MSK Direct Sequence Spread Spectrum System Incorporating SAW Tapped Delay Line and SAW Shaping Filter
Abstract
The present work deals with theIF section of an indoor wireless TDMA/CDMA(TCDMA) communication system operating inthe 2.45 GHz ISM band [1, 2]. Spreading anddespreading is done at the 361 MHz IF withan extremely wide bandwidth of 65 MHz whichis greater than the coherence bandwidth of theUHF indoor radio channel. For the modulationof the data we employ on o_ keying (OOK), andfor the spreading minimum shift keying (MSK).Two types of MSK modulators have been devel-oped, one with a SAW tapped delay line (TDL)and one with a SAW shaping _lter. Despreadingis done by another SAW TDL, which operates asa matched _lter.
INTRODUCTION
In recent years a growing demand for wirelessLAN systems with high data rates can be ob-served. One attempt to achieve high data rateswithin the transmission bandwidth of 100 MHzin the ISM band at 2.45 GHz is the time codedivision multiple access (TCDMA) scheme. Incontrast to other wireless modems which use thespread spectrum technique only for interferencereduction [3, 4], the TCDMA system has a realCDMA component. For spreading of the databit stream we employ MSK modulation ratherthan the commonly used binary phase shift key-ing (BPSK) since MSK shows the advantage oflower spectral sidelobes and a low inuence ofclipping and distortion e_ects in the power am-pli_er on the system performance. However, thehardware implementation of MSK is more com-plicated than that of BPSK, especially at thePAPLL Synth.BB2,1 GHz350 MHz 43,75 MHzPCMSK-Mod. 2MSK-Mod. 1Fig. 1: Transmitter of the TCDMA downlink (PA: powerampli_er, BB: baseband circuit, PC: personal computer)PLL Synth.BB2,1 GHz350 MHz 43,75 MHzLNA PCSAW-MF 1SAW-MF 2Demod.Demod.Fig. 2: Receiver of the TCDMA downlink (LNA: low noiseampli_er, MF: matched _lter)given high data rates.As a result of extensive system simulations [1],for data transmission we use OOK instead of dif-ferential BPSK, since OOK o_ers an increasedsystem capacity.
BASIC CONCEPT OF TCDMA
The system downlink incorporates a synchroniza-tion channel with an m-sequence code and userchannels with selected gold codes. Since the chiprates are very high, only solutions with the highanalog signal processing capacity of SAW de-vices are of interest. Figures 1 and 2 show thetransmitter and the receiver of the TCDMA sy tem. A PLL synthesizer generates a frequencyof 2.1 GHz for up- and downconversion of the IFsignal, a 350 MHz signal for the modulators anda 43.75 MHz signal, which is the chiprate for thebaseband circuit. In the receiver an envelope de-tector is used for demodulating the correlationpeaks. Only two modulators/demodulators forthe synchronizetion channel and one user chan-nel are drawn in Figures 1 and 2. In the followingwe will discuss the implementation of the SAW_lters, the MSK modulators (spreading) and thecorrelator (despreading).
MSK SAW TAPPED DELAY LINESFOR SPREADING ANDDESPREADING
The _rst approach for spreading and despread-ing the data bits is using SAW TDL, both in thetransmitter and in the receiver. The impulse re-sponse of the receiver TDL is the time inverseimpulse response of the transmitter TDL. OurTDL design method is described in Reference[5]. All TDLs were fabricated from LiTaO3 sub-strates. Code length, chip rate, center frequency,and bandwidth are 127, 43.75 MChip/s, 361 MHzand 65 MHz (null-to-null bandwidth of the MSKspectrum), respectively. >From the chiprate andthe center frequency of the MSK spectrum wecalculate the two frequencies f1=350 MHz and-3 dBTTLDriverDBM350 MHzSAW ShapingFilterFig. 5: MSK modulator with SAW shaping _lter (DBM:double balanced mixer)IN OUTFig. 6: Structure of the MSK shaping _lterf2=372 MHz which are used to modulate the codeinformation onto the carrier. The TDL process-ing gain is 21 dB. The matched and unmatchedinsertion loss is 28 dB and 39 dB, respectively.To achieve broadband matching, a network of sixLC elements at the input and six LC elements atthe output was used. The MSK modulator isshown in Figure 3, and Figure 4 gives the struc-ture of the SAW TDL. A sine burst generatorprovides a burst for every data bit. The outputof the TDL is a complete MSK modulated PN(pseudo noise) code sequence. The electromag-netic feedthrough at the output of the TDL hadto be gated because of the high amplitude whichwould drive the subsequent ampli_er into satu-ration. One disadvantage of this modulator isthe _xed PN code sequence which is determinedby the structure of the output transducer of theTDL. Variable codes are allowed by a modulatorwith a SAW shaping _lter.


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