1. Introduction to Mobile Communications
GMU - TCOM 552, Fall 2006
09/18/2006
Introduction to Mobile Communications
TCOM 552, Lecture #3
Hung Nguyen, Ph.D.
18 September, 2006
(C) Hung Nguyen, 2006

2. Outline Channel Capacity Signal-to-Noise Ratio (SNR) Multiplexing
GMU - TCOM 552, Fall 2006
Outline
09/18/2006
Channel Capacity
Signal-to-Noise Ratio (SNR)
Multiplexing
Digital Modulation
Analog Modulation
Coding
Simplex and Duplex Transission
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006
(C) Hung Nguyen, 2006

3. About Channel Capacity
Impairments, such as noise, limit data rate that can be achieved
Channel Capacity – the maximum rate at which data can be transmitted over a given communication path, or channel, under given conditions
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

4. Transmission Impairments
GMU - TCOM 552, Fall 2006
Transmission Impairments
09/18/2006
Signal received may differ from signal transmitted
Analog - degradation of signal quality
Digital - bit errors
Caused by
Attenuation and attenuation distortion
Delay distortion
Noise
Just what happens to signals as they propagate over a medium.
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006
(C) Hung Nguyen, 2006

5. Attenuation Signal strength falls off with distance Depends on medium
GMU - TCOM 552, Fall 2006
Attenuation
09/18/2006
Signal strength falls off with distance
Depends on medium
Received signal strength:
must be enough to be detected
must be sufficiently higher than noise to be received without error
Attenuation is an increasing function of frequency
Attenuation in dB’s, factors of 10 (so 20 dB is 10 twice or 100, 30 is 1000, 10 is 10)
In phone lines higher frequencies attenuate more. They would distort the signal, as in Fig. 3-12a if not equalized.
In vacuum there is not distortion by attenuation, only free space attenuation (1/R^2). In guided media distortion occurs. In air, water vapor absorbs some frequencies more than others, so do the other gases. Typically little absorption up to about 10GHz, gets worse higher, some windows.
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006
(C) Hung Nguyen, 2006

6. Noise (1) Additional EM energy and signals on the receiver
GMU - TCOM 552, Fall 2006
Noise (1)
09/18/2006
Additional EM energy and signals on the receiver
Thermal -- usually inserted by receiver circuits
Due to thermal agitation of electrons
Uniformly distributed
White noise
Intermodulation
Signals that are the sum and difference of original frequencies sharing a medium, and falling within the desired signal’s passband
Most noise is ‘thermal’, at receiver. Signal needs to be about x10 above is so effect is less noticeable, maybe 20 or 30. In dB, dB SNR’s.
Intermod is that signals couple and mix and cause sum and differences in freqs. Also harmonics (X2, X3, X4). Limiting intermod is part of good system design.
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006
(C) Hung Nguyen, 2006

7. Noise (2) Crosstalk Impulse Multipath
GMU - TCOM 552, Fall 2006
Noise (2)
09/18/2006
Crosstalk
A signal from one line or channel is picked up by another
Impulse
Irregular pulses or spikes
e.g. External electromagnetic interference
Short duration
High amplitude
Multipath
See in later Sessions, causes distortions
Coupling between near twisted pair in phone lines.
Impulse is noise spikes, turning something on and the spike getting coupled in. It’s a click or crackle. Good design eliminates most, not all lightning effects.
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006
(C) Hung Nguyen, 2006

8. Signal-to-Noise Ratio
Ratio of the power in a signal to the power contained in the noise that’s present at a particular point in the transmission
Typically measured at a receiver
Signal-to-noise ratio (SNR, or S/N)
A high SNR means a high-quality signal, low number of required intermediate repeaters
SNR sets upper bound on achievable data rate
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

9. Signals and Noise High SNR Lower SNR Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

10. Concepts Related to Channel Capacity
Data rate - rate at which data can be communicated (bps)
Bandwidth - the bandwidth of the transmitted signal as constrained by the transmitter and the nature of the transmission medium (Hertz)
Noise - average level of noise over the communications path
Error rate - rate at which errors occur
Error = transmit 1 and receive 0; transmit 0 and receive 1
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

11. Nyquist Bandwidth For binary signals (two voltage levels)
C = 2B
With multilevel signaling
C = 2B log2 M
M = number of discrete signal or voltage levels
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

12. Shannon Capacity Formula
Equation:
Represents theoretical maximum that can be achieved
In practice, somewhat lower rates achieved
Formula assumes white noise (thermal noise)
Worse when other forms of noise are included
Impulse noise
Attenuation distortion or delay distortion
Interference
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

13. Example of Nyquist and Shannon Formulations
Spectrum of a channel between 3 MHz and 4 MHz ; SNRdB = 24 dB
Using Shannon’s formula
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

14. Example of Nyquist and Shannon Formulations
How many signaling levels are required?
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

15. Multiplexing
Capacity of transmission medium usually exceeds capacity required for transmission of a single signal
Multiplexing - carrying multiple signals on a single medium
More efficient use of transmission medium
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

16. Multiplexing
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

17. Reasons for Widespread Use of Multiplexing
Cost per kbps of transmission facility declines with an increase in the data rate
Cost of transmission and receiving equipment declines with increased data rate
Most individual data communicating devices require relatively modest data rate support
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

18. Multiplexing Techniques
Frequency-division multiplexing (FDM)
Takes advantage of the fact that the useful bandwidth of the medium exceeds the required bandwidth of a given signal --- different users at different frequency bands or subbands
Time-division multiplexing (TDM)
Takes advantage of the fact that the achievable bit rate of the medium exceeds the required data rate of a digital signal --- different users at different time slots
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

19. Frequency-division Multiplexing
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

20. Time-division Multiplexing
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

21. Multiplexing and Multiple Access
Both refer to the sharing of a communications resource, usually a channel
Multiplexing usually refers to sharing some resource by doing something at one site --- e.g., at the multiplexer
Often a static or pseudo-static allocation of fractions of the multiplexed channel, e.g., a T1 line. Often refers to sharing one resource. The division of the resource can be made on frequency, or time, or other physical feature
Multiple Access shares an asset in a distributed domain
i.e., multiple users at different places sharing an overall media, and using a scheme where it is divided into channels based on frequency, or time or another physical feature
Usually dynamic
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

22. Factors Used to Compare Modulation and Encoding Schemes
Signal spectrum
With fewer higher frequency components, less bandwidth required --- Spectrum Efficiency
For wired comms: with no DC component, AC coupling via transformer possible --- DC components cause problems
Transfer function of a channel is worse near band edges -- always better to constrain signal spectrum well inside the spectrum available
Synchronization and Clocking
Determining when 0 phase occurs -- carrier synch
Determining beginning and end of each bit position -- bit sync
Determining frame sync --- usually layer above physical
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

23. Signal Modulation/Encoding Criteria: Demodulating/Decoding Accurately
What determines how successful a receiver will be in interpreting an incoming signal?
Signal-to-noise ratio = SNR
Signal power/noise power
Note: power = energy per unit time
Data rate (R)
Bandwidth (BW)
An increase in data rate increases bit error rate
An increase in SNR decreases bit error rate
An increase in bandwidth allows an increase in data rate
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

24. Factors Used to Compare Modulation/Encoding Schemes
Signal interference and noise immunity ---
Performance in the presence of interference and noise
For a given signal power level, the effect of noise and interference is then labeled the Power Efficiency
For digital modulation, Prob. Of Bit Error = function (SNR) where N includes the interference terms
More exactly, Prob. Bit Error = function (Energy per bit/Noise power density, with noise including interference and other noise like terms) --- see next chart
Cost and complexity
Usually the higher the signal and data rates require a higher complexity and greater the cost
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

25. A Figure of Merit in Communications: Noise Immunity
For digital modulation one bottom line Figure of Merit (FOM) is Probability of Bit Error (Pe) -- Lowest for Most Accurate Decoding of Bit Stream
Prob. Bit Error= function of (Eb/N0)
Many functions for many different modulation and coding types have been computed - usually decreases with increasing Eb/N0
Eb = energy per bit
N0 = noise spectral density; Noise Power N = (N0)* BW
Note: Includes Interference and Intermodulation and Crosstalk
(Eb/N0) is a critically important number for digital comms
Eb/N0 =(SNR)*(BW/R) ---- important formula -- derive it
SNR is signal to noise ratio, a ratio of power levels
BW is signal bandwidth, R is data rate in bits/sec
For analog modulation the FOM is SNR
Signal quality given by subjective statistical scores -- voice: 1-5 (high)
FM requires a lower SNR than AM for the same signal quality
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

26. Basic Modulation/Encoding Techniques
Digital data to analog signal --- Digital Modulation
Amplitude-shift keying (ASK)
Amplitude difference of carrier frequency
Frequency-shift keying (FSK)
Frequency difference near carrier frequency
Phase-shift keying (PSK)
Phase of carrier signal shifted
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

27. Basic Encoding Techniques
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

28. Amplitude-Shift Keying
One binary digit represented by presence of carrier, at constant amplitude
Other binary digit represented by absence of carrier
where the carrier signal is A*cos(2πfct)
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

29. Amplitude-Shift Keying
Susceptible to sudden gain changes
Inefficient modulation technique
On voice-grade lines, used up to 1200 bps
Used to transmit digital data over optical fiber
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

30. Binary Frequency-Shift Keying (BFSK)
Two binary digits represented by two different frequencies near the carrier frequency
where f1 and f2 are offset from carrier frequency fc by equal but opposite amounts
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

31. Binary Frequency-Shift Keying (BFSK)
Less susceptible to error than ASK
On voice-grade lines, used up to 1200bps
Used for high-frequency (3 to 30 MHz) radio transmission
Can be used at higher frequencies on LANs that use coaxial cable
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

32. Multiple Frequency-Shift Keying (MFSK)
More than two frequencies are used
More bandwidth efficient but more susceptible to error
fi = fc + (2i – 1 – M)fd
fc = the carrier frequency
fd = the difference frequency
M = number of different signal elements = 2 L
L = number of bits per signal element
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

33. Multiple Frequency-Shift Keying (MFSK)
To match data rate of input bit stream, each output signal element is held for:
Ts = LT seconds
where T is the bit period (data rate = 1/T)
So, one signal element encodes L bits
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

34. Multiple Frequency-Shift Keying (MFSK)
Total bandwidth required
2Mfd
Minimum frequency separation required 2fd = 1/Ts
Therefore, modulator requires a bandwidth of
Wd = 2L/LT = M/Ts
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

35. Multiple Frequency-Shift Keying (MFSK)
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

36. Phase Shift Keying (PSK)
GMU - TCOM 552, Fall 2006
Phase Shift Keying (PSK)
09/18/2006
The signal carrier is shifted in phase according to the input data stream
2 level PSK, also called binary PSK or BPSK or 2-PSK, uses 2 phase possibilities over which the phase can vary, typically 0 and 180 degrees -- each phase represents 1 bit
can also have n-PSK -- 4-PSK often is 0, 90, 180 and 270 degrees --- each phase then represents 2 bits
Each phase called a ‘symbol’
Each bit or groups of bits can be represented by a phase value (e.g., 0 degrees, or 180 degrees), or bits can be based on whether or not phase changes (differential keying, e.g., no phase change is a 0, a phase change is a 1) --- DPSK
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006
(C) Hung Nguyen, 2006

37. Phase-Shift Keying (PSK)
Two-level PSK (BPSK)
Uses two phases to represent binary digits
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

38. Phase-Shift Keying (PSK)
Differential PSK (DPSK)
Phase shift with reference to previous bit
Binary 0 – signal burst of same phase as previous signal burst
Binary 1 – signal burst of opposite phase to previous signal burst
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

39. Phase-Shift Keying (PSK)
Four-level PSK (QPSK)
Each element represents more than one bit
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

40. Quadrature PSK
More efficient use by each signal element (or symbol) representing more than one bit
e.g. shifts of /2 (90o)
In QPSK each element or symbol represents two bits
Can use 8 phase angles and have more than one amplitude -- then becomes QAM then (combining PSK and ASK)
QPSK used in different forms in a many cellular digital systems
Offset-QPSK: OQPSK: The I (0 and 180 degrees) and Q (90 and 270 degrees) quadrature bits are offset from each other by half a bit --- becomes a more efficient modulation, with phase changes not so abrupt so better spectrally, and more linear
p/4-QPSK is a similar approach to OQPSK, also used
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

41. Multilevel Phase-Shift Keying (MPSK)
Multilevel PSK
Using multiple phase angles multiple signals elements can be achieved
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements or symbols = 2L
L = number of bits per signal element or symbol
e.g., 4-PSK is QPSK, 8-PSK, etc
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

42. Quadrature Amplitude Modulation
QAM is a combination of ASK and PSK
Two different signals sent simultaneously on the same carrier frequency
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

43. Quadrature Amplitude Modulation
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

44. Quadrature Amplitude Modulation (QAM)
The most common method for quad (4) bit transfer
Combination of 8 different angles in phase modulation and two amplitudes of signal
Provides 16 different signals (or ‘symbols’), each of which can represent 4 bits (there are 16 possible 4 bit combinations)
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

45. GMU - TCOM 552, Fall 2006
Quadrature Amplitude Modulation Illustration -- Example of Constellation Diagram
09/18/2006 90 45
135
180
225
270
315
amplitude 1
amplitude 2
Notice that there are 16 circles or nodes, each represents a possible amplitude and phase, and each represents 4 bits
Obviously there are many such constellation diagrams possible --- the technical issue winds up being that as the nodes get closer to each other any noise can lead to the receiver confusing them, and making a bit error
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006
(C) Hung Nguyen, 2006

46. Performance of Digital Modulation Schemes
Bandwidth or Spectral Efficiency
ASK and PSK bandwidth directly related to bit rate
FSK bandwidth related to data rate for lower frequencies, but to offset of modulated frequency from carrier at high frequencies
Determined by C/BW i.e. bps/Hz
Noise Immunity or Power Efficiency: In the presence of noise, bit error rate of PSK and QPSK are about 3dB superior to ASK and FSK ---- i.e., x2 less power for same performance
Determined by BER as function of Eb/N0
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

47. Spectral Performance Bandwidth of modulated signal (BT)
ASK, PSK BT = (1+r)R
FSK BT = 2DF+(1+r)R
R = bit rate
0 < r < 1; related to how signal is filtered
DF = f2-fc = fc-f1
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

48. SPECTRAL Performance Bandwidth of modulated signal (BT) MPSK MFSK
L = number of bits encoded per signal element
M = number of different signal elements
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

49. BER vs.. Eb/N0 In Stallings Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

50. BER vs.. Eb/N0 (cont’d) In Stallings Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

51. Power-Bandwidth Efficiency Plane
From Bernard Sklar
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

52. Analog Modulation Techniques
Analog data to analog signal
Also called analog modulation
Amplitude modulation (AM)
Angle modulation
Frequency modulation (FM)
Phase modulation (PM)
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

53. AM Modulation & Demodulation
Top left: source (baseband) signal to be modulated;
Bottom left: modulated signal, carrier lines inside white;
Right: demodulated after it is transmitted and received (note after 1.e-3 similarity except for attenuation)
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

54. FM Modulation & Demodulation
Input Voice and Received Voice after Transmission and Reception, Using FM --- Only a Little Noise -- Notice Similarity
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

55. Input Voice and Received Voice after Transmission and Reception, Using FM
--- Lots More Noise in Channel -- Notice that Received Signal is NOT What Was Transmitted
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

56. Amplitude Modulation Amplitude Modulation
cos2fct = carrier
x(t) = input signal
na = modulation index
Ratio of amplitude of input signal to carrier
a.k.a double sideband transmitted carrier (DSBTC)
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

57. Spectrum of AM signal
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

58. Amplitude Modulation Transmitted power
Pt = total transmitted power in s(t)
Pc = transmitted power in carrier
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

59. Single Sideband (SSB) Variant of AM is single sideband (SSB)
Sends only one sideband
Eliminates other sideband and carrier
Advantages
Only half the bandwidth is required
Less power is required
Disadvantages
Suppressed carrier can’t be used for synchronization purposes
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

60. Angle Modulation Angle modulation Phase modulation
Phase is proportional to modulating signal
np = phase modulation index
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

61. Angle Modulation Frequency modulation
Derivative of the phase is proportional to modulating signal
nf = frequency modulation index
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

62. Angle Modulation
Compared to AM, FM and PM result in a signal whose bandwidth:
is also centered at fc
but has a magnitude that is much different
Angle modulation includes cos(f (t)) which produces a wide range of frequencies
Thus, FM and PM require greater bandwidth than AM
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

63. Angle Modulation Carson’s rule The formula for FM becomes where
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

64. Coding
Encoding sometimes is used to refer to the way in which analog data is converted to digital signals
e.g., A/D’s, PCM or DM
Source Coding refers to the way in which basic digitized analog data can be compressed to lower data rates without loosing any or to much information -- e.g., voice, video, fax, graphics, etc.
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

65. Coding (cont’d)
Channel coding refers to signal transformations used to improve the signal’s ability to withstand the channel propagation impairments --- two types
Waveform coding --- transforms signals (waveforms) into better ones --- able to withstand propagation errors better --- this refers to different modulation schemes, M-ary signaling, spread spectrum
Forward Error coding (FEC), also called Sequence coding, transforms data bits sequences into those that are less error prone, by inserting redundant bits in a smart way -- e.g., block and convolutional codes
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

66. Basic Encoding Techniques
Analog data to digital signal
Used for digitization of analog sources
Pulse code modulation (PCM)
Delta modulation (DM)
After the above, usually additional processing done to compress signal to achieve similar signal quality with fewer bits --- called source coding
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

67. Analog to Digital Conversion
Once analog data have been converted to digital signals, the digital data:
can be transmitted using NRZ-L
can be encoded as a digital signal using a code other than NRZ-L
can be modulated to an analog signal for wireless transmission, using previously discussed techniques
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

68. Pulse Code Modulation Based on the sampling theorem
Each analog sample is assigned a binary code
Analog samples are referred to as pulse amplitude modulation (PAM) samples
The digital signal consists of block of n bits, where each n-bit number is the amplitude of a PCM pulse
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

69. Pulse Code Modulation
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

70. Pulse Code Modulation
By quantizing the PAM pulse, original signal is only approximated
Leads to quantizing noise
Signal-to-noise ratio for quantizing noise
Thus, each additional bit increases SNR by 6 dB, or a factor of 4
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

71. Delta Modulation Analog input is approximated by staircase function
Moves up or down by one quantization level () at each sampling interval
The bit stream approximates derivative of analog signal (rather than amplitude)
1 is generated if function goes up
0 otherwise
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

72. Delta Modulation
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

73. Delta Modulation Two important parameters
Size of step assigned to each binary digit ()
Sampling rate
Accuracy improved by increasing sampling rate
However, this increases the data rate
Advantage of DM over PCM is the simplicity of its implementation
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

74. Source Coding Voice or Speech or Audio
Basic PCM yields 4 KHz*2 samples/Hz*8 bits/sample=64 Kbps -- music/etc up to 768 Kbps
Coding can exploit redundancies in the speech waveform -- one way is LPC, linear predictive coding --- predicts what’s next, sends only the changes expected
RPE and CELP (Code Excited LPC) used in cell phones, using LPC, at rates of 4 to 9.6 to 13 kbps
Graphics and Video: e.g., JPEG or GIF, MPEG
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

75. Reasons for Growth of Digital Modulation and Transmission
Cheaper components used in creating the modulations and doing the encoding, and similarly on the receivers
Best performance in terms of immunity to noise and in terms of spectral efficiency --- improved digital modulation and channel coding techniques
Great improvements in digital voice and video compression
Voice to about 8 Kbps at good quality, video varies to below 1 Mbps provide increased capacity in terms of numbers of users in given BW
Dynamic and efficient multiple access and multiplexing techniques using TDM, TDMA and CDMA, even when some larger scale Frequency Allocations (FDMA) -- labeled as combinations
Easier and simpler implementation interfaces to the digital landline networks and IP
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

76. Duplex Modes
Duplex modes refer to the ways in which two way traffic is arranged
One way vs. two way:
Simplex (one way only),
Half duplex (both ways, but only one way at a time),
Duplex (two ways at the same time)
If duplex, question is then how one separates the two ways
In wired systems, it could be in different wires (or cables, fibers, etc)
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006

77. Duplex Modes (cont’d)
FDD, frequency division duplex. Both wired and wireless one way is to separate the two paths in frequency. If two frequencies, or frequency bands, are separate enough, no cross interference
Cellular systems are all FDD
It’s clean and easy to do, good performance, but it limits channel assignments and is not best for asymmetric traffic
TDD is time division duplex, same frequencies are used both ways, but time slots are assigned one way or the other
Good for asymmetrical traffic, allows more control through time slot reassignments
But strong transmissions one way could interfere with other users
Mostly not used in cellular, but 3G has one such protocol, and low tier portables also
Hung Nguyen, TCOM 552, Fall 2006
09/18/2006