modulation methods for medium-access wireless medium access

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Wireless Medium Access Control and CDMA-based Communication

Lesson 01Modulation Methods for Medium-access


Number of signal sources access to wireless medium simultaneously

• Simultaneously transmitted Signals (actually electromagnetic radiation) may interference with each other, when they travel through a medium

• Network has to receive signals from each radio carrier distinctly


Modulation with radio carrier frequency (ies)

• Voice-data or data signals propagate through the medium after modulation

• Wireless station accesses a medium by modulation of radio carrier(s) with the signal symbols


Symbols

• Digitized form of the analog signals• Symbol─ bit(s) prepared for transmission

after encoding of data bits and insertion of the error control and other bits


Instantaneous value of signal amplitude, s(t) at an instant t

• s(t) = S × s0 sin (2π ×c/λc × t + φt0) • s(t) = S × s0 sin (2π × fc × t + φt0)where S is the symbol to be transmitted, 1 or

0


Amplitude shifted keying (ASK) modulation

s0 = A0 when Symbol = 0 (1)s0 = A1 when Symbol = 1 (2)


Frequency shifted keying (FSK) Modulation─ BFSK

f = fc− fs when Symbol = 0 (1)f = fc+ fs when Symbol = 1 (2)


BFSK (binary frequency shift keying) offc when S = 1

• s0(t) = s0/√2 . sin (2π × fc × t + φt0) when S = 0

• s1(t) = s0/√2 . sin (2π × (fc + fm )× t + φt0) when S = 1

• Bandwidth > fc• Harmonics of (fc + 2 fm), (fc + 3 fm ) , (fc +

4 fm ) present


GMSK (Gaussian Minimum shift keying)

• DSP based Gaussian low pass filter• Bandwidth is 2 .fm plus guard band


QPSK

• One of the four possible distinct sequences (00, 01, 10, or 11) transmitted using a specific phase angle of the radio carrier frequency

• Symbols (00, 01, 10, or 11) represent a sequence

• Each symbol actually a sequence of 2 bits


OQPSK

• Each alternate symbol is in the next quadrature

• 90° are added to the phase angle, the second symbol shifts to the next quadrature during transmission


OQPSK

• An OQPSK receiver subtracts the phase angle by 90° and, therefore, receives the signal in the original quadrature and, therefore, also the original second symbol, and so on


Advantage of OQPSK

• In-phase and quadrature signals overlap, because now they are in the same phase quadrant

• Thus, the number of sharp transitions in the signals reduces to half of its original value


OQPSK

• Transmitted envelope smoother as compared to one transmitted through QPSK

• Utilization-efficiency of the bandwidth allotted to a mobile service improves


ππππ/4-QPSK

• A form of QPSK modulation in which the signal phase shifts by 45° after every two symbols


Advantage of ππππ/4-QPSK

• There are no sharp transitions of π in the phase angle

• The number of sharp transitions of the signals halves


ππππ/4-QPSK

• Lesser sharp transitions imply a lower number of significant higher harmonics, lesser bandwidth requirement per channel, and increased utilization of the allotted bandwidth to a wireless service provider

• Bandwidth utilization-efficiency improves


Example

• Symbol sequence 10 00 11 01 to be transmitted after QPSK modulation

• After each successive time interval of T, the phase angles of the transmitted signal s(t), which are 3π/4, –3π/4, –π/4, +π/4 become 3π/4, – π/2, 0°, π /2 in π /4-QPSK


Example

• The π/4-QPSK demodulator at the receiving end subtracts π/4 after each successive bit pair, the original QPSK angles

• 3π/ 4, – 3π / 4, – π/4, + π /4 found and the bits are recovered as 10 00 11 01


Four-bit per symbol 16-QAM method

• Each symbol actually a sequence of 4 bits• Four symbol sequences representing a 16

bits grouped and transmitted by phase shift keying

• One of the 16 possible distinct sequences transmits by a specific phase angle of the radio carrier frequency at a specific amplitude (one of the three values of amplitude s0)


64-QAM

• Two most significant bits for QPSK while reserving the remaining 4 for the 16-QAM signals

• 64-QAM thus transmits 6 symbols (bits) in a sequence

• When the bits are transmitted after 64-QAM, the spectrum bandwidth requirement reduces greatly


64-QAM Example

• For example, assume that a 64-QAM modulated signal is generated and transmitted at 19.2 ksymbol/s

• One of the 64 possible distinct sequences is transmitted at a specific phase angle, frequency, and amplitude

• Six symbols represent a sequence


64-QAM

• The bit transmission rate is 6 ×19.2 ksymbol/s = 115.2 kbps when 64-QAM is transmitted at 19.2 ksymbol/s

• Each symbol actually a sequence of 6 bits• The bandwidth requirement, in this case,

is thus reduced by a factor of 1/6


Summary

• Modulation methods, ASK, FSK, GMSK• QPSK for each symbol representing a pair

of bits• OQPSK• π/4 QPSK• 16 QAM for each symbol representing a

set of 4 bits• 64 QAM 16 QAM for each symbol

representing a set of 4 bits


End of Lesson 01Modulation Methods for Medium-access

modulation methods for medium-access wireless medium access

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