NAL JOURNAL OF SATELLITE COMMUNICATIONS, VOL. 6, 283-288 (1988)

i

A MODULAR MULTISTAGE APPROACH TO DIGITAL FDM DEMULTIPLEXING FOR MOBILE SCPC SATELLITE

COMMUNICATIONS

HEINZ GOCKLER ANT Nndiriclrterlreclrrlik Grnblrl, Post~clr 1120, 0-7150 Bnckrrnrrg, F. R. G e m n n y

SUMMARY A hierarchical multistage method (HMM) for digital clemultiplexing of an FDM signal composed of L adjacent SCPC signals is described; L is (preferably) a power of two, here L=32. This I-IMM approach to FDM dernultiplexing applies bandpass sampling and is based on the processing of complex- valued signals by linear-phase FIR filters, where at any stage of processing the respective signals are always oversampled by two. The simulation results fully confirm the predicted system performance. An electrical demonstration model constructed by cascading six identical specially designed signal processors is being built.

KEY WORDS FDM demultiplexing Multistage approach Analytic signal processing Oversampling Bandpass sampling scheme

INTRODUCTION

In North America, Japan and Europe digital com- munications with mobile vehicles via satellite is currently being investigated.l-' A forward link takes messages from an earth-station to the satellite, which retransmits to mobiles; a return link begins at the mobile, goes up to the satellite and is returned to the earth-station. The satellite will use spot-beams to achieve power gain and to facilitate frequency reuse. Forward links are expected to employ TDM techniques. A mobile will acquire one such TDM signal and extract its own traffic from it. Each active mobile within a spot-beam will be assigned a different operating centre frequency, applying a channel frequency spacing of width B. In essence, the mobile-generated signals gain simultaneous access to the system by frequency multiplexing and by space discrimination afforded by the satellite-antenna pattern.

Following Reference 5 , it is assumed that the satellite has 19 spot-beams, that up to 3600 mobiles are to be served simultaneously, and that up to 800 mobiles may be served in a single beam with a channel frequency spacing of

The last requirement is consistent with appropriately shaped QPSK signals having a data rate of 9.6 kbls to be applied in each mobile transmitter: square root of 40% cosine roll-off filtering in conjunction with a maxinium frequency offset of 2600 Hz due to factors such as Doppler shift and oscillator instabilities.

Simple translation of the FDM uplink at L-band to C-band would be an inefficient use of power and

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spectrum. Furthermore, not all signals in a beam are destined for the same earth-station, so instead individual TDM streams to each of the earth-stations are required. To accomplish the format change and necessary routing requires extensive signal processing on board the satellite.

Figure 1 shows a block diagram of the return- link on-board processing. The received signals are separated from one another in a frequency demultiplexer (FDM DEMUX). Each separated signal is passed to a receiving QPSK modem (DEMOD) which applies complex signal processing, producing a digital data stream. These many parallel streams are recombined into serial TDM streams for retransmission to the earth-stations.

PROBLEM STATEMENT

The object of this paper is to describe the hierarchical multi-stage method (HMM) for digital FDM-demulti- plexing of L=32 SCPC signals in conjunction with non-ideal band-limitation of the analogue FDM signal in front of the AID converter. This extended demultiplexer, in Figure 1 delineated by the dash-dotted line, is subsequently referred to as the transmultiplexer (TMUX).

The design of the TMUX (Figure 1) is now described in more detail. The oscillator frequency f, of the (analogue) down-converter has to be selected such that the desired L real signals from mobiles pass ~maffected through the analogue anti- aliasing bandpass filter (AAF), of passband width LB. The final down-conversion to baseband is achieved by sampling the continuous AAF output signal with a sampling rate of at least 2LB, consistent

Recei!~ed Febrwary 1988 Revised Marclz 1988

antenna I i &fG fsi b = LLB i I I Transrnultiplexer TMUX L J I

I I I I I Is-------------- Multicarrier Dernodulator MCD

Figure 1. Block diagram of a multi-carrier demodulator for L channels, applying cligital signal processing (return link). AAF, analogue anti-aliasing bandpass filter; fs,, f&,:,,,, input and output sampling frequency of demultiplcxer (FDM DEMUX); fA, centre frequency of

A A F

with the sampling theorem, using a fast sample-and- hold (SIH) circuit cascaded by a relatively slow analogue-to-digital (AID) converter. Rather than the minimum possible sampling rate, however, subsequently oversampling by a factor of two is applied at the DEMUX input. With this approach the specifications of the AAF and DEMUX filters are greatly relaxed; they are relaxed further if the transition from real to complex (analytic) signal processing is performed as close to the DEMUX input as possible. l

Therefore the present TMUX design problem may be stated as follows: Design a highly modular, 32-channel HMM DEMUX for real input and complex output sequences. Apply complex signal processing with oversampling by a factor of two throughout the DEMUX. Select the AAF centre frequency f, and the channel allocation within the AAF passband relative to the input sampling rate,

such that the most efficient HMM DEMUX implementation results.

THE DIGITAL HMM DEMULTIPLEXER The 32-channel analogue SCPC-FDM signal to be demultiplexed digitally is centred at an IF of about 17.5 MHz. In order to satisfy the sampling theorem, the FDM signal is band-limited by a crystal bandpass filter (AAF) such that it can be (over)-sampled with fs, given by equation (2). With this bandpass sampling scheme, which requires a fast, accurate S/ H circuit and a slow AID converter, the FDM spectrum of the digitized DEMUX input signal sD(kT) is folded down to a centre frequency

assumed that its 32-channel SCPC-FDM input signal is given by a complex-valued (analytic) sequence si(2kT), which is the result of preprocessing in a digital anti-aliasing filter (DAF). The DAF'O per- forms the transition from real to complex signal representation of the digitized FDM signal s,(kT), and decimates by two. In the following, underlining indicates that the associated signals or filter coef- ficients are complex-valued.

In the HMM shown in Figure 2 each cell (block) splits its complex input signal into two complex output sequences, each decimated by two. Different stages and their respective input sampling rates are distinguished by a (Roman) superscript:

K {O, I , 11,111, IV, V} (4)

where K = O stands for DAF = f,,). Each HMM cell delivers two different output signals. The two passbands of an HMM cell, each of bandwidth B" = &14, are distinguished by AE{O,l}. The two slot transfer functions H:(exp(jQK)) and RK=2vf1

to be realized by the various HMM cells are indicated in Figure 2 by the subscript A. It should be noted that all 31 HMM cells are identical, except for the DAF necessary for preprocessing.

A more profound understanding of the HMM can be gained from the associated spectral representation illustrated in Figure 3. Figure 3(a) shows the idealized frequency response,,of a linear-phase FIR half-band filter with the propertics"

and

The tree structure of the most efficient and highly modular hierarchical multi-stage method adopted for demultiplexing is shown in Figure 2.'- It is

MODULAR MULTISTAGE DIGITAL FDM DEMULTIPLEXING

Stage: K = 0 (DAF) I II Ill 1V

Figure 2. Overall block diagram of HMM-DEMUX with one real input and 32 complex output ports, constructed from one DAF for preprocessing and 31 identical I~ierarchically cascaded HMM cells, which all operate at a stage individual sampling rate AT1 = l/T"+'

where 1.1 represents the filter length. From such a prototype half-band filter are derived the DAF and all (identical) NMM cells. The various filter transfer functions with complex coefficients required for the D A F and HMM cells are immediately obtained by nlodulating a complex sinusoidal carrier of an appropriate frequency by the real impulse response of the prototype half-band filter. Figure 3(b,c) shows that these carrier frequencies are fJ4 for the DAF, and ~f$1/8 and 3J;,/8 for the HMM cells, with the sampling rate?;, given by equation (5), to which all filters of stage K are related.

As is obvious from Figure 3(c), each DEMUX output channel is still loaded by a repeatedly aliased spectral portion adjacent to the spectrum of the isa able slot signal. This calls for a final band-

limitation, which is accomplished by a non-decima- ting linear-phase FIR half-band filter with complex coefficients.12 (This half-band filter is not included in Figure 2.)

The filters of the HMM-DEMUX have been designed such that a ~ninimum signal-to-noise ratio of 30 dB is achievecl for all 32 DEMUX output signals after final band-limitation. Thereby an ideal FDM signal is anticipated at the input port of the A A F in front of the AID converter. As a result, the D A F and HMM prototype calls for a symmetric FIR half-band filter of length 11, whereas the length of the symmetric half-band filter for final band- limitation should not be shorter than 19. Figure 4 shows, as an example, the attenuation response of the DAF.

@ Halfband prototype filter for ail HMM s t a g e s and DAF (fsOi = f S i ) I H ( ~ ~ ~ ~ 11 K S {o,I,u,~,Iv,v}

t

@ DAF (x=O): Real input and complex o u t p d sequence , 5' = L5 , T ' =

I a d + l I S , [ ~ " ) I

Shift of spectrum by t fgf112 :

e ' ~ ( k T N " ) = (-ilk z;(kTXd) f

Figure 3. Spectral representation of HMM-DEMUX according to Figure 2: (a) prototype half-band filter for all stages, (b) pieprocessing by DAF (stage 1

MODULAR MULTISTAGE DIGITAL FDM DEMULTIPLEXING

Figure 4. Attenuation response o l DAF with coefficients scaled up by 2 and shortened to HI$' = 10 bit, showing rounded coefficients (solid line) and coefficients obtained by d~screte optimization (dashed line)

Table I. Signal-to-noise ratio obtained by simulation with QPSK stimulation of the DEMUX

Signal-to-noise ratio (dB) Signal word- Set 1 Set 2 Set 3 lengths w,=16 bit w,=10 bit w,=16 bit

Coefficient sets 1 and 2: lilter length r ~ = l l , coefficient set 3: r1=7 Signal word-lengths: W, AID conversion; W,,, between HMM cells; wi, cell inherent

Redundonl V5P Easily tsstobta

Stoge: OAF101 H M M I H M M I I H M M l l l HMMN H M M V Number of sepamtad slols: S l o t s o m p l i n g r o t e : fS1 Degree of rnultipl@xing: I 1 2 L 8 16

Figure 5. Hardware rnultiplexing scheme for HMM-TMUX

ation, high reliability with or without redundant VSPs (Figure 5 ) , and reduced overhead circuitry. In a preliminary assessment study the VLSI (gate array) implementation of the HMM-DEMUX has been considered in conjunction with the filters for final band-limitation. As a result, a power consumption of less than 50 mW per channel is expected for the transmultiplexer. This figure is based on the highly conservative assumption of a 2 p CMOS technology requiring 18 pW per switched gate and MHz.

ACKNOWLEDGEMENTS

This work was supported by the European Space Technology Centre (ESA-ESTEC), Noordwijk, The Netherlands, under Contract No. 6497/55. However, the opinion expressed in this paper is not necessarily shared by ESTEC. In particular the author is greatly indebted to G. Bjornstro~n, ESTEC, ancl P. Enders and H. Eyssele, ANT, for their support in promoting the reported work, and to his colleagues M. I-Iagen, H . Scheuermann and.A. Szillus for various stimulati~lg discussions on tbpics treated in this paper.

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