VHDL-AMS Behavioral Modeling and Simulation of M-QAM transceiver system

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1 SETT 7 th nternational Conference: Sciences Of Electronic, Technologies Of nforation And Telecounications March 5-9, 7 TUNSA VHDL-AMS Behavioral Modeling and Siulation of M-AM transceiver syste Kari JABER, Ahed FAKHFAKH and Nouri MASMOUD Laboratoire d Electronique et des Technologies de l nforation Ecole Nationale d ngénieurs de Sfax BP W, 338 SFAX TUNSE, Tel (+16) Fax: (+16) karijaber@yahoo.fr a_fakhfakh@yahoo.co Nouri.Masoudi@enis.rnu.tn Abstract: The size, coplexity and perforances of odern wireless counication circuits are getting ore and ore challenging whereas arketing tie and cost ust be reduced as uch as possible. n order to fill the current lack in CAD tools, this work is dedicated to the ixed siulation of coplete counication systes driven by digital and digitally odulated RF signals. This paper describes a ethodology for top-down design, odeling, and siulation of coplete RF syste using hardware description language VHDL-AMS (Very high speed integrated circuit Hardware Description Langage Analog Mixed Signal). As application, we consider a coplex M-AM syste (transitter, channel, and receiver) and we show soe details of VHDL-AMS ipleentation for each eleentary block (sequence of bit, ixer, oscillator, filter, M-level, etc.). Using these behavioral blocks, we siulate our M-AM syste and evaluate syste perforance. We show that the results of VHDL-AMS siulations atch both Agilent ADS results and theoretical calculations. The developed library of RF blocks is destinated to engineers who work on behavioral odeling and siulation of coplete RF systes using hardware description languages. Key words: Modulation, digital, ixed, odelling, siulation, behavioral, VHDL-AMS, AM, M-AM. NTRODUCTON As the deand for syste-on-chip (SoC) ipleentations increases, the need to accurately odel ixed-signal designs becoes ore iportant. Digital designs have been highly autoated, and the prevalence of top-down design is very strong in this area. n contrast, traditional analog RF designs are norally botto-up, starting at the transistor level. Mixed-signal designers ust then take a cobination of hierarchical design approaches, and effort is being ade to autoate this design flow in a siilar anner as seen for current digital systes. The overall goal is to provide designers tools to allow the cobination of digital and RF odels at the netlist level, creating a physical SoC odel fro which asks can be ade for quick prototyping and fabrication. The ability to odel and co-siulate digital and RF coponents together was ade possible by the creation of hardware description languages (HDLs) such as VHDL-AMS [EE 99] and Verilog-A. That requires the developent of high-level behavioral odels for ixed-signal systes blocks. Later, the abstraction levels of these odels can be reduced to ore accurately odel physical circuit ipleentations [NOR ]. n this paper, we concentrate on VHDL-AMS behavioural odeling and siulation of coplete RF systes, a quadrature aplitude odulation (M- AM) transceiver/receiver. We present a library of siple RF blocks that can be run in any HDL siulator with proper language support functionality. Any additional level of detail can be further added to these blocks, down to the circuit level inclusively [NK ]. For VHDL-AMS siulations, we use Siplorer 6.

2 We try here to achieve a generic odulator, which can be adjusted for a big variety of odulations and copatible with needs of telecounications today. When the receiver exploits knowledge of the carrier s phase to detect the signals, the process is called coherent deodulation/detection [SKL 98]. Figure shows a block diagra of a AM coherent 1. DESCRPTON OF MXED MODULATOR / DEMODULTEUR M-AM 1.1. Modulator M-AM The baseband equivalent representation, u (, of the AM signal can be expressed as u ( = ( A ja ) g( =1,,..,M (1) + Where A, A {±1,±3,...,±( M 1) } ( is a constant whose value is deterined by the average transitted power). The baseband signal S (, which is chosen fro one of M possible signalling wavefors is given by: s ( = Re{ u j πfct ( e }= A g( cos( fct θ) π + () f c is the interediate carrier frequency. Alternatively, the bandpass AM signal in () can be expressed equivalently in ters of its quadrature coponents as: s ( = A g( cos( πf A g( sin( πf (3) c This representation leads to the ost coon functional representation of the AM odulator, which is shown in Figure 1. Binary sequence (R b) Constellation encoder (R) (R) A g( g( Figure 1: Typical AM odulator. First, the input binary baseband sequence with bit rate R b bits/second is encoded into two quadrature M -level pulse aplitude odulation (PAM) signals, each having a sybol rate of R= R b / K sybols/second ( K = log M ). These two coponents of and are then filtered by pulse-shaping lowpass filters (s), g(, to liit the transission bandwidth. Finally the quadrature signals odulate the and carriers for transission. The transitted bandpass signal S (, which is the suation of all sybols represented by the M possible signalling wavefors for AM [PRO 95], [CHE ]. 1.. Deodulator M-AM Since the inforation is carried in the phase and aplitude of the odulated carrier for the AM signal, the receiver is assued to be able to generate a reference carrier whose frequency and phase are identical to those of the carrier at the transitter. LO 9 c cos(πf c -sin(πf c s ( deodulator. Figure : Block diagra of a AM coherent deodulator. At the receiver, the received high frequency signal is first down-converted to a lower interediate frequency (F) before being further processed. The deodulator perfors the ajority of its work at an interediate or baseband frequency. The ixer in the coherent deodulator converts the F signal to a baseband signal, by ultiplying the incoing F signal with a locally generated carrier reference and the product is passed through a lowpass filter (). The reoves the high-frequency coponents and selects the difference coponent fro the ixer output. These s also perfor as atched filters whose ipulse responses are atched to the transitted signal to provide the axiu signal-to-noise ratio (SNR) at their output. Then detectors decide which of the possible signal wavefors was transitted fro the output of the s.. VHDL-AMS MPLEMENTATON OF M- AM SYSTEM Our work is focused on the developent of a coplete VHDL-AMS library containing high level descriptions of the ost coun odulation and deodulation techniques like ASK, PSK, PSK, and AM. We will only focused in this paper on the presentation of a generic M-AM odulator/deodulator. A highly ideal syste, odeled after the theoretical AM syste, is ipleented in VHDL-AMS. To better anage the nuber of coponents involved in this design, the code hierarchy in Figure 3 is ipleented. Each block contains the coponents needed to perfor its intended function. The AM syste wrapper is used to initialize variables concerning only the RF syste: transitter, and receiver.. Binary sequence Transitter M-AM Channel Figure 3: Basic M-AM code hierarchy of VHDL- AMS odel. Receiver

3 .1. Transitter M-AM The transitter block diagra is shown on Figure. Figure : Transitter bloc diagra. The architecture of the odulator 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= N ). -The generation block of M-levels (odulator i(/q() produces the two ways n-phase i( and uadraturephase q(. t integrates a digital to analog converter -The odulation block produces the odulated signal. The goal is to support a coding adapted to several types of odulations. The coder will accept odulations PSK and -AM until 56-AM Serial to parallel The binary input strea is subdivided into block of N bits, called sybols, and each sybol is represented by one of M= N pulse aplitude values. The incoing N bits are considered in groups of N bits. The block diagra of a M-AM transitter is shown on Figure. The input binary data are divided into N channels:, 1. (N/)-1, and, 1,, (N/)-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 M-level AM n M-level AM the bit data is suitably assebled into N sybols (M= N ) and each sybol transitted by a carrier wave having a unique aplitude and phase. The duration of each sybol deterines the bandwidth of the AM signal. Fig.6 shows a M-level constellation where each dot represents the position of the phasor relative to the intersection of the axes arked (for in phase) and (for uadrature) [HAN 9]. The phasors of the M-level constellation ay be decoposed into N/ independent N-level AM signals that are transitted on quadrature coponents of the sae carrier. Each AM carrier is transitted with an aplitude 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 coponents are binary encoded using N/ Gray coded bits for each level. For exaple, the -level AM coponents of 16-AM are binary encoded using two Gray coded bits for each level; Gray codes 1,, 1 and 11, are assigned to levels 3d, d, -d and 3d, respectively [YEA 3]. Origin M-AM (M=16) 3d d -d -3d d d 3d Origin -3d -d d 3d d Modulation AM of N states Horloge T Horloge NT Shift register to N bits Binary sequence Latch D Figure 5: Serial to parallel block. 1 Figure 6: M-ary AM constellation, M=16. The apping of the bits into sybols is frequently done in accordance with the Gray code which helps to iniize the nuber of bit errors occurring for every sybol error. Because Gray-coding is given to a bit assignent where the bit patterns in adjacent sybols only differ by one bit [BAT 99], this code ensures that a single sybol in error likely corresponds to a single bit in error. Finally, we ixed the channel with cos(ω c and channel with sin(ω c. The resulting signals are then sued and aplified by an ideal power aplifier. 3

4 . Receiver The receiver diagra block is depicted on Figure 7. The signal is first aplified and then down converted by ixing it with the sine and cosine of the carrier frequency. Then, the obtained signals passe through low-pass filters, creating the reconstructed and channels. The and channels output fro the deodulator are then digitized in N bit and passed to the parallel to serial converter, where they are sapled using the recovered sybol clock, hard liited, and output serially fro the binary sequence u u 6u 8u 1u t [s] Figure 9(a): nphase and uadrature for - AM(M=). gen_bit.val inphase.val quadrature.val inp_estie.val quad_estie.val nphase.input - s ( LO cos(πf c A/D Parallel to serial - - uadrature.input _Filter.VAL 9 -sin(πf c - 5u 1u 15u u 5u 9u t [s] _Filter.VAL Figure 7: Block diagra of VHDL-AMS M-AM receiver. 1-1 Figure 9(b): nphase and uadrature for 16- AM (M=16). nphase.input 3. SMULATON RESULTS The VHDL-AMS description developed for the transitter/receiver M-AM syste is based on the principe detailed above. The obtained high level odel is generic and gives the possibility to the user to choice the M-AM odulation technique. Figure 8 shows four wavefors. Digital data, inphase, quadrature and the -AM odulated signal obtained after the transitter odel siulation u 1u 15u u 6u uadrature.input _Filter.VAL _Filter.VAL Figure 9(c): nphase and uadrature for 6-AM (M=6). Figure 1 shows -AM the constellation in the transitter and in the receiver (after filtering). t [s] gen_bit.val - inphase.val 1.3 _AM - quadrature.val 1 - signal_od.val.5 1u u 3u u 5u 6u t [s] -.5 Figure 8: -AM Transitter. Figure 9 (a,b,c) shows the signals and in the transitter, and channels in the receiver (after filtering) respectively for AM, 16 AM and 6 AM. We can see that in all cases we receive the sae transitted inforation Figure 1: Constellation -AM (M=)

5 With the generic odel that we have developped and siulated, we can draw the spectru of the odulated signal. Figures 11(a), (b) and (c) represent respectively spectrus of the odulation 16 AM, 3 AM and 6 AM -.17Meg Meg 1.Meg 15.Meg.Meg 5.Meg su Meg 5.Meg 1.Meg 15.Meg.Meg 5.Meg -.5Meg 1. 5.Meg 1.Meg (a) 15.Meg.Meg 5.Meg su Meg 5.Meg 1.Meg 15.Meg.Meg 5.Meg -.Meg 1. 5.Meg 1.Meg (b) 15.Meg.Meg 5.Meg su Meg 5.Meg 1.Meg 15.Meg.Meg 5.Meg We have shown that we are able to obtain a transient response, a constellation or a spectru of the odulated and deodulated signals after the siulation of our VHDL-AMS description. All these results allow to study and to optiize the transitter/receiver at a high level of design which constitutes an iportant step in the top-down hierarchical design flow. REFERENCES [BAT 99] A. Batean, Digital Counications: Design for the real world, Addison-Wesley Longan Liited, New York, NY, [CHE ] J. Chen, CARRER RECOVERY N BURST- MODE 16-AM, thesis, University of Saskatchewan, June. [HAN 9] L. Hanzo, R. Steele, and P.M. Fortune A Subband Coding, BCH Coding, and 16-AM Syste for Mobile Radio Speech Counications, EEE Transactions on Vehicular Technologies, 199. [EE 99] VHDL Analog and Mixed-Signal Extensions: EEE standard , [NK ] P. Nikitin, E. Norark, C. Wakayaa, and R. Shi VHDL-AMS odeling and siulation of a BPSK transceiver syste, Proceedings of EEE nternational Conference on Circuits and Systes for Counications,. [NOR ] E. Norark, L. Yang, C. Wakayaa, P. Nikitin, and R. Shi VHDL-AMS Behavioural Modeling ad Siulation of a π/ DPSK transceiver syste,. BMAS. Proceedings of the Fifth EEE nternational Workshop on, 1-1 Oct.. [PRO 95] J. G. Proakis, Digital Counications, McGraw-Hill nc., New York, NY, 1995 (Third Edition). [SKL 98] B. Sklar, Digital Counications: Fundaentals and Applications, PrenticeHall nc., Englewood Cliffs, NJ, [YEA 3] Bee Leong Yeap, Choong Hin Wong, and Lajos Hanzo, Reduced Coplexity n-phase/uadrature- Phase M-AM Turbo Equalization Using terative Channel Estiation, EEE Transactionson Wireless Counications, VOL., NO. 1, JANUARY 3. (c) Figure 11: Spectru of ((a):16 AM, (b):3 AM, (c): 6 AM) We can see that when the nuber of bits increases (or sybols), the spectral clutter increases.. CONCLUSON n this paper, we described a ethodology for odelling and siulation of coplete RF syste using VHDL-AMS. As a deonstration exaple, we considered an M-AM syste. We siulated and evaluated syste perforance and a high level of description. 5