VHDL-AMS Behavioral Modeling and Simulation of M-QAM ...

SETIT 2007
4 th International Conference: Sciences Of Electronic,
Technologies Of Information And Telecommunications
March 25-29, 2007 – TUNISIA
VHDL-AMS Behavioral Modeling and Simulation of
M-QAM transceiver system
Karim JABER, Ahmed FAKHFAKH and Nouri MASMOUDI
Laboratoire d’Electronique et des Technologies de l’Information
Ecole Nationale d’Ingénieurs de Sfax BP W, 3038 SFAX TUNISIE,
Tel (+216) 74 274 088 - Fax: (+216) 74 275 595
karimjaber200@yahoo.fr
a_fakhfakh@yahoo.com
Nouri.Masmoudi@enis.rnu.tn
Abstract: The size, complexity and performances of modern wireless communication circuits are getting more and more
challenging whereas marketing time and cost must be reduced as much as possible. In order to fill the current lack in
CAD tools, this work is dedicated to the mixed simulation of complete communication systems driven by digital and
digitally modulated RF signals.
This paper describes a methodology for top-down design, modeling, and simulation of complete RF system using
hardware description language VHDL-AMS (Very high speed integrated circuit Hardware Description Langage –
Analog Mixed Signal).
As application, we consider a complex M-QAM system (transmitter, channel, and receiver) and we show some details of
VHDL-AMS implementation for each elementary block (sequence of bit, mixer, oscillator, filter, M-level, etc.). Using
these behavioral blocks, we simulate our M-QAM system and evaluate system performance. We show that the results of
VHDL-AMS simulations match both Agilent ADS results and theoretical calculations.
The developed library of RF blocks is destinated to engineers who work on behavioral modeling and simulation of
complete RF systems using hardware description languages.
Key words: Modulation, digital, mixed, modelling, simulation, behavioral, VHDL-AMS, QAM, M-QAM.
INTRODUCTION
As the demand for system-on-chip (SoC)
implementations increases, the need to accurately
model mixed-signal designs becomes more important.
Digital designs have been highly automated, and the
prevalence of top-down design is very strong in this
area. In contrast, traditional analog RF designs are
normally bottom-up, starting at the transistor level.
Mixed-signal designers must then take a combination
of hierarchical design approaches, and effort is being
made to automate this design flow in a similar manner
as seen for current digital systems. The overall goal is
to provide designers tools to allow the combination of
digital and RF models at the netlist level, creating a
physical SoC model from which masks can be made
for quick prototyping and fabrication. The ability to
model and co-simulate digital and RF components
together was made possible by the creation of
hardware description languages (HDLs) such as
VHDL-AMS [IEE 99] and Verilog-A. That requires
the development of high-level behavioral models for
mixed-signal systems blocks. Later, the abstraction
levels of these models can be reduced to more
accurately model physical circuit implementations
[NOR 04].
In this paper, we concentrate on VHDL-AMS
behavioural modeling and simulation of complete RF
systems, a quadrature amplitude modulation (M-
QAM) transceiver/receiver. We present a library of
simple RF blocks that can be run in any HDL
simulator with proper language support functionality.
Any additional level of detail can be further added to
these blocks, down to the circuit level inclusively
[NIK 04]. For VHDL-AMS simulations, we use
Simplorer 6.

We try here to achieve a generic modulator, which can
be adjusted for a big variety of modulations and
compatible with needs of telecommunications today.
When the receiver exploits knowledge of the carrier’s
phase to detect the signals, the process is called
coherent demodulation/detection [SKL 98]. Figure 2
shows a block diagram of a QAM coherent
1. DESCRIPTION OF MIXED MODULATOR /
DEMODULTEUR M-QAM
1.1. Modulator M-QAM
The baseband equivalent representation, u m (t), of
the QAM signal can be expressed as
I Q
u ( t)
= ( A jA ) g(
t)
m=1,2,…..,M
(1)
m m
+
Where
m
I Q
A , A ∈ {±1∆,±3∆,...,±( M −1)∆} (∆ is a
m
m
constant whose value is determined by the average
transmitted power).
The baseband signal S m (t), which is chosen from one
of M possible signalling waveforms is given by:
s m
(t) =
Re{ u
j 2πfct
m
( t)
e }= Am
g( t)cos(2
fct
θm)
π + (2)
f c is the intermediate carrier frequency. Alternatively,
the bandpass QAM signal in (2) can be expressed
equivalently in terms of its quadrature components as:
I
Q
s m
(t) = A g( t)cos(
2πf
t)
− A g(
t)sin(
2πf
t)
(3)
m
c
This representation leads to the most common
functional representation of the QAM modulator,
which is shown in Figure 1.
Binary
sequence
(R b)
Constellation
encoder
(R)
(R)
Q
A
m
LPF
g(t)
LPF
g(t)
Figure 1: Typical QAM modulator.
First, the input binary baseband sequence with bit rate
R b bits/second is encoded into two quadrature M -level
pulse amplitude modulation (PAM) signals, each
having a symbol rate of R= R b / K symbols/second ( K
= log 2 M ). These two components of I and Q are then
filtered by pulse-shaping lowpass filters (LPFs), g(t) ,
to limit the transmission bandwidth. Finally the
quadrature signals modulate the I and Q carriers for
transmission. The transmitted bandpass signal S m (t) ,
which is the summation of all symbols represented by
the M possible signalling waveforms for QAM [PRO
95], [CHE 04].
1.2. Demodulator M-QAM
Since the information is carried in the phase and
amplitude of the modulated carrier for the QAM
signal, the receiver is assumed to be able to generate a
reference carrier whose frequency and phase are
identical to those of the carrier at the transmitter.
m
LO
90°
c
cos(2πf ct)
-sin(2πf ct)
s m (t)
demodulator.
Figure 2: Block diagram of a QAM coherent
demodulator.
At the receiver, the received high frequency signal is
first down-converted to a lower intermediate frequency
(IF) before being further processed. The demodulator
performs the majority of its work at an intermediate or
baseband frequency. The mixer in the coherent
demodulator converts the IF signal to a baseband
signal, by multiplying the incoming IF signal with a
locally generated carrier reference and the product is
passed through a lowpass filter (LPF). The LPF
removes the high-frequency components and selects
the difference component from the mixer output.
These LPFs also perform as matched filters whose
impulse responses are matched to the transmitted
signal to provide the maximum signal-to-noise ratio
(SNR) at their output. Then detectors decide which of
the possible signal waveforms was transmitted from
the output of the LPFs.
2. VHDL-AMS IMPLEMENTATION OF M-
QAM SYSTEM
Our work is focused on the development of a
complete VHDL-AMS library containing high level
descriptions of the most commun modulation and
demodulation techniques like ASK, PSK, QPSK, and
QAM. We will only focused in this paper on the
presentation of a generic M-QAM
modulator/demodulator.
A highly ideal system, modeled after the theoretical
QAM system, is implemented in VHDL-AMS. To
better manage the number of components involved in
this design, the code hierarchy in Figure 3 is
implemented. Each block contains the components
needed to perform its intended function. The QAM
system wrapper is used to initialize variables
concerning only the RF system: transmitter, and
receiver.
. Binary
sequence
Transmitter
‘M-QAM’
Channel
Figure 3: Basic M-QAM code hierarchy of VHDL-
AMS model.
Receiver
2

2.1. Transmitter M-QAM
The transmitter block diagram is shown on Figure 4.
Figure 4: Transmitter bloc diagram.
The architecture of the modulator can be separate
in several functional blocks, witch can be studied
separately:
-The block of serial to parallel: the binary sequence
(serial data) is converted to N-bit (M=2 N ).
-The generation block of M-levels (modulator i(t)/q(t))
produces the two ways In-phase i(t) and Quadraturephase
q(t). It integrates a digital to analog converter
-The modulation block produces the modulated signal.
The goal is to support a coding adapted to several
types of modulations. The coder will accept
modulations QPSK and 4-QAM until 256-QAM.
2.1.1. Serial to parallel
The binary input stream is subdivided into block of
N bits, called symbols, and each symbol is represented
by one of M=2 N pulse amplitude values.
The incoming N bits are considered in groups of N
bits. The block diagram of a M-QAM transmitter is
shown on Figure 4. The input binary data are divided
into N channels: I 0 , I 1 …. I (N/2)-1 , and Q 0 , Q 1 ,……,
Q (N/2)-1 . The bit rate in each channel is fb/N. N bits
are serially clocked into the serial-to-parallel block.
The serial to parallel block used shift register to N bits
and N latch D shown on Figure 5.
2.1.2. M-level QAM
In M-level QAM the bit data is suitably assembled
into N symbols (M=2 N ) and each symbol transmitted
by a carrier wave having a unique amplitude and
phase. The duration of each symbol determines the
bandwidth of the QAM signal. Fig.6 shows a M-level
constellation where each dot represents the position of
the phasor relative to the intersection of the axes
marked I (for in phase) and Q (for Quadrature) [HAN
90].
The phasors of the M-level constellation may be
decomposed into N/2 independent N-level AM signals
that are transmitted on quadrature components of the
same carrier.
Each AM carrier is transmitted with an amplitude of
either -(N-1)d,……, -3d, -d, d, 3d,…….., (N-1)d,
where d is the coordinate spacing shown in Fig.6.
The N-level AM components are binary encoded using
N/2 Gray coded bits for each level. For example, the
4-level AM components of 16-QAM are binary
encoded using two Gray coded bits for each level;
Gray codes 01, 00, 10 and 11, are assigned to levels
3d, d, -d and 3d, respectively [YEA 03].
Origin
Q
01
00
10
11
M-QAM (M=16)
3d
d
-d
Q
-3d d d 3d
Origin
-3d -d d 3d
11 10 00 01
I
I
-3d
Modulation QAM of 2 N states
Horloge T
Horloge NT
Shift register to N bits
Binary sequence
Latch D
Figure 5: Serial to parallel block.
I0Q0I1Q2
Figure 6: M-ary QAM constellation, M=16.
The mapping of the bits into symbols is frequently
done in accordance with the Gray code which helps to
minimize the number of bit errors occurring for every
symbol error. Because Gray-coding is given to a bit
assignment where the bit patterns in adjacent symbols
only differ by one bit [BAT 99], this code ensures that
a single symbol in error likely corresponds to a single
bit in error.
Finally, we mixed the I channel with cos(ω c t) and Q
channel with sin(ω c t). The resulting signals are then
summed and amplified by an ideal power amplifier.
3

2.2 Receiver
The receiver diagram block is depicted on Figure 7.
The signal is first amplified and then down converted
by mixing it with the sine and cosine of the carrier
frequency.
Then, the obtained signals passe through low-pass
filters, creating the reconstructed I and Q channels.
The I and Q channels output from the demodulator are
then digitized in N bit and passed to the parallel to
serial converter, where they are sampled using the
recovered symbol clock, hard limited, and output
serially from the binary sequence.
2
-2
2
-2
2
-2
2
-2
0 2u 4u 6u 8u
10u t [s]
Figure 9(a): Inphase and Quadrature for 4-
QAM(M=4).
gen_bit.val
inphase.val
quadrature.val
inp_estime.VAL
quad_estime.VAL
4
Inphase.input0
0
-4
s m (t)
LO
cos(2πf ct)
LPF
A/D
Parallel
to serial
4
0
-4
2
0
-2
Quadrature.input0
I_Filter.VAL
90°
-sin(2πf ct)
LPF
2
0
-2
5u 10u 15u 20u 25u
0 29u
t [s]
Q_Filter.VAL
Figure 7: Block diagram of VHDL-AMS M-QAM
receiver.
10
0
-10
Figure 9(b): Inphase and Quadrature for 16-
QAM (M=16).
Inphase.input0
3. SIMULATION RESULTS
The VHDL-AMS description developed for the
transmitter/receiver M-QAM system is based on the
principe detailed above.
The obtained high level model is generic and gives
the possibility to the user to choice the M-QAM
modulation technique.
Figure 8 shows four waveforms. Digital data,
inphase, quadrature and the 4-QAM modulated signal
obtained after the transmitter model simulation.
10
0
-10
5
0
-5
4
0
-4
5u 10u 15u 20u
0 26u
Quadrature.input0
I_Filter.VAL
Q_Filter.VAL
Figure 9(c): Inphase and Quadrature for 64-QAM
(M=64).
Figure 10 shows 4-QAM the constellation in the
transmitter and in the receiver (after filtering).
t [s]
gen_bit.val
2
-2
inphase.val
1.3
4_QAM
2
-2
quadrature.val
1
2
-2
signal_mod.val
0.5
0
0 1u 2u 3u 4u 5u
6u t [s]
-0.5
Figure 8: 4-QAM Transmitter.
Figure 9 (a,b,c) shows the signals I and Q in the
transmitter, I and Q channels in the receiver (after
filtering) respectively for 4 QAM, 16 QAM and 64
QAM. We can see that in all cases we receive the
same transmitted information.
-1
-1.3
0
-1.5 -1 -0.5 0.5 1 1.5
-2 2
Figure 10: Constellation 4-QAM (M=4)
4

With the generic model that we have developped and
simulated, we can draw the spectrum of the modulated
signal. Figures 11(a), (b) and (c) represent respectively
spectrums of the modulation 16 QAM, 32 QAM and
64 QAM
-0.17Meg
1.003
5.00Meg
10.00Meg
15.00Meg
20.00Meg
25.00Meg
sum1....
0.750 0.750
0.500 0.500
0.250 0.250
0 0
-0.17Meg 5.00Meg
10.00Meg
15.00Meg
20.00Meg 25.00Meg
-0.25Meg
1000.0m
5.00Meg
10.00Meg
(a)
15.00Meg
20.00Meg
25.02Meg
sum1....
750.0m 750.0m
500.0m 500.0m
250.0m 250.0m
0 0
-0.25Meg 5.00Meg
10.00Meg
15.00Meg
20.00Meg 25.02Meg
-0.24Meg
1.002
5.00Meg
10.00Meg
(b)
15.00Meg
20.00Meg
25.02Meg
sum1....
0.750 0.750
0.500 0.500
0.250 0.250
0 0
-0.24Meg 5.00Meg
10.00Meg
15.00Meg
20.00Meg 25.02Meg
We have shown that we are able to obtain a transient
response, a constellation or a spectrum of the
modulated and demodulated signals after the
simulation of our VHDL-AMS description.
All these results allow to study and to optimize the
transmitter/receiver at a high level of design which
constitutes an important step in the top-down
hierarchical design flow.
REFERENCES
[BAT 99] A. Bateman, Digital Communications: Design for
the real world, Addison-Wesley Longman Limited, New
York, NY, 1999.
[CHE 04] J. Chen, “CARRIER RECOVERY IN BURST-
MODE 16-QAM,” thesis, University of Saskatchewan, June
2004.
[HAN 90] L. Hanzo, R. Steele, and P.M. Fortune “A
Subband Coding, BCH Coding, and 16-QAM System for
Mobile Radio Speech Communications,” IEEE Transactions
on Vehicular Technologies, 1990.
[IEE 99] “VHDL Analog and Mixed-Signal Extensions:
IEEE standard 1076.1-1999,”
[NIK 04] P. Nikitin, E. Normark, C. Wakayama, and R. Shi
“VHDL-AMS modeling and simulation of a BPSK
transceiver system,” Proceedings of IEEE International
Conference on Circuits and Systems for Communications,
2004.
[NOR 04] E. Normark, L. Yang, C. Wakayama, P. Nikitin,
and R. Shi “VHDL-AMS Behavioural Modeling ad
Simulation of a π/4 DQPSK transceiver system,” 2004.
BMAS 2004. Proceedings of the Fifth IEEE International
Workshop on, 10-12 Oct. 2004.
[PRO 95] J. G. Proakis, Digital Communications,
McGraw-Hill Inc., New York, NY, 1995 (Third Edition).
[SKL 98] B. Sklar, Digital Communications:
Fundamentals and Applications, PrenticeHall Inc.,
Englewood Cliffs, NJ, 1998.
[YEA 03] Bee Leong Yeap, Choong Hin Wong, and
Lajos Hanzo, “Reduced Complexity In-Phase/Quadrature-
Phase M-QAM Turbo Equalization Using Iterative Channel
Estimation,” IEEE Transactionson Wireless
Communications, VOL. 2, NO. 1, JANUARY 2003.
(c)
Figure 11: Spectrum of ((a):16 QAM, (b):32
QAM, (c): 64 QAM)
We can see that when the number of bits increases (or
symbols), the spectral clutter increases.
4. CONCLUSION
In this paper, we described a methodology for
modelling and simulation of complete RF system using
VHDL-AMS. As a demonstration example, we
considered an M-QAM system. We simulated and
evaluated system performance and a high level of
description.
5