Frequency-Shift Keying Demodulation and …

1 Frequency-Shift Keying Demodulation and manchester -BitDecoding Using a digital Radio and digital signal ProcessingTechniquesAuthor: James M. ShimaINTRODUCTIONThis paper discusses a method of Frequency-Shift Keying (FSK) Demodulation andManchester-bit decoding using a digital signal processing (DSP) approach. Thedemodulator is implemented on a single-channel high-speed digital radio board. Theboard architecture contains a high-speed A/D converter, a digital receiver chip, a hostDSP processing chip, and a back-end D/A converter [2]. The demodulator software isbooted off an on-board EPROM and run on the DSP chip [3]. The algorithm acceptscomplex digital baseband data available from the front-end digital receiver chip [2].

2 Thetarget FSK modulation is assumed to be in the RF range (VHF or UHF signals). A blockdiagram of the single-channel digital radio is shown in Figure 1 [2].high-speedA/D converterGray GC1011digital receiver chipHost DSPDACRF inputAnalog outputSingle channel digital radio block diagramFigure 1 BACKGROUNDFSKF requency-shift Keying is a type of digital binary communication technique . It is identicalto FM modulation using a digital binary signal as the message m(t) [1]. Thus, a binary 1represents one frequency, and a binary 0 represents another frequency. The FSK signaldeviates from the carrier frequency depending on the binary message m(t). For example,2assume m(t) can take on the values 1 or -1.

3 When m(t) = 1, the FSK signal would deviateon the high side of the carrier frequency. When m(t) = -1, the FSK signal would deviateto the low side of the carrier frequency. The frequency deviation of the FSK signal isgiven by the FM equation: FVDpf= 2 In this example, Vp = 1 and -1. So, the deviation of the FSK signal is Df2 about thecarrier. The modulation parameter Df is chosen to give the desired amount of deviationin the FSK modulated bit formatIn manchester binary signaling, each binary digit is represented by a positive half-bitperiod and a negative half-bit period [1]. The bit period is split in two, thereforeguaranteeing a zero crossing during each bit time.

4 For example, a binary 1 is representedby a positive half-bit interval followed by a negative half-bit interval. Likewise, a binary 0is represented by a negative half-bit interval followed by a positive half-bit interval [1]. Anexample of a manchester encoded bit stream of 110001 is demonstrated in Figure 2. Figure 2By examining the manchester code, it satisfies the self-synchronization property of linecodes [1]. The advantage of manchester encoding is that a zero crossing always occurs ina bit interval, so bit synchronizers are able to extract the data without timing errors. Forexample, in standard RS-232 digital communication protocols, a long stream of 1's or 0's110001 manchester bit format bit interval0A-A3might cause bit errors because the receiver clock may migrate due to sampling the binarysignal at a non-integer rate.

5 With manchester coding, a long stream of 1's or 0's alwaysguarantees two zero crossings (one at the half-bit interval and one at the start of the nextbit interval). Alternating 1's and 0's guarantee one zero crossing (at the half-bit interval),thus allowing the receiver clock to resynchronize on this edge to correctly extract the manchester bit synchronizer utilizes these facts to correctly synchronize to theguaranteed half-interval bit edge and decode the original information bits. Thedisadvantage of manchester coding is that the bandwidth of the signal doubles since onepiece of information (each original bit) is represented by two levels in the DEVELOPMENTFSK demodulationUsing the background information on FSK and manchester coding, a demodulator isdeveloped to demodulate a FSK signal , with the binary message m(t) manchester encodedbefore it is modulated onto the carrier.

6 In other words, the binary message m(t) isManchester encoded and then FM modulated onto a carrier frequency. The frequencydeviation of the resulting FSK signal is assumed to be 10kHz. Thus, a negativeManchester pulse causes the FSK signal to deviate -10kHz from the carrier , a positive manchester pulse causes the FSK signal to deviate +10kHz from thecarrier demodulator was developed using the complex data mode on the digital radio board[2]. This results in a complex quadrature down conversion of the sampled RF , the DSP retrieves real and imaginary data from the digital receiver chip. The DSPuses the complex data to demodulate the FSK polar discriminator was utilized to retrieve the phase difference between consecutivecomplex samples of the RF signal [2].

7 Recall, the polar discriminator multiplies the newsample by the conjugate of the old sample. The complex resultant vector gives the phasedifference of the two samples through the relation tan-1 (Im / Re). Also, the samplingrate governs the maximum deviation a vector can rotate between samples. Note, the polardiscriminator vector resides at the origin for zero phase difference (constant sinusoid, nophase modulation). When the FSK signal deviates from the carrier, the polar discriminator4vector will migrate to its corresponding quadrant. For example, if the incoming FSKsignal is 4x oversampled, we know the polar discriminator vector is constrained to rotatein quadrants I and IV [2].

8 Thus, if the polar discriminator vector is in quadrant I, the FSKsignal deviates to the high side of the carrier. This corresponds to a positive Manchesterpulse. Likewise, if the polar discriminator vector is in quadrant IV, the FSK signaldeviates to the low side of the carrier. This corresponds to a negative manchester , by checking the imaginary part of the polar discriminator result we can determinewhat quadrant the polar discriminator vector is in. This in turn determines whether wehave a positive or negative manchester , using the above concept we can determine the zero crossings of the manchester bitstream by observing the polar discriminator results. If the polar discriminator vectorchanges polarity ( , it migrates from quadrant I to quadrant IV), we know a zerocrossing occurs.

9 This is because the FSK signal switches frequencies when theManchester code changes polarity. The polar discriminator detects this switch by passingthrough the origin (of the complex plane) and rotating quadrants. Therefore, this quadrantswitch by the polar discriminator vector corresponds to a zero crossing in the Manchestercode. Figure 3 demonstrates this concept of FSK Demodulation using the polardiscriminator resultant vector. Figure 3 FSK signalf1f2tIm(z)Re(z) manchester tPolar discriminator resultf1 freq vector f2 freq vectorUse of polar discriminator to demodulate FSK(positive manchester pulse)(negative manchester pulse)origin encoded "1"5 Again, we are assuming a 4x oversampling rate.

10 In this figure, a binary 1 is Manchesterencoded and then FSK modulated. Figure 3 shows how the quadrants relate to the FSKfrequencies. These FSK frequencies are directly related to the manchester code since theymodulate the FSK signal . In other words, if the polar discriminator detects frequency f1,we have a manchester positive pulse. If the polar discriminator detects frequency f2, wehave a manchester negative pulse. The manchester encoded "1" causes the polardiscriminator to rotate from quadrant I to IV when the zero crossing concept essentially demodulates the FSK signal . Using the polar discriminator, wecan determine the zero crossings and the polarity of the manchester pulses.