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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