image transmission over a wireless channel

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Development and Performance Evaluation of Hierarchical Quadrature Amplitude

Modulation (HQAM) for Image Transmission over Wireless Channels

Md. Abdul Kader, Farid Ghani and R. Badlishah

School of Computer and Communication EngineeringUniversiti Malaysia Perlis (UniMAP)

Kanger, Perlis, [email protected], [email protected] and [email protected]

Abstract —Asymmetric modulation techniques like Quadrature

Amplitude Modulation system is one of the key techniques in

building a broadband mobile communication network because

of increasing shortage of wireless communication channels. It

provides alternative means of Equal Error Protection (EEP) to

the transmitted bits without increasing the bandwidth.Hierarchical Quadrature Amplitude Modulation (HQAM), a

modification of QAM, provides Unequal Error Protection

(UEP) to the transmitted bits for increasing the protection to

the sensitive and non-sensitive bits and is also efficient in

power and bandwidth. This paper presents an analysis and

simulation of HQAM in order to bring out its merits over

QAM. The simulation is carried out using MATLAB &

Simulink for different values of modulation parameter. A

Simulink-based simulation system is designed for the

transmission of gray image as test image over AWGN wireless

channel.

Keywords-Wireless Channels, HQAM, Error Protection

I.

I NTRODUCTION With the fast development of modern communication

techniques, the demand for reliable high data ratetransmission is increased significantly, which stimulate muchinterest in modulation techniques. Different modulationtechniques allow us to send different bits per symbol andthus achieve different throughputs or efficiencies. QAM isone of widely used modulation techniques because of itsefficiency in power and bandwidth [1]. This modulationtechnique assigns equal priority to all the generated bits,hence are classified as EEP method of modulation [2]. Theuse of EEP methods of modulation has the disadvantage thatit provides the same degree of protection to both thesignificant and the non-significant bits of data. Thus UEP

modulation methods are to be preferred particularly forimage data transmission, as these will allow gradual protection of the data with regard to its importance.

This paper has been proposed a simulink-based model ofthe Hierarchical Quadrature Amplitude Modulation, HQAMfor the transmission of images over wireless channels,HQAM that is a modification of QAM, is a method that

provides UEP. This is a simple and efficient approach inwhich non-uniform signal-constellation is used to givedifferent degrees of protection. The advantage of this methodis that different degrees of protection are achieved without an

increase in bandwidth in contrast to channel coding thatincreases the data rate by adding redundancy [3, 4].

To better understand the HQAM system, a MATLAB/Simulink-based simulation system is designed and shown inthis paper. Actual performance is carried out through

computer simulation using 16-HQAM technique and usinggray image as test image. In case of HQAM bit error rate isstudied for different values of the modulation parameter.

The paper has been organized as follows: In Section IIand Section III, an overview of M-ary QAM and HQAM areconsidered, respectively. 16-HQAM is briefly described inSection IV. Simulink-based simulation is discussed inSection V. Based on Simulinke-based simulation results; the

performance of HQAM is given in Section VI.

II. M -ARY QUADRATURE AMPLITUDE MODULATION

Modern modulation techniques exploit the fact thatdigital baseband data may be sent by varying both envelopeand phase/frequency of a carrier wave. Because the envelope

and phase offer two degrees of freedom, such modulationtechniques map baseband data into four or more possiblecarrier signals. Such modulation techniques are called M -arymodulation, since they can represent more signals than if justthe amplitude or phase were varied alone.

In an M -ary signaling scheme, two or more bits aregrouped together to form symbols and one of M possiblesignals is transmitted during each symbol period. Usually,the number of possible signals is M =2

n, where n is an

integer. Depending on whether the amplitude, phase, orfrequency is varied, the modulation technique is called M -aryASK, M -ary PSK, or M -ary FSK. Modulation which alters

both amplitude and phase is M -ary QAM. [1]As with many digital modulation techniques, the

constellation diagram is a useful representation. It provides a

graphical representation of the complex envelop of each possible symbol state. The constellation diagram of 16-QAMis shown in Fig. 1. The constellation consists of a squarelattice of signal points. The general form of an M -ary signalcan be defined as [12] in (1).

M iT t

t f bT

E t f a

T

E t s

s

i

s

i

s

i

,.....2,1,0

)2sin(2

)2cos(2

)(0

min0

min

=≤≤

+= π π (1)

2011 Third International Conference on Computational Intelligence, Modelling & Simulation

978-0-7695-4562-2/11 $26.00 © 2011 IEEE

DOI 10.1109/CIMSim.2011.47

227



Where E min is the energy of the signal with the lowestamplitude. ai and bi are a pair of independent integers chosenaccording to the location of the particular signal point; f 0 isthe carrier frequency; T s is the symbol period.

Figure 1. 16-QAM Constellation Diagram

The coordinates of the ith message points are min E ai and

min E biwhere (ai ,bi) is an element of the L by L matrix

given by

( )[ ]

)3,1()1,3()1,1(

)3,1()3,3()3,1(

)1,1()1,3()1,1(

,

−−+−+−−+−

−−−+−−+−

−−−+−−+−

=

L L L L L L

L L L L L L

L L L L L L

ba ii

(2)

Where M L =

It can be shown that the average probability of error in anAWGN channel for M -ary QAM, using coherent detection,can be approximated by (3) [13]

( ) ⎟⎟ ⎠

⎞⎜⎜⎝

⎛

−⎟ ⎠

⎞⎜⎝

⎛ −≅

01

3114

N M

E Q

M P av

e

(3)

Where E av / N 0 is the average signal to noise ratio.Although QAM appears to increase the efficiency of

transmission by utilizing both amplitude and phasevariations, it has a number of limitations specially in thetransmission of image data.

The first is that it is more susceptible to noise because thestates are closer together so that a lower level of noise isneeded to move the signal to a different decision point.Receivers for use with phase or frequency modulation are

both able to use limiting amplifiers that are able to removeany amplitude noise and thereby improve the noise reliance.This is not the case with QAM [5].

The second limitation is also associated with theamplitude component of the signal. When a phase orfrequency modulated signal is amplified in a transmitter,there is no need to use linear amplifiers, whereas when usingQAM that contains an amplitude component, linearity must

be maintained. Unfortunately linear amplifiers are lessefficient and consume more power, and this makes them lessattractive for mobile applications [5].

In order to overcome the limitations of QAM that provides equal error protection (EEP) in the transmitted bits,Hierarchical Quadrature Amplitude Modulation (HQAM)can be effectively used. HQAM is a modification of QAMand provides unequal error protection (UEP) in thetransmitted bits. The advantages of using Hierarchical QAMover QAM summarized below and the details are consideredin the next section.

• HQAM provides different degree of protectionwithout increase in bandwidth

• More effective transmission through noisy wirelesschannel

• Enable greater coverage area for importantinformation

• Provides basic communication in all channelconditions

• It works on the existing QAM transmission system

III.

HIERARCHICAL QUADRATURE AMPLITUDEMODULATION (HQAM)

A more spectrally efficient, dc-free modulation scheme isHierarchical Quadrature Amplitude Modulation (HQAM)[5]. Hierarchical transmission system is composed of ahierarchical source coder and the corresponding channelcoder divides the information into several layers according totheir significance, and transmits each layer with differentreliability according to the layers [6].

HQAM is another way of providing UEP to thetransmitted data bits, in which the high priority data bits(HP) of the image are mapped to the most significant bits(MSB) in the modulation constellation points while the low

priority data bits (LP) of the image are mapped to the least

significant bits (LSB) in the modulation constellation points[7-9]. Using HQAM will, therefore, result in improvedimage quality at low channel signal to noise ratio (SNR)conditions, since the highly sensitive data bits are mapped tothe MSBs of the HQAM with low bit error rate (BER). M -tuple Hierarchical Quadratic Amplitude Modulation ( M -HQAM) is an efficient modulation mode that achievesadditional compression by assigning more than one bit toeach transmission symbol. However, for the sake ofsimplicity only 16-HQAM is considered in this paper.

IV. 16-HQAM

The conventional HQAM with signal constellation size M ( M -HQAM) offers two levels of priority. HP dataoccupies the first two most significant bits (MSBs) of each

point Fig. 2(a) for 16-HQAM. Gray code labelling permitsall points belonging to the same quadrant to have the sameHP bits. This means that if the received point is de-mappederroneously to a neighbouring point but remains within itsconstellation quadrant, the HP bits will remain uncorrupted.LP data occupies the rest of the bits in the point label. For M -

point constellation, the number of LP bits in each symbol isgiven by log2 M – 2. [14]

Hierarchical 16-QAM constellation is shown in Fig. 2(a),among the four bits of each symbol, the first two bits

♦ ♦ ♦ ♦

♦ ♦ ♦ ♦

♦ ♦ ♦ ♦

♦ ♦ ♦ ♦

Q

I

228



represent the HP bits which have lower bit error rates thanthe last two bits. Last two bits are representing LP bits [10].

The modulation location of HP bits and LP bits aredepicted in Fig. 2(a) for the fixed value of signal power

(k =9). This mapping scheme is different to the traditionalgray mapping, it can be seen that the four symbols in everyquadrant have the same HP bits but different LP bits; this isalso called constellation overlapping which ensure the HP

bits to be transmitted correctly [11].

(a)

(b)

Figure 2. Constellation diagram of (a) 16-HQAM for α = 1, k =9 (b) 16-

HQAM for α = 2, k =9

In Hierarchical 16-QAM, it is possible to give the higher protection to the most important data (significant bits) bychanging the value of modulation parameter α. Where α isratio of the distance between quadrants b to the distance

between the points within a quadrant c in the constellationdiagram with b/2+c being a constant (k - maximum signal

power) Fig. 2(b) shows the constellation diagram for α=2 i.e.b=2c and k =9.

There is a limit for the maximum value of α. It must beless than the square root of the carrier power. Otherwise, theconstellation points of the same quadrant will overlap. Bymapping the high priority (HP) and the low priority (LP)data to the constellation points as shown in the Fig. 2(a), then

by varying the ratio of α one can distribute the transmitter power between the HP and LP data.

When α = 1, i.e. b=c HQAM results in QAM as can beseen from Fig. 2(a) for k =9. When α increases, the distance

between the quadrants constellation points increases. Higher portion of the transmitter power means better protection

against the channel errors than the lower portion of thetransmitter power. This is a simple unequal error protectionwithout introducing any redundancy to the modulated data.

In this UEP scheme, the performance of the HP will be

improved at the expense of LP. However, by increasing thedegree of non-uniformity, (b > c) the improvement of the HP

performance is significant, at the expense of the LP sub-channel.

V. HQAM SIMULATION MODEL

Simulink (simulation and link), developed by TheMathWorks, is an environment for multi-domain simulationand Model-based Design for dynamic and embeddedsystems. It provides an interactive graphical environment anda customizable set of block libraries that let us design,simulate, implement, and test a variety of time-varyingsystems, including communications, controls, signal

processing, video processing, and image processing [1].

With Simulink, models are built by dragging anddropping blocks from the library browser onto the graphicaleditor and connecting them with lines that establishmathematical relationships between blocks. It can be set upsimulation parameters by double clicking the blocks.

The modulation library in Communication Blockset ofSimulink contains four sub-libraries: digital basebandmodulation, analog baseband modulation, digital passbandmodulation, and analog passband modulation. For a givenmodulation technique, two ways to simulate modulationtechniques are called baseband and passband. Passbandsimulation requires higher sample rate since it contains thecarrier wave. Baseband simulation, also known as thelowpass equivalent method, requires less computation.Because baseband simulation is more prevalent [1]. This

paper focuses on baseband simulation.The baseband simulation model of 16-HQAM is given in

Fig. 3. The parameter settings for each block are given inTable 1 to Table 6.

TABLE I. PARAMETER SETTING FOR IMAGE SOURCE BLOCK

Parameter Value

File Name Lena.pgm

Sample Time inf

Image Signal One multidimentional Signal

Output Data Types Unsigned Integer (uint8)

TABLE II. PARAMETER SETTING FOR OUTPUT IMAGE VIEWER BLOCK

Parameter Value

Colourmap Matrix Gray(256)Minimum Input Value 0

Maximum Input Value 255

Axis Origin Upper Left Corner

TABLE III. USER DEFINED SETTINGS FOR PARTITIONING AND

MERGING BLOCK

Parameter Value

Data Type Conversion Decimal to Binary and Binery to Decimal

Data Partioning Higher Priority (HP) and Lower Priority (LP)

Symbol Size 4 bits

Q

♦ ♦ ♦ ♦

♦ ♦ ♦ ♦

♦ ♦ ♦ ♦

♦ ♦ ♦ ♦

I

-4.5-9 94.5

9

4.5

9

-4.5

c b

HP LP

♦ ♦ ♦ ♦

♦ ♦ ♦ ♦

♦ ♦ ♦ ♦

♦ ♦ ♦ ♦

9

3

-9

-3-3-9 93

1001

1000

1101

1100

1011

1010

1111

1110

0001

0000

0101

0100

00 11

0010

0111

0110

Q

I

229



TABLE IV. USER DEFINED SETTINGS FOR 16-HQAM MODULATION

AND DEMODULATION TECHNIQUE

Parameter ValueM-ary Number 16

Input(Mod.)/Output(Demod.) Type Integer

Constellation Ordering User-defined

Constellation Mapping [0:15]

Phase Offset 0

Modulation Parameter 1 to 5

Input(Demod.)/Output(Mod.) Type Double

TABLE V. PARAMETER SETTING FOR AWGN CHANNEL

Parameter Value

Initial seed Any positive integer

Mode Signal to Noise Ratio (SNR)

SNR (dB) 17

Input signal power (watts) 1

Symbol period 1/symbol rate

TABLE VI. PARAMETER SETTING FOR ERROR R ATE CALCULATION

Parameter Value

Receive delay 0

Computation delay 0

Computation mode Entire frame Output data Workspace Variable name Name of a variable

VI. SIMULATION R ESULTS

In Table I and II, the parameters are shown for thetransmission of gray image. Table III shows user definedsettings for partitioning and merging transmitted and

received data, respectively. Table IV considers the userdefined parameters used for the 16-HQAM technique.AWGN channel parameters have been set according to theTable V. Error rate calculation block calculates bit error ratesand the parameters are shown in Table VI.

Fig. 4 shows the Bit Error Rate (BER) vs. channel SNR(for AWGN channel) for High Priority (HP) and LowPriority (LP) data using hierarchical 16-QAM for differentvalue of modulation parameter, α = 1 to 5. It is seen that byincreasing the value of α, the BER of the HP bits decreasesand for LP bits increases. However, HP (significant) data arehighly protected for the larger value of α.

Simulation of hierarchical 16-QAM (α = 1 to 5)techniques were used to transmit the gray image via anAWGN channel. The simulation results are shown in Fig.

5(b) to 5(f). From the simulation results of the hierarchical16-QAM modulations in Fig. 5, it is easy to observe that thehierarchical modulation leads to a better quality ofreconstructed image without increasing the channel SNR. Incontrast, when applying hierarchical modulation, it is seen toyield considerable improvement in the noisy situation.

VII. CONCLUSIONS

MATLAB/Simulink is a very powerful tool that can be

used for simulation in communication, control, DSP, etc.This paper builds a simple simulation model to illustrate theHQAM techniques and how the Communication Blockset ofthe Simulink allow to implement it. The simulation modelverified the theory of HQAM and shows the suitability ofusing it from the simulation results for image transmissionover erroneous wireless channels.

R EFERENCES

[1] Xiaolong Li, “Simulink-based Simulation of Quadrature AmplitudeModulation (QAM) System”, Proceedings of IAJC-IJMEInternational Conference, USA, ISBN Paper 205, ENG105, 2008.

[2] Yoong Choon Chang; Sze Wei Lee; Komiya, R.; , "A low-complexityunequal error protection of H.264/AVC video using adaptive

hierarchical QAM," Consumer Electronics, IEEE Transactions on ,vol.52, no.4, pp.1153-1158, Nov. 2006.

[3] B.Barmada, M.M.Gandhi, E.V.Jones, M.Ghanbari, “Prioritizedtransmission of Data Partitioned H.264 Video with HQAM” IEEE,August 2005.

[4] M. Mahdi Ghandi and M. Ghanbari, "Layered H.264 videotransmission with hierarchical QAM," Elsevier J. Visual Commun.Image Representation, Special issue on H.264/AVC, vol. 17,no. 2, pp. 451 - 466, April 2006.

[5] Mirabbasi, S.; Martin, K.; , "Hierarchical QAM: a spectrally efficientdc-free modulation scheme," Communications Magazine, IEEE ,vol.38, no.11, pp. 140- 146, Nov 2000.

[6] M. Morimoto, M. Okada, S. Komaki, “Robust mobile imagetransmission using hierarchical QAM”, Signal Processing: ImageCommunication, Volume 12, Issue 2, Pages 127-134, 27 April 1998.

[7] Lee-Fang Wei, “Coded modulation with unequal error protection,”

IEEE Transactions on Communications, vol. 41, no. 10, October1993.

[8] Seamus O’Leary, “Hierarchical transmission and COFDM systems,”IEEE Transactions on Broadcasting, vol. 43, no. 2, June 1997.

[9] Chee-Siong Lee, Thoandmas Keller and Lajos Hanzo, “OFDM-BasedTurbo-Coded hierarchical and Non-Hierarchical terrestrial mobiledigital video broadcasting,” IEEE Transactions on Broadcasting,vol.46, no. 1, March 2000.

[10] L. Hanzo, W. Webb, T. Keller, “Single and multi carrier quadratureamplitude modulation: principles and applications for personalcommunications, WLANs and broadcasting”, John Wiley and Sons,2000.

[11] 3GPP TR 25.814 V7.1.0.Physical layer aspects for evolved UniversalTerrestrial Radio Access(UTRA)(Release 7)

[12] Rappaport, T. S., “Wireless Communications: Principles & Practice”,2nd edition, Prentice Hall, 2003.

[13]

Ziemer, R. E. and Peterson, R. L., “Introduction to DigitalCommunications”, Macmillan Publishing Company, 1992.

[14] Khan, M.A.; Hasan, T.; Moinuddin, A.A.; Khan, E.; , "Physical layerunequal error protection of wavelet coded video using HierarchicalQAM," Industrial Electronics & Applications (ISIEA), 2010 IEEESymposium on , vol., no., pp.217-221, 3-5 Oct. 2010.

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Figure 3. Hierarchical Quadrature Amplitude Modulation (16-HQAM) Simulation Model

Figure 4. Bit Error Rate (BER) vs. Channel SNR for High Priority (HP) and Low Priority (LP) Data Using Hierarchical 16-QAM for α = [1,…, 5]

231



(a) Original transmitted image (b) received noisy image with α =1, SNR=17 dB

(c) received noisy image with α =2, SNR=17 dB (d) received noisy image with α =3, SNR=17 dB

(e) received noisy image with α =4, SNR=17 dB (f) received noisy image with α =5, SNR=17 dB

Figure 5. Simulation results (reconstructed images) for different values of modulation parameter

232

image transmission over a wireless channel

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