Block 1 Part 2 Flashcards
Carrier waves (b-r-s)
- basic radio waves that are sinusoidal
time domain representation (h-s-s-v-t)
- how strength of signal varies over time
Frequency domain representation (s-l-f-p-a)
- single spike or line placed at its frequency with peak amplitude
Discrete spectrum (a-p-s-t), (s-r-l-s-f)
- all periodic signals have this
- means spectrum can be represented by line or series of lines at specific frequencies
harmonic (f-c-p-s-m-h-f-f)
- each frequency component of periodic signal is a multiple, known as harmonic, of fundamental frequency f
Fourier series (s-c-j-t-f-f)
series of components that join together to fundamental frequency
Three basic modulation schemes
- Amplitude-shift keying (ASK)
- Frequency-shift keying (FSK)
- Phase-shift keying (PSK)
Desirable characteristics of digital modulation schemes (b-e), (p-e), (p-w-m-f-c), (c-e-s-I)
- bandwidth efficient
- power efficient
- performs well in multipath fading channels
- cost effective and simple to implement
Fourier transform (f-d-r-n-p-s)
- gives frequency domain representation of non-periodic signals
Non-periodic signals (s-c-c-s, c-d-f-c)
- have spectra comprising a continuous spectrum, means continuous distribution of frequency components
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Three basic digital modulation schemes
- amplitude-shift keying (ASK)
- frequency-shift keying (FSK)
- phase-shift keying (PSK)
Desirable qualities when choosing digital modulation scheme
- Bandwidth efficient
- power efficient
- performs well in multipath fading channels
- cost effective and simple to implement
Fourier transform
- gives frequency domain representation of non-periodic signals
- have spectra comprising a continuous spectrum
- means continuous distribution of frequency components
Important inverse relationship between time and frequency domains (n-p-b-r), (d-r-I-b-r)
- the narrower the pulse, the broader is spectrum
- as data rate increases, more bandwidth is required
ASK
- special case of AM
- amplitude of carrier signal is modified
On-off keying (OOK) (s-f-a), (b-t-n), (d-a-d-1-0),
- simplest form of ASK
- either bursts of waves are transmitted or they aren’t
- this done according to whether input data is 1 or 0
- other versions of ASK use differing (non-zero) amplitudes to represent 1 and 0
Advantages of ASK (s-l-c), (s-a-z-c-g)
- simple and low cost to implement
- switching between amplitudes usually done at zero crossings on graph
FSK
- type of frequency modulation
- frequency of carrier signal that is modified
- bursts of data transmitted at one frequency or another
What is an important parameter of FSK
- the frequency separation between the two sinusoids
- this determines the bandwidth of modulated signal
What are advantages of using two frequencies close together in FSK
- reduces the separation of the two signals
- therefore the overall bandwidth
- receiver still ahs to be able to distinguish both signals
- limit to how close signals can be
Minimum-shift keying
- specific case of FSK
- uses smallest possible separation between two sinusoids that still works
Gaussian minimum-shift keying (GMSK)
- used in second generation mobile cellular system
- special case of MSK that uses gaussian pulse
PSK
- most widely use digital modulations scheme
- in BSPK 0 and 1 represented by segments of sinusoids that differ in their phase
- normally separated by half a cycle
- BSPK signal almost as simple to produce as ASK signal but less susceptible to noise
Bit error rate (BER)
- defined as number of bits received in error divided by number of bits transmitted
- low BER desirable
Symbols
- different carrier states known as symbols i.e. 1 and 0
- if more than two possible carrier states, then more than two symbols available
Baud
- refers to number of symbols per second
- one baud is one symbol per second
Two most widely used modulation schemes used in wireless communications
- quadrature phase-shift keying (QPSK)
- quadrature amplitude modulation (QAM)
Quadrature phase-shift keying (QPSK)
-application of complementary pairs of amplitudes to two simultaneous sinusoidal waves differing in phase by one quarter cycle
I and Q waves
- sinusoidal waves (of same frequency) with phase difference of quarter or three quarters of cycle said to be in quadrature phase relationship
- I wave refers to in phase wave or component of these waves
- Q wave refers to quadrature wave or component of two waves
What can I wave and Q wave be thought of
- I wave also thought of as sine wave
- Q wave also thought of as cosine wave
Quadrature relationship
- despite inversion of either or both I and Q waves both wave forms can be modulated using BPSK
- relationship is preserved between them
Orthogonal
- way to describe I and Q waves
- means signals are transmitted simultaneously
- one can be fully recovered at receiver without interference from other one
QPSK data rate
- can achieve twice the data rate of BSPK for given bandwidth
How to increase number of symbols further from QPSK
- amplitude of modulated signal remains fixed but number of different phases carrier wave can take continues to increase
- or combine QPSK with ASK and introduce more amplitude and phases
