1 ec 723 satellite communication systems mohamed khedr khedr
TRANSCRIPT
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EC 723 Satellite Communication Systems
Mohamed Khedrhttp://webmail.aast.edu/~khedr
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Syllabus
Tentatively
Week 1 Overview
Week 2 Orbits and constellations: GEO, MEO and LEO
Week 3 Satellite space segment, Propagation and
satellite links , channel modelling Week 4 Satellite Communications Techniques
Week 5 Satellite Communications Techniques II
Week 6 Satellite Communications Techniques III
Week 7 Satellite error correction Techniques
Week 8 Satellite error correction TechniquesII
Week 9 Satellite error correction TechniquesIII
Week 10 MidTerm Exam
Week 11 Multiple access
Week 12 Presentations
Week 13 Presentations
Week 14 Presentations
Week 15 Presentations
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Interleaving
Convolutional codes are suitable for memoryless channels with random error events.
Some errors have bursty nature:Statistical dependence among successive error events (time-correlation) due to the channel memory.
• Like errors in multipath fading channels in wireless communications, errors due to the switching noise, …
“Interleaving” makes the channel looks like as a memoryless channel at the decoder.
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Interleaving …
Interleaving is done by spreading the coded symbols in time (interleaving) before transmission.The reverse in done at the receiver by deinterleaving the received sequence.“Interleaving” makes bursty errors look like random. Hence, Conv. codes can be used.Types of interleaving:
Block interleavingConvolutional or cross interleaving
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Interleaving …
Consider a code with t=1 and 2 coded bits.A burst error of length 3 can not be corrected.
Let us use a block interleaver 3X3
A1 A2 A3 B1 B2 B3 C1 C2 C3
2 errors
A1 A2 A3 B1 B2 B3 C1 C2 C3
Interleaver
A1 B1 C1 A2 B2 C2 A3 B3 C3
A1 B1 C1 A2 B2 C2 A3 B3 C3
Deinterleaver
A1 A2 A3 B1 B2 B3 C1 C2 C3
1 errors 1 errors 1 errors
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Trellisdiagram forK = 2, k = 2,n = 3convolutionalcode.
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State diagram forK = 2, k = 2, n = 3convolutional code.
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MULTIPLE ACCESS - 1THE PROBLEM:HOW DO WE SHARE ONE TRANSPONDER BETWEEN SEVERAL EARTH STATIONS?
Satellite Transponder
f1 f2
IT IS AN OPTIMIZATION PROBLEM
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MULTIPLE ACCESS - 2NEED TO OPTIMIZE
Satellite capacity (revenue issue)Spectrum utilization (coordination issue)Interconnectivity (multiple coverage issue)Flexibility (demand fluctuation issue)Adaptability (traffic mix issue)User acceptance (market share issue)Satellite powerCost
Very, VERY, rarely a simple optimum; nearly
always a trade-off exercise
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HOW DO YOU SEPARATE USERS?LABEL THE SIGNAL IN A UNIQUE WAY AT THE TRANSMITTER
UNIQUE FREQUENCY SLOT FDMAUNIQUE TIME SLOT TDMAUNIQUE CODE CDMA
RECOGNIZE THE UNIQUE FEATURE OF EACH SIGNAL AT THE RECEIVER
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CHANNEL RECOGNITION?FDMA
BAND PASS FILTER EXTRACTS SIGNAL IN THE CORRECT FREQUENCY SLOT
TDMADE-MULTIPLEXER “GRABS” SIGNAL IN THE CORRECT TIME SLOT
CDMADE-SPREADER OR DE-HOPPER EXTRACTS SIGNAL WITH THE CORRECT CODE
Direct Sequence Frequency-Hopped
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Multiple access techniques: FDMA, TDMA, and CDMA. Note that in the direct sequence form of CDMA shown here, all the channels overlap in both time and frequency.
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MULTIPLE ACCESSIf the proportion of the resource (frequency, time, code) is allocated in advance, it is called PRE-ASSIGNED MULTIPLE ACCESS or FIXED MULTIPLE ACCESSIf the proportion of the resource is allocated in response to traffic conditions in a dynamic manner it is called DEMAND ASSIGNED MULTIPLE ACCESS - DAMA
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FDMA
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FDMA
SHARE THE FREQUENCYTIME IS COMMON TO ALL SIGNALS
DEVELOP A FREQUENCY PLAN FROM USER CAPACITY REQUESTSTRANSPONDER LOADING PLAN USED TO MINIMIZE IM PRODUCTS
TRANSPONDER LOADING PLAN
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FDMA TRANSPONDER LOADING PLAN
Available transponder bandwidth typically 27 to 72 MHz
Four medium-sized FM signals
One large and four small digital signals
IMPORTANT TO CALCULATE INTERMODULATION PRODUCTS
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INTERMODULATION
INTERMODULATIONWHEN TWO, OR MORE, SIGNALS ARE PRESENT IN A CHANNEL, THE SIGNALS CAN “MIX” TOGETHER TO FORM SOME UNWANTED PRODUCTSWITH THREE SIGNALS, 1, 2 AND 3, PRESENT IN A CHANNEL, IM PRODUCTS CAN BE SECOND-ORDER, THIRD-ORDER, FOURTH-ORDER, ETC.ORDER OF IM
PRODUCTS
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IM PRODUCT ORDER
Second-order is 1 + 2, 2 + 3, 1 + 3
Third-order is 1 + 2 + 3, 21 - 2, 22 - 1..Usually, only the odd-order IM products fall within the passband of the channelAmplitude reduces as order risesOnly third-order IM products are usually important3-IM products very sensitive to small signal
changes. Hence, IM ‘noise’ can change sharply with output amplifier back-off
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IM EXAMPLEThere are two 10 MHz signals at 6.01 GHz and 6.02 GHz centered in a 72 MHz transponder2-IM product is at 12.03 GHz3-IM products are at [2(6.01) - 6.02] = 6.00 and [2(6.02) - 6.01] = 6.03 GHz
3-IM products
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FDMA LIMITATIONSIntermods cause C/N to fallBack-Off is needed to reduce IMParts of band cannot be used because of IMTransponder power is shared amongst carriersPower balancing must be done carefully
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FDMA Techniques
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TDMA
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TDMA
SHARE THE TIMEFREQUENCY IS COMMON TO ALL SIGNALS
DEVELOP A BURST TIME PLAN FROM USER CAPACITY REQUESTSLARGE SYSTEM BURST TIME PLANS CAN BE COMPLICATED AND DIFFICULT TO CHANGE
BURST TIME PLAN
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BURST TIME PLAN
timeFrame Time for Burst Time Plan
#1 #2 #3 #N
USERS OCCUPY A SET PORTION OF THE FRAME ACCORDING TO THE BURST TIME PLAN
NOTE: (1) GUARD TIMES BETWEEN BURSTS
(2) LENGTH OF BURST BANDWIDTH ALLOCATED
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TDMA - 1THE CONCEPT:
Each earth station transmits IN SEQUENCE
Transmission bursts from many earth stations arrive at the satelliteIN AN ORDERLY FASHION and IN THE CORRECT ORDER
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TDMA - 2
NOTE: Correct timing accomplished using Reference Transmission
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TDMA - 3FRAME
“Pre-amble”
“Traffic Burst”
Pre-amble in each traffic burst provides synchronization, signaling information
(e/s tx, e/s rx, etc.), and data
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TDMA - 4
Timing obtained byorganizing TDMA transmission into frameseach e/s transmits once per frame such that its burst begins to leave the satellite at a specified time interval before (or after) the start of a reference burst
Minimum frame length is 125 s125 s 1 voice channel sampled at 8 kHz
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TDMA - 5Reference burst(s) and pre-amble bits are system overhead and earn no revenueTraffic bits earn the revenueNeed to minimize system overheadComplicated system trade-off with number of voice (or data) channels, transmission bit rate, number of bursts, etc.
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TDMA - 6
V
T
NPR
n F
Number of
voice channels
Transmission bit rate
Bit rate for one voice channel
Number of bursts in a frame
Number of bits in each pre-amble
Frame period
For INTELSATR = 120 Mbit/s and TF = 2 ms No allowance for guard times
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TDMA - 7
PROBLEMDelay time to GEO satellite is 120 msTDMA Frame length is 125 s to 2 msThere could be almost 1000 frames on the path to the satellite at any instant in time
Timing is therefore CRUCIAL in a TDMA system
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LONG TDMA FRAMES
To reduce overhead, use longer frames125 s frame: 1 Word/Frame500 s frame: 4 Words/Frame2000 s frame:16 Words/Frame
2000 s = 2 ms = INTELSAT TDMA standard
NOTE: 1 Word is an 8-bit sample of digitized speech, a “terrestrial channel”, at 64 kbit/s
8 kHz × 8 bits = 64 kbit/s
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TDMA EXAMPLE - 1Transponder bandwidth =36 MHzBit rate (QPSK) 60 Mbit/s = 60 bits/sFour stations share transponder in TDMA using 125 s framesPre-amble = 240 bitsGuard time = 1.6 s
Assuming no reference burst we have
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TDMA EXAMPLE - 2
#1 #2 #3 #4
FRAME= 125 s
Pre-amble 240 bits= 4 s @ 60 bits/ s
Traffic: N bitslet it = T s
Guard time 96 bits = 1.6 s
First thing to do: draw the Timing Recovery Diagram
to get a picture of the way the frame is put together
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TDMA EXAMPLE - 3WITH THE TDMA EXAMPLE
(a) What is the transponder capacity in terms of 64 kbit/s speech channels?(b) How many channels can each earth station transmit?
ANSWER(a) There are four earth stations transmitting within the 125 s frame, so we have
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TDMA EXAMPLE - 4
125 s frame gives125 = (44 s) + (41.6 s) + (4T s)
Four earth stations, 4 s pre-amble, 1.6 s guard time, T s traffic bits
This gives T = (125 - 16 - 6.4)/4 = 25.65 s60 Mbit/s 60 bits/s, thus 25.65 s = 1539 bitsHence channels/earth station = 1539/8 = 192(.375) 8 bits/word for a voice channel
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TDMA EXAMPLE - 5(a) What is the transponder capacity in terms of 64 kbit/s speech channels?
Answer: 768 (64 kbit/s) voice channels
(b) How many channels can each earth station transmit?
Answer: 192 (64 kbit/s) voice channels
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TDMA EXAMPLE - 6
What happens in the previous example if we use an INTELSAT 2 ms frame length?2 ms = 2,000 s = 44 + 41.6 + 4T
Therefore, T = 494.4 sand, since there are 60 bits/s (60 Mbit/s), we have T 29,664 bits
Remember we have 128 bits for a satellite channel
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TDMA EXAMPLE - 7With 128 bits for a satellite channel we haveNumber of channels/access = 29,664/128
= 231(.75)Capacity has increased due to less overhead
125 s frame 192 channels/access2 ms frame 231 channels/access
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TDMA SYNCHRONIZATION
Start-up requires care!!Need to find accurate range to satellite
Loop-back (send a PN sequence)Use timing information from the controlling earth station
Distance to satellite varies continuouslyEarth station must monitor position of its burst within the frame AT ALL TIMES
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Structure of an Intelsat traffic data burst. A satellite channel is a block of sixteen 8-bit samples from one terrestrial speech channel. Other blocks in the traffic burst are used to synchronize the PSK demodulator, the bit clock, and the frame clock in the receiver (CBTR, UW) and to provide communication links between earth stations (TTY, SC, and VOW). CBTR, carrier and bit timing recovery; UW, unique word; TTY, teletype; SC, satellite channel; VOW, voice order wire.
Carrier and bitTime recovery
Unique word
TelegraphyTelephonyOrder wires
Service channel
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Unique word correlator. The example shown here has a 6 bit unique word for illustration—practical satellite systems use unique words of 24-48 bits. The bits stream from the receiver output is clocked into the shift register serially. When the contents of the shift register match the stored unique word the output of the summer is a maximum and exceeds the threshold, marking the end of the unique word. This provides a time marker for the remainder of the earth station’s transmission.
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TDMA SUMMARY - 1
ADVANTAGESNo intermodulation products (if the full transponder is occupied)Saturated transponder operation possibleGood for dataWith a flexible Burst Time Plan it will optimize capacity per connection
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TDMA SUMMARY - 2
DISADVANTAGESComplexHigh burst rateMust stay in synchronization
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CDMA
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CDMA - 1SHARE TIME AND FREQUENCY
SEPARATION OF SIGNALS IS THROUGH THE USE OF UNIQUE CODES
EACH USER IS ASSIGNED A CODESTATION 1 CODE 1STATION 2 CODE 2
RECEIVER SEARCHES FOR CODESCODE RATE >> DATA RATE
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CDMA - 2
SYSTEM OPERATOR - OR INDIVIDUAL PAIRS OF USERS - ASSIGN UNIQUE SPREADING OR HOPPING CODES TO EACH DUPLEX LINKCDMA IS A SOLUTION FOR SEVERE INTERFERENCE ENVIRONMENTS, USUALLY AT A CAPACITY LOSS COMPARED WITH TDMA AND FDMA
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CDMA - 3
TRANSPONDER BANDWIDTH
POWER
Users #1, #2, #3, and #4
User #N
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CODE DIVISION MULTIPLE ACCESS - CDMA
ALL USERS SHARE THE SAME TIME AND FREQUENCYSIGNALS ARE SEPARATED BY USING A UNIQUE CODE
Codes must be “orthogonal” so that User A does not respond to a code intended for User BCodes are usually very long : PN sequence, Gold, or Kasami codes
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CDMA - 1
CDMA CAN BE ONE OF THREE TYPESDirect Sequence (Spread Spectrum)• Occupies full bandwidth all the time
Frequency Hopping• A pair of frequencies (one for “1” and one for
“0”) hop over the full bandwidth randomly
A hybrid of Direct Sequence and Frequency Hopping
We will concentrate on Direct Sequence
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DIRECT SEQUENCE CDMA - 1
Multiply the information stream (the data) by a high speed PN codeUse two codes: one for a “1” and one for a “0”1 data bit many “Chips”e.g. 2.4 kbit/s 1 Mbit/s
The “Spreading factor” is 400, can think of this as coding
gain
The Chip Rate is essentially the code
rate from the PN sequence generator
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DIRECT SEQUENCE CDMA - 2
Narrow-band dataNarrow-band data “spread”
over the full bandwidth
Other spread signals added, filling up the channel with
many noise-like signals
De-spreading process brings the wanted channel out of the noise
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DIRECT SEQUENCE CDMA - 2
Figure 6.16 in the text
Each incoming bit is multiplied by the PN sequence
Spreading Sequence
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DIRECT SEQUENCE CDMA - 3
De-spreading Sequence
Figure 6.18 in the text
Incoming bit-stream multiplied by a synchronized
copy of the PN sequence
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CDMA SPECTRUM
FLAT - usually below the noiseCode must be compressed (de-spread) to raise the signal above the noiseReceiver must synchronize to a code sequence which is below the noiseRequires the use of a correlator, a generator, and ….. patience
Other users in channel just look like noise
Takes a while to “pull in”
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CDMA APPLICATIONS
MILITARYAnti-Jam (AJ)Low Probability of Intercept (LPI)
COMMERCIALVSATs (due to wide beams)GPSMicrowave Cellular Systems
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MF-TDMA INTERNET S/C
A B C D E
In-bound, downlink TDM stream to the hub
Hub
In-bound, uplink MF-TDMA VSAT bursts
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MF-TDMA ADVANTAGES
Relatively narrow-band uplinkDetection of signal at satellite enables
U/L power control to be exercisedOn-board routing of trafficError detection and correction of the u/l signals
TDM downlink enables relatively easy capture of desired return signal at the terminal