENCE FOR ISI FREE COMMUNICATION. Figure- 1: Flowchart for converting serial data to parallel data. 3.2 Mapping the parallel data on to Constellation diagram A constellation diagram is a plot that gives the position of a symbol in terms of phase angle and amplitude. For a 16-QAM, the constellation diagram has 4 points in each quadrant of x-y plane. The x axis represents the real and Inphase components and the y axis represents the imaginary and Quadrature components. For each symbol there is a fixed point on the constellation diagram. In order to find the phase angle and the amplitude, an imaginary line is drawn from the origin. The angle from the x-axis is the phase of the symbol and the distance from the origin gives the amplitude of the symbol. Since the 16-QAM is both the phase modulation as well as amplitude modulation, the constellation diagram provides useful information about the phase and amplitude of a particular symbol at any point of time. The flowchart in [Figure- 2] illustrates the second step. Initialize the input, clock and reset, then if reset is high, the output will be 16 bit zeros. If reset is low then at rising edge of the clock the 4-bit parallel data is mapped to its location on constellation diagram whose position is written in terms of real and imaginary co-ordinates in the program. Initiate the input, clock and reset Initialize Real and Imaginary register Check if Reset=1 Yes Real=16 bit zeros Imag=16 bit zeros No At rising edge of the clock Map the 4-bit input ( ) to their exact location in the constellation diagram 56 Output

5 International Journal of Computer Engineering and Applications, Volume VI, Issue I, April 14 ISSN Figure- 2: Flowchart for mapping parallel data on constellation diagram. 3.3Combining the above two steps to form a 16-QAM Finally, [Figure- 4] illustrates the QAM generator block. The QAM output is generated by generating the parallel data and mapping it to constellation diagram. The programs for these are joined in the QAM program by using component instantiation generally used in structural style programming in HDL. The output is finally displayed in terms of parallel data and real and imaginary co-ordinate values. The [Figure- 5] illustrates the interior view of [Figure- 4], showing the combination of first two steps. The flowchart in [Figure- 3] illustrates the third step. The same constellation diagram can also be generated using Matlab software which has an in-built program for generating constellation diagram, in order to verify the exact location for mapping the data points on the constellation diagram. Start Initiate the input, clock and reset Convert Serial data to parallel data Temporary parallel data output Map the parallel data to its position on the constellation diagram Display the results End Chethan B, Ravisimha B N and Dr. M Z Kurian 57

DESIGN OF QAM MODULATOR AND GENERATION OF QAM SEQUENCE FOR ISI FREE COMMUNICATION. Figure- 3: Flowchart for generation of QAM and constellation diagram.

6 DESIGN OF QAM MODULATOR AND GENERATION OF QAM SEQUENCE FOR ISI FREE COMMUNICATION. Figure- 3: Flowchart for generation of QAM and constellation diagram. Figure- 4: The RTL Schematic of the QAM generator. Figure- 5: The interior RTL schematic of QAM generator [4] RESULTS The result of the first step is the generation of 4-bit parallel data out of serial input using clock, reset and count values. The second step involves the mapping of parallel data onto the constellation diagram. The results give the 4- bit data and the corresponding X and Y co-ordinate values that represent the exact values of the data point.the QAM generator is the combination of the above two results. It gives the parallel data output and the X-Y coordinate values of the data points in the constellation diagram on to which the parallel data is mapped which is illustrated in the [Figure- 6] 58

International Journal of Computer Engineering and Applications, Volume VI, Issue I, April 14 www.ijcea.com ISSN 2321 3469 Figure- 6: Modelsim results of generation of QAM and constellation diagram.

Figure- 7: Matlab results for 16-QAM Constellation. After the generation of QAM sequences, then it is transmitted through open air medium which acts as channel.

7 International Journal of Computer Engineering and Applications, Volume VI, Issue I, April 14 ISSN Figure- 6: Modelsim results of generation of QAM and constellation diagram. The Matlab representation of the constellation diagram is illustrated in [Figure- 7], which shows the exact locations of particular data points in the constellation diagram. Figure- 7: Matlab results for 16-QAM Constellation. After the generation of QAM sequences, then it is transmitted through open air medium which acts as channel. In the open air, the QAM modulated wave is gets disturbed by the noises and non linearities that make the QAM wave corrupt and the information gets lost due to corruption by noise. Sometimes the original square pulse gets de-shaped and gets combined and merged with its neighboring pulse making the data corrupt. This phenomenon is called Inter Symbol Interference (ISI) [8]. This phenomenon is eliminated at the receiver by using the filters. Chethan B, Ravisimha B N and Dr. M Z Kurian 59

8 DESIGN OF QAM MODULATOR AND GENERATION OF QAM SEQUENCE FOR ISI FREE COMMUNICATION. [6] CONCLUSION From the above discussion it is clear that QAM is a better modulation technique than QPSK technique since the QAM technique has many advantages over QPSK. Also QAM is bandwidth efficient and requires fewer resources such as cost and memory to operate. The generation of QAM involves two major steps serial to parallel data generation and the mapping of parallel data to the constellation diagram. The process of modulation is complete after the mapping on to constellation diagram. Finally when the modulated wave is transmitted through the channel the noise gets added making the data corrupt which is received and eliminated at the receiver. 60

9 International Journal of Computer Engineering and Applications, Volume VI, Issue I, April 14 ISSN REFERENCES [1] Everlong, David Cary, Communication systems, Page no [2] Micheal P Fitz, Analog Communication Theory, 2001, Page no: [3] John G.Proakis and Massoud Salehi, Digital Communications, 5th ed. New York: McGraw Hill Higher Education, [4] Analog Modulation Fundamentals, [Available online]: modulation.pdf/ [5] D.K.Sharma, A. Mishra & Rajiv Saxena, Analog & Digital modulation techniques: an overview, TECHNIA International Journal of Computing Science and Communication Technologies, VOL. 3, NO. 1, July (ISSN ), Pg No [6] Simon Haykin, Digital Communications, Chapter 6, Base band shaping for Data Transmission, pg no , 5th ed, John Wiley & sons, 2004 [7] Simon Haykin, An Introduction to Analog and Digital Communications, Chapter 6, Inter Symbol Interference and its Cures, pg no , 6th ed, John Wiley & Sons, [8] Charan Langton, Inter Symbol Interference and Root Raised Cosine filtering [Available online: Chethan B, Ravisimha B N and Dr. M Z Kurian 61

10 DESIGN OF QAM MODULATOR AND GENERATION OF QAM SEQUENCE FOR ISI FREE COMMUNICATION. Corresponding Address- Chethan B, s/o V N Basavaraju, CHETHANA, 3 rd Block, 1 st Main, 1 st Cross, Guru Layout, Kuvempunagar, Tumkur Mobile:

EC 6501 DIGITAL COMMUNICATION UNIT - IV PART A

EC 6501 DIGITAL COMMUNICATION UNIT - IV PART A 1. Distinguish coherent vs non coherent digital modulation techniques. [N/D-16] a. Coherent detection: In this method the local carrier generated at the receiver