Question

2. The different types of modulation schemes have different distance between the transmitted signals. The distance for BPSK, QPSK, M-PSK, BFSK, and M-FSK is shown below. Modulation Scheme Minimum Distance between Constellation Points Bandwidth BPSK d=2./PT= 2/E B = 2f = 21 QPSK d=2/PTo = 2/E B = 2/3 = fo M-PSK d = 54€, sin? ) = 4.NE, sin? (75) B = 2f: = 2 16-QAM d = 2/0.4E BFSK d= 2E (Orthogonal BFSK) B = 45 M-FSK d = /2E, = 2NE B = 2Mfs= 2N+170 N M Eb = energy contained in a bit duration, N = number of bits per symbol, = 2 (number of symbols) fo = T (bit rate), fs = = = (symbol rate) A. What are the advantages and disadvantages of using BPSK over BFSK? B. Calculate the distance for the BPSK, QPSK, 16-PSK, 16-QAM, BFSK, and 16-FSK if Ev = 1 m). C. Suppose you want to change your modulation scheme from QPSK to BFSK. If it is required that the minimum distance between the constellation points and the bandwidth used for the modulation remain the same, what is the necessary change in the energy contained in bit duration (Eb) and the bit rate ()?

Answer 1

In general, FSK modulations tend to be worse than comparable linear modulations by several criteria. Probability of Bit Error: Binary Modulations Noncoherent BFSK, Coherent BFSK and Coherent BPSK 1.E+00 -NC-BFSK --C-BFSK/BASK C-BPSKIQPSK Probability of Bit Error: Pe eric 2 VN 16 1.E-07- 0 2 4 6 8 10 12 14 EJN, dB EJN, - Signal energy per bit over Noise density per bit June 2013 www.AranaRF.com 18 20 22 24 Atlanta RF Services, Software & Designs Unfortunately, I couldn't find a nice picture for this, so the one above from AtlantaRF will have to do, but in telecommunications literature, you have these bit error rate versus per bit signal to noise ratio graphs for various modulation schemes. One thing you always see in those graphs is that you need higher Eb/No for same BER in FSK compared to PSK. In amateur radio, (clean) power is often relatively limited and getting more with less is respected, giving advantage to PSK. One big advantage of FSK is that we don't need clean power for it! It can work with very non-linear power amplifiers, so a set power level is easier to reach with FSK than it is with PSK.

One big advantage of FSK is that we don't need clean power for it! It can work with very non-linear power amplifiers, so a set power level is easier to reach with FSK than it is with PSK. Next, we have the spectral efficiency as well. FSK is less spectrally efficient compared to PSK with same number of modulation levels. For example, for non-coherent orthogonal FSK, the null-to-null bandwidth is B= (M + 1)Rs, while for PSK (which is same as for non-coherent DPSK as used in say BPSK31) it's 2R,, so in binary case, we have 3Rcompared to 2R) for BPSK. If number of modulation levels increases, FSK starts to lose very quickly, since in PSK case, bandwidth does not depend on number of modulation levels.

Advantage of BPSK:

=It is most robust modulation technique due to the fact that binary 1 and 0 are separated by 180 degree phase shift of the carrier. Due to this property, BPSK modulated data can travel longer distances when transmitted from base station or subscriber stations. Hence BPSK modulation is employed in pilot carrier as well as in preamble sequences. These are used for time/frequency synchronization and channel estimation/equalization purpose. Due to above, BPSK modulation is used by most of the cellular towers for long distance communication or transmission of the data. BPSK demodulator requires to make only two decisions in order to recover original binary information. Hence BPSK receiver is very simple compare to other modulation types.

-BPSK is power efficient modulation technique as less power is needed to transmit the carrier with less number of bits.

Disadvantages of BPFK

– In BPSK modulation, one bit is carried by one single analog carrier. Hence data rate in bits per second is same as the symbol rate. This is half in comparison to the QPSK modulation technique and many times less compare to other higher modulation techniques such as 16QAM, 64QAM etc. Due to above reason, BPSK is not bandwidth efficient modulation technique compare to other modulation types.

Benefits or advantages of FSK Following are the benefits or advantages of FSK: It has lower probability of error (Pe). It provides high SNR (Signal to Noise Ratio). It has higher immunity to noise due to constant envelope. Hence it is robust against variation in attenuation through channel. FSK transmitter and FSK receiver implementations are simple for low data rate application.

Drawbacks or disadvantages of FSK Following are the disadvantages of FSK: It uses larger bandwidth compare to other modulation techniques such as ASK and PSK. Hence it is not bandwidth efficient The BER (Bit Error Rate) performance in AWGN channel is worse compare to PSK modulation. In order to overcome drawbacks of BFSK, multiple FSK modulation techniques with more than two frequencies have been developed. In MFSK (Multiple FSK), more than one bits are represented by each signal elements. Refer 2FSK and 4FSK Modulation types>>

B)

Na - 1-391. d= нх PSK x -PSk Eb=lm; Isase d = JANEG SAYYO d=25Eb d=1-391m take a d=25103 d = 54x1x10 sm (2) = 54x10 = 2-390m * 16-QAM * BFSK * Misk d=280.46h d=SED = 2 504x603 -0-044m = 0.045h da RX10 d= ANG - SSG 20-04m

C)

c) when changing modulation scheme from Beyk. Opsk -> BFSK I QPSK Brsk de 26 de sen Bodo B = 4 fo If it is required that the minimum distance between the constellation points and the the band width bandwidth wod ful modwlation remain same, then cheegy contained in bit duation & ) should be doubled (286) and the bit rate (&) should be fore times (4хfo)