Telecommunications

1
Telecommunications

• AERSP 401B

2
Communication System Designers Goal

• Maximize information transfer
• Minimize errors/interference
• Minimize required power
• Minimize required system bandwidth
• Maximize system utilization
• Minimize cost

3
Useful Relationships

• Decibels
• A logarithmic unit originally devised to express
  power ratios but used today to express a variety
  of other ratios as well
• where P1 and P2 are the two power levels being
  compared

4
Examples

• Loss
• 1,000 watts (P1) 10 watts (P2)
• Gain
• 10 watts (P1) 1,000 watts (P2)

• Telephone cable line

P1
P2
Power in
Power out
The unit decibel was named after Alexander Graham
Bell. The unit originated as a measure of power
loss in one mile of telephone cable. Also,
hearing is based on decibel levels.
20 dB means 100 times more
5
Derived Decibel Units

• The dBm
• Example 20W is what in dBm?
• The dBW
• Example conversions

6
Voltages (examples)
7
Gains and Losses

• Power is gained via amplification and lost via
  absorption or resistance
• Gains and losses are expressed in dB (usually the
  W or m are dropped)

8
Communications Example
Attenuation x dB
Gain b dB
Attenuation y dB
Pin
Gain c dB
Gain a dB
9
Special Values
10
Other Examples

• Sound levels
• If Pref is the sound power resulting in a barely
  audible sound,

11
Radio Frequency Radiation

• RF signals travel at the speed of light in air
  (atmosphere) and space (vacuum)
• c speed of light in vacuum
• 2.998x108 m/sec (186,200 miles/sec)
• Wavelength, ?c/f
• f frequency
• Beam width ? (rad) ? ?/D
• D aperture width or diameter
• Defines how spread out the beam is

12
Half Power

• A 3 dB drop in power represents the half-power
  point

13
Isotropic Radiation

• Aperture area of a receiving or transmitting
  antenna through which all signal is assumed to
  pass.
• If transmitting antenna radiates equally in all
  directions, it is called isotropic
• The fraction of power received from an isotropic
  radiator at a distance, d, is
• where Ar is the aperture area of the receiving
  antenna

14
Isotropic Radiation (contd)

• Receiver is not 100 efficient, so including
  efficiency factor, z,
• Z ? 0.55
• Transmitting antenna designed to focus radiation
  (i.e. not isotropic)
• Can also be expressed in dB

15
Typical Antenna Patterns

• Slot
• Glt 10 dBi

• Dipole
• Glt10 dBi

• Horn
• G10-20 dBi

16
Parabolic Reflector Antenna
parallel beams
focal point

• D diameter
• - wavelength
• z - efficiency

17
Lobes

• Backlobes
• Sidelobes

18
Cassegrain Reflector Antenna
19
Modulation

• Definition
• Altering a signal to make it convey information
  (either analog or digital)
• AM (Amplitude Modulation)
• Changes amplitude (frequency constant)
• FM (Frequency Modulation)
• Changes frequency (amplitude constant)

Frequency modulation
20
Modulation (contd)

• Changing the phase of the signal
• For digital data, these methods are also called
• ASK amplitude shift keying
• FSK frequency shift keying
• PSK phase shift keying

21
Link Budget

• Allocation of various losses and gains in the
  communication link between Earth and the
  spacecraft
• Similar to signal-to-noise ratio, but Eb/No
  pertains to digital data

22
Link Requirements

• For data
• (Eb/No)estimated (Eb/No)required ? 3 dB
• For commands
• (Eb/No)estimated (Eb/No)required ? 20 dB
• This difference is known as the link margin

23
Terms

• P transmitter power
• Ll line loss (between transmitter and antenna)
• Gt transmitter antenna gain
• Ls space loss (inverse square in distance)
• k Boltzmanns constant

• La transmission path loss (atmosphere and rain
  absorption)
• Gr receive antenna gain
• Ts system noise temperature
• R data rate
• Li implementation loss (?-2 dB)

24
More Details
25
More Details
26
More Details
27
More Details

• Calculate Link Margin (Eb/No)est (Eb/No)req

Fig. 13-9, SMAD
Acceptable BER
28
Example

• If acceptable BER (bit error rate) is one bit
  error in every 100,000 bits, then BER10-5
• Using BPSK modulation with Reed/Solomon coding,
  this requires an Eb/No2.5dB
• If BPSK is used without coding, Eb/No9.5dB
• Increase transmitter power by 7 dB
• Multiplicative factor of 100.75
• Increasing the transmitter and receiver antenna
  gains by 7dB (combined)
• Antennas then more sensitive to pointing errors

29
Data Rates

• For each sensor, data rate
• Sample size is determined based upon required
  level of accuracy
• Example temperature sensor needed to monitor
  propellant tank temperature in range -10?C to
  80?C
• Amplitude range80?C-(-10?C) 90?C

30
Data Rates (contd)

• Sensor generates voltage proportional to
  temperature
• A/D converter generates a digitized
  representation of this temperature an n-bit
  word
• Number of quantized levels that are represented
  2n
• Quantization step here

Quantized steps
31
Data Rates (contd)

• Example continued

So, if n8, then quantized step 0.35oC and Eq
0.175oC Typically, one needs to find the required
value of n. Using same example, if required Eq ?
0.05oC, then quantized step 0.1oC and
32
Sample Rate

• Determined based upon estimated rate of change of
  quantity being measured
• Examples
• Thermal sensors typically sample at low rates
  (once per minute)
• Attitude sensors sample at high rates, especially
  during attitude maneuvers (1-5 samples/sec)

33
Sampling Oscillatory Phenomena

• Must sample at 2.2 times the highest frequency
  present
• Human voice has frequency range of 3.5 KHz
• Sample at 7.7 KHz (7,000 samples/second)
• Commercial audio (telephony) requires 8
  bits/sample
• Data rate 7,700 samples/sec x 8 bits/sample
• 62,000 bits/sec (bps)

34
Data Compression

• Compression/encoding allow lower data rates
• Make use of repeated patterns in the data and/or
  transmit only parts of data that changes since
  previous sample
• Voice data can be reduced to 9.6 Kbps
• Compressed video (videophone) 28 Kbps
• Full video with color 256 Mbps (40 Mbps with
  coding)

35
Telemetry

• Packet telemetry format
• Each sensor forms packet of data
• When packet complete, microcomputer interrupts
  main computer
• Main computer formats main block
• Main block transmitted
• Advantages
• Flexible data rates for sensors
• Disadvantages
• Spacecraft processing more complex
• Ground station equipment more complex

36
Error Detection and Correction

• Once our telemetry data is set to transmit, we
  must concern ourselves with possible induced
  errors in the transmission
• With digital data, there are several ways to
  check for errors
• Parity check (with retransmission)
• Error correction (without retransmission)
• Ref Spacecraft Attitude Determination and
  Control, J.R. Wertz (ed), Reidel Publishing Co.,
  1978

37
Parity Check

• Simplest method of detection
• Example
• M 1,1,0,0 original message
• Add another parity bit to M
• M now becomes 1,1,0,0,p
• Even parity scheme
• m1 m2 m3 m4 p even number ?p0
• Odd parity scheme
• m1 m2 m3 m4 p odd number ?p1
• Receiving equipment then checks each message
  vector

38
Parity Check

• Suppose receiving equipment receives
• M 1,1,0,0,1
• If both transmitter and receiver are employing
  even parity scheme, then an error has occurred
• m1 m2 m3 m4 p 3 not an even number
• Receiver requests retransmission
• What if two bits are flipped?
• Parity scheme fails (much lower probability of
  two bit flips than one bit flip)

39
Error Correction without Retransmission

• Example self-correcting developed by Hamming
• Extra set of bits equal in number to the original
  message bits added to message vector
• Before M a,b,c,d
• After M p1,p2,p3,a,p4,b,c,d

40
Hamming (contd)

• Multiply MT by the Hamming matrix,
• to get SHMT (syndrome vector)

41
Hamming (contd)

• Need to determine the values of p1,p2,p3, and p4
• Set these such that S 0,0,0,0T (mod 2)
• (Any even number 0 mod 2)
• Arrangement of parity bits in M so that only one
  new parity bit is involved in each successive
  calculation of p1,p2,p3,p4

42
Hamming Example

• Intended message vector Mo0,0,1,1,1,1,0,0
• Received message vector M10,0,1,1,1,0,0,0
• Correction scheme
• If s4 0, then a, b, c, and d are correct
• If s4 1, then error occurred in message bit
  s1s2s3 (101)25

43
Hamming Example (contd)

• M b0 b1,b2 b3,b4 b5,b6 ,b7
• M10, 0, 1, 1, 1, 0, 0, 0
• Correct M1 is M10, 0, 1, 1, 1, 1, 0, 0
• So the original message data is 1 1 0 0
• 
  a b c d
  

Error
44
Probability of Errors Simple Parity

• If probability of error in 1 bit is 1,
  probability of at least one error in a 4-bit
  message is 4
• Adding one parity bit increases error rate to 5
• Can detect, but not correct this error
• Need to retransmit 5 of the data
• Probability of 0.25 that error occurs in 2 or
  more of the original 4 bits

45
Probability of Errors Hamming Code

• Using the 8-bit Hamming code will increase
  probability of error to 8
• One bit error can be corrected
• Errors in 2 bits of M will occur in 0.64 of
  messages received
• Two bit errors cannot be corrected
• Hamming will detect two errors, so retransmission
  can be requested
• Undetected errors in 3 or more bits will occur in
  0.051 of the messages received.