1. Wireless Communication Technology
Chapter 3
Modulation/Multiple Access Techniques
Asst. Prof. Bijaya Shrestha
Department of Electronics & Communication Engineering
nec

2. Amplitude Modulation (AM)
By Asst. Prof. Bijaya Shrestha, nec

3. Amplitude Modulation (AM)
By Asst. Prof. Bijaya Shrestha, nec

4. Amplitude Modulation (AM)
The process in which the amplitude of a carrier signal is varied in accordance with the message signal is called the amplitude modulation.
If Accos(2πfct) is the carrier signal and m(t)is the message signal, then the AM signal can be represented as
sAM(t)=[Ac+m(t)] cos(2πfct)
For a message signal Amcos(2πfmt), the modulation index is defined as
μ=Am/Ac
By Asst. Prof. Bijaya Shrestha, nec

5. Amplitude Modulation (AM)
The modulation is often expressed as a percentage, and is called percentage modulation.
The spectrum of an AM signal can be shown to be
SAM(f)=Ac/2[δ(f-fc)+M(f-fc)+δ(f+fc)+M(f+fc)]
The bandwidth of an AM signal is twice the bandwidth of the message signal.
The total power in an AM signal can be shown to be
PAM=Pc(1+μ2/2)
Pc =Ac2/2 is the carrier power.
By Asst. Prof. Bijaya Shrestha, nec

6. Frequency Modulation (FM)
By Asst. Prof. Bijaya Shrestha, nec

7. Frequency Modulation (FM)
By Asst. Prof. Bijaya Shrestha, nec

8. Frequency Modulation (FM)
Frequency modulation (FM) is a form of angle modulation in which the instantaneous frequency of the carrier signal is varied linearly with the message signal.
The FM signal can be expressed as
where kf is the frequency sensitivity of the modulator (measured in Hz/V).
For message signal Amcos(2πfmt), the FM wave may be expressed as
By Asst. Prof. Bijaya Shrestha, nec

9. Frequency Modulation (FM)
Here,
is the peak frequency deviation and
is the modulation index of frequency modulation.
The bandwidth of an FM wave can be estimated by using Carson’s rule
BFM=2(βf+1)fm
By Asst. Prof. Bijaya Shrestha, nec

10. Phase Modulation (PM)
Phase modulation (PM) is a form of angle modulation in which the angle θ(t) of the carrier signal is varied linearly with the message signal.
The PM signal can be expressed as
sPM(t)=Accos[2πfct+kpm(t)]
kp is the phase sensitivity of the modulator (measured in rad./V).
For message signal m(t)=Amcos(2πfmt), the PM wave can be expresssed as
sPM(t)=Accos[2πfct+kpAmcos(2πfmt)]
βp=kpAm is the modulation index of phase modulation
By Asst. Prof. Bijaya Shrestha, nec

11. Binary Amplitude Shift Keying (BASK)
Binary ASK modulation can be defined by
By Asst. Prof. Bijaya Shrestha, nec

12. Binary Amplitude Shift Keying (BASK)
Generation of Binary ASK wave
Detection of Binary ASK wave
By Asst. Prof. Bijaya Shrestha, nec

13. Binary Frequency Shift Keying (BFSK)
In a binary FSK system, two sinusoidal waves of the same amplitude Ac but different frequencies f1 and f2 are used to represent binary symbols 1 and 0, respectively.
By Asst. Prof. Bijaya Shrestha, nec

14. Binary Frequency Shift Keying (BFSK)
Generation of Binary FSK wave
Detection of Binary FSK wave
By Asst. Prof. Bijaya Shrestha, nec

15. Binary Phase Shift Keying (BPSK)
In a binary PSK system, the phase of a carrier signal is switched between two values (normally separated by 180˚) according to the two possible signals binary 1 and binary 0.
If the carrier signal is
and the energy per bit is
then, the BPSK signal can be expressed as
By Asst. Prof. Bijaya Shrestha, nec

16. Binary Phase Shift Keying (BPSK)
The power spectral density (PSD) of a BPSK signal is given by
By Asst. Prof. Bijaya Shrestha, nec

17. Binary Phase Shift Keying (BPSK)
The null-to-null bandwidth is equal to twice the bit rate. That is,
By Asst. Prof. Bijaya Shrestha, nec

18. Binary Phase Shift Keying (BPSK)
Generation of binary PSK wave
Detection of binary PSK wave
By Asst. Prof. Bijaya Shrestha, nec

19. Binary Phase Shift Keying (BPSK)
The probability of bit error for BPSK transmission is
N0 is the noise spectral density.
The Q-function is defined as
By Asst. Prof. Bijaya Shrestha, nec

20. Quadrature Phase Shift Keying (QPSK)
QPSK has twice the bandwidth efficiency of BPSK, since two bits are transmitted in a single modulation symbol.
The phase of the carrier takes on one of four equally spaced values, such as π/4, 3π/4, 5π/4, and 7π/4, where each value of phase corresponds to a unique pair of message bits.
The QPSK signal may be defined as
By Asst. Prof. Bijaya Shrestha, nec

21. Quadrature Phase Shift Keying (QPSK)
Where Ts is the symbol duration and is equal to twice the bit period.
Using trigonometric identities, the above equation can be rewritten as
The basis functions are defined as
and
Now, the QPSK signal can be written as
By Asst. Prof. Bijaya Shrestha, nec

22. Quadrature Phase Shift Keying (QPSK)
The constellation diagram of a QPSK signal is shown in the figure below.
The distance between the
adjacent points is
Since each symbol
corresponds to two bits,
then
Thus the distance between two neighboring points, in term of Eb, is equal to
By Asst. Prof. Bijaya Shrestha, nec

23. Quadrature Phase Shift Keying (QPSK)
The average probability of bit error in the AWGN channel can be obtained as
The bit error probability of QPSK is identical to that of BPSK, but twice as much data can be sent in the same bandwidth.
By Asst. Prof. Bijaya Shrestha, nec

24. Quadrature Phase Shift Keying (QPSK)
The PSD of a QPSK signal using rectangular pulses can be expressed as
The null-to-null RF
bandwidth of a QPSK
signal is equal to the
bit rate Rb, which is half
that of a BPSK signal.
By Asst. Prof. Bijaya Shrestha, nec

25. Quadrature Phase Shift Keying (QPSK)
QPSK Transmitter
By Asst. Prof. Bijaya Shrestha, nec

26. Quadrature Phase Shift Keying (QPSK)
QPSK Transmitter:
The unipolar binary message stream has bit rate Rb and is first converted into a bipolar non-return-to-zero (NRZ) sequence using a unipolar to bipolar converter.
The bit stream m(t) is then split into two bit streams mI(t) and mQ(t) (in-phase and quadrature streams), each having a bit rate of Rs=Rb/2.
The bit stream mI(t) is called the “even” stream and mQ(t) is called the “odd” stream.
The two binary sequences are separately modulated by two carriers Φ1(t) and Φ2(t), which are in quadrature.
The two modulated signals, each of which can be considered to be a BPSK signal, are summed to produce a QPSK signal.
The filter at the output of the modulator confines the power spectrum of the QPSK signal within the allocated band.
By Asst. Prof. Bijaya Shrestha, nec

27. Quadrature Phase Shift Keying (QPSK)
QPSK Receiver
By Asst. Prof. Bijaya Shrestha, nec

28. Quadrature Phase Shift Keying (QPSK)
QPSK Receiver:
The frontend bandpass filter removes the out-of-band noise and adjacent channel interference.
The filtered output is split into two parts, and each part is coherently demodulated using the in-phase and quadrature carriers.
The outputs of the demodulators are passed through decision circuits which generate the in-phase and quadrature binary streams.
The two components are then multiplexed to reproduce the original binary sequence.
By Asst. Prof. Bijaya Shrestha, nec

29. Offset QPSK (OQPSK)
The difference between the QPSK and the OQPSK is only in the alignment of the two bit streams mI(t) and mQ(t).
By Asst. Prof. Bijaya Shrestha, nec

30. Offset QPSK (OQPSK)
In QPSK signaling, the bit transitions of the even and odd bit streams occur at the same time instants.
But in OQPSK, the even and odd bit streams are offset in their relative alignment by one bit period (half-symbol period).
In QPSK, phase transitions occur only once every 2T seconds, and will be a maximum of 180˚ if there is a change in the value of both streams mI(t) and mQ(t).
However, in OQPSK, bit transitions occur every T seconds and at any given time only one of the two bit streams can change values thereby occuring maximum phase shift of ± 90˚ at any given time.
By Asst. Prof. Bijaya Shrestha, nec

31. Offset QPSK (OQPSK)
In QPSK, the occasional phase shifts of 180˚ in the carrier will make the envelope go to zero.
Though the amplitude of a QPSK signal is constant, after bandlimiting, it losses the constant envelope property.
Moreover, the hardlimiting of QPSK signal brings back the filtered sidelobes.
On the other hand, bandlimiting and hardlimiting operations do not produce the same deleterious effect on an OQPSK signal.
For this reason, OQPSK is very attractive for mobile communications systems.
By Asst. Prof. Bijaya Shrestha, nec

32. Offset QPSK (OQPSK)
Furthermore, OQPSK signals perform better in the presence of phase jitter due to noisy reference signals at the receiver.
However, OQPSK signal is identical to QPSK signal in terms of its spectrum.
By Asst. Prof. Bijaya Shrestha, nec

33. Minimum Shift Keying (MSK)
MSK is a special type of continuous phase-frequency shift keying (CPFSK) wherein the peak frequency deviation is equal to ¼ the bit rate Rb (or modulation index βMSK is 0.5).
Modulation index of MSK is expressed as
βMSK=2Δf/Rb
A modulation index of 0.5 corresponds to the minimum frequency spacing that allows two FSK signals to be coherently orthogonal.
Two FSK signals s1(t) and s2(t) are said to be orthogonal if
By Asst. Prof. Bijaya Shrestha, nec

34. Minimum Shift Keying (MSK)
MSK is sometimes referred to as fast FSK.
MSK modulation technique is very attractive for use in mobile radio communication systems due to the properties of constant envelope, spectral efficiency, good BER performance, and self-synchronizing capability.
MSK signal can be thought of as a special case of OQPSK with sinusoidal pulse weighting.
Considering OQPSK signal, if sinusoidal pulses are employed instead of rectangular pulses, the modified signal can be defined as MSK and equals
By Asst. Prof. Bijaya Shrestha, nec

35. Minimum Shift Keying (MSK)
Using trigonometric identity, MSK signal can be seen as a special type of a CPFSK signal expressed as
where φk is 0 or π depending on whether mI(t) is 1 or -1 and mI(t) and mQ(t) are even and odd bit streams respectively.
In order to make the phase continuous at bit transitions, the carrier frequency should be chosen to be qual to an integral multiple of one fourth the bit rate.
By Asst. Prof. Bijaya Shrestha, nec

36. Minimum Shift Keying (MSK)
We can deduce from the equation that the MSK signal is an FSK signal with binary signaling frequencies of fc+1/4T and fc-1/4T.
By Asst. Prof. Bijaya Shrestha, nec

37. MSK Power Spectrum The power spectral density for MSK is given by
By Asst. Prof. Bijaya Shrestha, nec

38. MSK Power Spectrum
The MSK spectrum has lower sidelobes than QPSK and OQPSK.
99 % of the MSK power is contained within a bandwidth B=1.2/T, while for QPSK and OQPSK, the 99 % bandwidth is equal to 8/T.
The main lobe of MSK is wider than that of QPSK and OQPSK, and hence when compared in terms of first null bandwidth, MSK is less spectrally efficient.
By Asst. Prof. Bijaya Shrestha, nec

39. MSK Transmitter
By Asst. Prof. Bijaya Shrestha, nec

40. MSK Transmitter
Multiplying a carrier signal with c0s[πt/2T] produces two phase-coherent signals at fc+1/4T and fc-1/4T.
These two FSK signals are separated using two narrow bandpass filters and appropriately combined to form the in-phase and quadrature carrier components x(t) and y(t) respectively.
These carriers are multiplied with even and odd bit streams, mI(t) and mQ(t), to produce the MSK modulated signal sMSK(t).
By Asst. Prof. Bijaya Shrestha, nec

41. MSK Receiver
By Asst. Prof. Bijaya Shrestha, nec

42. MSK Receiver
The received signal sMSK(t) (in the absence of noise and interference) is multiplied by the respective in-phase and quadrature carrier components x(t) and y(t).
The output of the multipliers are integrated over two bit periods and dumped to a decision circuit at the end of each two bit periods.
Based on the level of the signal at the output of the integrator, the threshold detector decides whether the signal is a 0 or a 1.
The output data streams correspond to mI(t) and mQ(t), which are offset combined to obtain the demodulated signal.
By Asst. Prof. Bijaya Shrestha, nec

43. Gaussian Minimum Shift Keying (GMSK)
GMSK may be viewed as a derivative of MSK.
In GMSK, the sidelobe levels of the spectrum are further reduced by passing the modulating NRZ data waveform through a predmodulation Gaussian pulse-shaping filter.
Baseband Gaussian pulse shaping smooths the phase trajectory of the MSK signal and hence stabilizes the instantaneous frequency variations over time.
GMSK can be coherently detected just as an MSK signal, or noncoherently detected as simple FSK.
By Asst. Prof. Bijaya Shrestha, nec

44. Gaussian Minimum Shift Keying (GMSK)
In practice, GMSK is most attractive for its excellent power efficiency (due to the constant envelope) and its excellent spectral efficiency.
The premodulation Gaussian filtering introduces ISI in the transmitted signal, but it can be shown that the degradation is not severe if the 3-dB bandwidth-bit duration product (BT) of the filter is greater than 0.5.
The GMSK premodulation filter has an impulse response given by
By Asst. Prof. Bijaya Shrestha, nec

45. Gaussian Minimum Shift Keying (GMSK)
and the transfer function given by
The parameter α is related to B, the 3-dB baseband bandwidth of HG(f), by
α=0.5887/B
By Asst. Prof. Bijaya Shrestha, nec

46. GMSK Power Spectrum
By Asst. Prof. Bijaya Shrestha, nec

47. GMSK Power Spectrum
We can see from the graph that as the BT product decreases, the sidelobe levels fall off very rapidly.
But, reducing BT increases the irreducible error rate due to ISI.
The BER degradation due to ISI casued by filtering is minimum for a BT value of
By Asst. Prof. Bijaya Shrestha, nec

48. GMSK Bit Error Rate The bit error probability for GMSK is given by
where γ is a constant related to BT by
By Asst. Prof. Bijaya Shrestha, nec

49. GMSK Transmitter
A GMSK signal can be generated by passing a NRZ message bit stream through a Gaussian baseband filter followed by an FM modulator.
This modulation technique is used in US Cellular Digital Packet Data (CPDP) system and GSM system.
By Asst. Prof. Bijaya Shrestha, nec

50. GMSK Receiver
GMSK signals can be detected using orthogonal coherent detectors.
By Asst. Prof. Bijaya Shrestha, nec

51. Quadrature Amplitude Modulation (QAM)
In QAM, both the amplitude and phase are varied.
It is possible to code n bits into one symbol such that there can have 2n QAM symbols.
The general form of an M-ary QAM signal can be defined as
where Emin is the energy of the signal with the lowest amplitude, and ai and bi are a pair of independent integers chosen according to the location of the particular signal point.
By Asst. Prof. Bijaya Shrestha, nec

52. Quadrature Amplitude Modulation (QAM)
Let
then,
The coordinates of the ith message point are
and
By Asst. Prof. Bijaya Shrestha, nec

53. Quadrature Amplitude Modulation (QAM)
(ai, bi) is an element of the L by L matrix given by
where
For 16-QAM, the L by L matrix is given as
By Asst. Prof. Bijaya Shrestha, nec

54. Quadrature Amplitude Modulation (QAM)
By Asst. Prof. Bijaya Shrestha, nec

55. Multiple Access Techniques
By multiple access techniques we understand the techniques which enable multiple users to share the common medium.
Frequency Division Multiple Access (FDMA)
Time Division Multiple Access (TDMA)
Code Division Multiple Access (CDMA)
Space Division Multiple Access (SDMA)
By Asst. Prof. Bijaya Shrestha, nec

56. Frequency Division Multiple Access (FDMA)
By Asst. Prof. Bijaya Shrestha, nec

57. Frequency Division Multiple Access (FDMA)
FDMA assigns individual channels to individual users.
Each user is allocated a unique frequency band or channel.
During the period of the call, no other user can share the same channel.
The FDMA channel carries only one phone circuit at a time.
FDMA is usually implemented in narrowband systems.
If an FDMA channel is not in use, then it is a wasted resource.
FDMA has to bear guardband overhead.
FDMA requires tight RF filtering to minimize adjacent channel interference.
By Asst. Prof. Bijaya Shrestha, nec

58. Time Division Multiple Access (TDMA)
By Asst. Prof. Bijaya Shrestha, nec

59. Time Division Multiple Access (TDMA)
TDMA systems divide the radio spectrum into time slots, and in each slot only one user is allowed to either transmit or receive.
Each user occupies a cyclically repeating time slot.
TDMA systems transmit data in a buffer-and-burst method, thus the transmission for any user is noncontinuous.
Unlike in FDMA systems, digital data and digital modulation must be used with TDMA.
It is possible to allocate different numbers of time slots per frame to different users.
By Asst. Prof. Bijaya Shrestha, nec

60. Space Division Multiple Access (SDMA)
By Asst. Prof. Bijaya Shrestha, nec

61. Space Division Multiple Access (SDMA)
SDMA controls the radiated energy for each user in space.
SDMA serves different users by using spot beam antennas.
The different areas covered by the antenna beam may be served by the same frequency (in a TDMA or CDMA system) or different frequencies (in an FDMA system).
With SDMA, all users within the system would be able to communicate at the same time using the same channel.
In addition, a perfect adaptive antenna system would be able to track individual multipath components for each user and combine them in an optimal manner to collect all of the available signal energy from each user.
By Asst. Prof. Bijaya Shrestha, nec

62. Orthogonal Frequency Division Multiplexing (OFDM)
OFDM is a multi-carrier modulation scheme that transmits data over a number of orthogonal subcarriers.
OFDM breaks the data to be sent into small chunks, allocating each sub-data stream to a subcarrier and the data is sent in parallel orthogonal subcarriers.
Splitting the channel into narrowband channels enables significant simplification of equalizer design in multipath environments.
By Asst. Prof. Bijaya Shrestha, nec

63. Orthogonal Frequency Division Multiplexing (OFDM)
By Asst. Prof. Bijaya Shrestha, nec

64. Spread Spectrum Modulation Techniques
Spread spectrum modulation technique uses transmission bandwidth that is several times greater than the minimum required signal bandwidth.
Many users can simultaneously use the same bandwidth without significantly interfering with one another.
Spread spectrum signals are pseudorandom and have noise-like properties.
The spreading waveform is controlled by a pseudo-noise (PN) sequence or pseudo-noise code, which is a binary sequence that appears random but can be reproduced in a deterministic manner by intended receivers.
By Asst. Prof. Bijaya Shrestha, nec

65. Spread Spectrum Modulation Techniques
Spread spectrum signals are demodulated at the receiver through cross-correlation with a locally-generated version of the pseudorandom carrier.
Cross-correlation with the correct PN sequence despreads the spread spectrum signal and restores the modulated message in the narrow band as the original data.
Cross-correlating the signal from an undesired user results in a very small amount of wideband noise at the receiver.
By Asst. Prof. Bijaya Shrestha, nec

66. Spread Spectrum Modulation Techniques
Since each user is assigned a unique PN code which is approximately orthogonal to the codes of other users, the receiver can separate each user based on their codes, even though they occupy the same spectrum at all times.
Since all users are able to share the same spectrum, spread spectrum may eliminate frequency planning, since all cells can use the same channels.
Spread spectrum systems are not affected by multipath fading; the reason being that the delayed versions of the transmitted PN signal will have poor correlation with the original PN sequence.
By Asst. Prof. Bijaya Shrestha, nec

67. Spread Spectrum Modulation Techniques
Pseudo-Noise (PN) Sequences
A pseudo-noise (PN) or pseudorandom sequence is a binary sequence with an autocorrelation that resembles, over a period, the autocorrelation of a random binary sequence and bandlimited white noise.
Although it is deterministic, a pseudonoise sequence has many characteristics that are similar to those of random binary sequences, such as having a nearly equal number of zeros and ones, very low correlation between shifted versions of the sequence, very low crosscorrelation between any two sequences, etc.
By Asst. Prof. Bijaya Shrestha, nec

68. Direct Sequence Spread Spectrum (DS-SS)
A DS-SS system spreads the baseband data by directly multiplying the baseband data pulses with a pseudo-noise sequence that is produced by a pseudo-noise code generator.
Then, the spread spectrum signal is PSK modulated.
A single pulse or symbol of the PN waveform is called a chip.
The received spread spectrum signal for a single user can be represented as
By Asst. Prof. Bijaya Shrestha, nec

69. Direct Sequence Spread Spectrum (DS-SS)
By Asst. Prof. Bijaya Shrestha, nec

70. Direct Sequence Spread Spectrum (DS-SS)
By Asst. Prof. Bijaya Shrestha, nec

71. Direct Sequence Spread Spectrum (DS-SS)
where m(t) is the data sequence and p(t) is the PN spreading sequence.
m(t) and p(t) use NRZ bipolar signaling.
Each symbol in m(t) represents a data symbol with duration Ts.
Each pulse in p(t) represents a chip with duration Tc.
The transitions of the symbols and chips coincide such that the ratio Ts to Tc is an integer.
At the receiver, the received signal passses through the wideband filter and is multiplied by a local replica of the PN code sequence p(t)
By Asst. Prof. Bijaya Shrestha, nec

72. Direct Sequence Spread Spectrum (DS-SS)
If p(t)=±1, then p2(t)=1, and this multiplication yields the despread signal s(t) given by
at the input of the demodulator.
Then the signal s1(t) is demodulated to extract m(t).
By Asst. Prof. Bijaya Shrestha, nec

73. Frequency Hopped Spread Spectrum (FH-SS)
Frequency hopping involves a periodic change of transmission frequency.
A frequency hopping signal may be regarded as a sequence of modulated data bursts with time-varying, pseudorandom carrier frequencies.
The set of possible carrier frequencies is called the hopset.
Hopping occurs over a frequency band that includes a number of channels.
Each channel is defined as a spectral region with a central frequency in the hopset and a bandwidth large enough to include most of the power in a narrowband modulation burst (usually FSK) having the corresponding carrier frequency.
By Asst. Prof. Bijaya Shrestha, nec

74. Frequency Hopped Spread Spectrum (FH-SS)
Data is sent by hopping the transmitter carrier to seemingly random channels which are known only to the desired receiver.
On each channel, small bursts of data are sent using conventional narrowband modulation before the transmitter hops again.
By Asst. Prof. Bijaya Shrestha, nec

75. Frequency Hopped Spread Spectrum (FH-SS)
By Asst. Prof. Bijaya Shrestha, nec

76. Frequency Hopped Spread Spectrum (FH-SS)
By Asst. Prof. Bijaya Shrestha, nec

77. References
Theodore S. Rappaport, Wireless Communications, second edition, Prentice Hall, 2012.
S. Haykin, An Introduction to Analog and Digital Communications, John Wiley & Sons, Inc., 1994.
Subbarayan Pasupathy, “Minimum Shift Keying: A Spectrally Efficient Modulation”, IEEE Communications Magazine, August 1979.
Per Hjalmar Lehne and Frode Bohagen, “OFDM (A) for Wireless Communication”, R & I Research Report, 2008.
By Asst. Prof. Bijaya Shrestha, nec