Acces Techniques Unit5

8/3/2019 Acces Techniques Unit5

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Agenda

Multiple Access Concept

FDMA TDMA CDMA

On Board Processing


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Multiple Access Concept


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MULTIPLE ACCESS - 1THE PROBLEM:HOW DO WE SHARE ONE TRANSPONDER BETWEEN SEVERAL EARTH STATIONS?

SatelliteTransponder

f 1 f 2

IT IS AN OPTIMIZATION PROBLEM


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MULTIPLE ACCESS - 2NEED TO OPTIMIZESatellite 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 ATTHE TRANSMITTER

UNIQUE FREQUENCY SLOT FDMA UNIQUE TIME SLOT TDMA UNIQUE CODE CDMA

RECOGNIZE THE UNIQUE FEATURE OFEACH SIGNAL AT THE RECEIVER


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CHANNEL RECOGNITION?FDMA BAND PASS FILTER EXTRACTS SIGNAL IN

THE CORRECT FREQUENCY SLOTTDMA DE-MULTIPLEXER GRABS SIGNAL IN THECORRECT TIME SLOT

CDMA DE-SPREADER OR DE-HOPPER EXTRACTSSIGNAL WITH THE CORRECT CODE

Direct Sequence Frequency-Hopped


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MULTIPLE ACCESS - 3A

Fig. 6.1 (top part) in text


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MULTIPLE ACCESS - 3B

Fig. 6.1 (bottom part) in text


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MULTIPLE ACCESS - 4If the proportion of the resource (frequency,time, code) is allocated in advance, it is called

PRE-ASSIGNED MULTIPLE ACCESS orFIXED MULTIPLE ACCESS If the proportion of the resource is allocated inresponse to traffic conditions in a dynamic

manner it is called DEMAND ASSIGNED MULTIPLE ACCESS - DAMA


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FDMA


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FDMASHARE THE FREQUENCY

TIME IS COMMON TO ALL SIGNALS

DEVELOP A FREQUENCY PLAN FROMUSER CAPACITY REQUESTSTRANSPONDER LOADING PLAN USED TOMINIMIZE IM PRODUCTS

TRANSPONDER LOADING PLAN


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FDMA TRANSPONDER LOADING PLAN

Available transponder bandwidthtypically 27 to 72 MHz

Four medium-sizedFM signals

One large and foursmall digital signals

IMPORTANT TO CALCULATEINTERMODULATION PRODUCTS


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INTERMODULATION INTERMODULATION

WHEN TWO, OR MORE, SIGNALS AREPRESENT IN A CHANNEL, THE SIGNALSCAN MIX TOGETHER TO FORM SOMEUNWANTED PRODUCTSWITH THREE SIGNALS,

1,

2AND

3,

PRESENT IN A CHANNEL, IM PRODUCTSCAN BE SECOND-ORDER, THIRD-ORDER,FOURTH-ORDER, ETC. ORDER OF IM

PRODUCTS


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IM PRODUCT ORDERSecond-order is 1 + 2, 2 + 3, 1 + 3 Third-order is 1 + 2 + 3, 2 1 - 2, 2 2 -

1..Usually, only the odd-order IM productsfall within the passband of the channel

Amplitude reduces as order risesOnly third-order IM products are usuallyimportant 3-IM products very sensitive to small signal

changes. Hence, IM noise can change

sharply with output amplifier back-off


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IM EXAMPLE There are two 10 MHz signals at 6.01 GHz and6.02 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 IM

Parts of band cannot be used because of IMTransponder power is shared amongst carriersPower balancing must be done carefully

Frequencies get tied to routes

Patterned after terrestrial analog telecoms and so does notconfer the full benefit of satellite broadcast capabilities


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TDMA


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TDMASHARE THE TIME

FREQUENCY IS COMMON TO ALL SIGNALS

DEVELOP A BURST TIME PLAN FROMUSER CAPACITY REQUESTSLARGE SYSTEM BURST TIME PLANS CANBE COMPLICATED AND DIFFICULT TOCHANGE

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 FRAMEACCORDING 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 earthstations arrive at the satelliteIN AN ORDERLY FASHION and IN THE CORRECT ORDER


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TDMA - 2

Figure 6.6in the text

NOTE: Correct timing accomplished using Reference Transmission


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TDMA - 3

Figure 6.7in the text

FRAME

Pre -amble

Traffic Burst

Pre-amble in each traffic burst providessynchronization, signaling information

(e/s tx, e/s rx, etc.), and data


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TDMA - 4Timing obtained by

organizing TDMA transmission into frameseach e/s transmits once per frame suchthat its burst begins to leave the satellite ata specified time interval before (or after)

the start of a reference burstMinimum frame length is 125 s

125 s 1 voice channel sampled at 8 kHz


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TDMA - 5Reference burst(s) and pre-amble bits aresystem overhead and earn no revenue

Traffic 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

NP R

n F Number of voice channels

Transmission

bit rate

Bit rate for one voice channel

Number of burstsin a frame

Number of bits in

each pre-amble

Frame period

Equation 6.18

in the text

For INTELSATR = 120 Mbit/sand T F = 2 ms No allowance for guard times


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TDMA - 7 PROBLEM

Delay time to GEO satellite is 120 msTDMA Frame length is 125 s to 2 msThere could be almost 1000 frames on thepath to the satellite at any instant in time

Timing is therefore CRUCIAL in aTDMA system


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LONG TDMA FRAMESTo reduce overhead, use longer frames

125 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 digitizedspeech, 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/ s

Four stations share transponder in TDMA using125 s frames

Pre-amble = 240 bits

Guard 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 time96 bits = 1.6 s

First thing to do: draw theTiming Recovery Diagram

to get a picture of the way theframe 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 earthstation transmit?

ANSWER (a) There are four earth stationstransmitting within the 125 s frame, sowe have


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TDMA EXAMPLE - 4125 s frame gives125 = (4 4 s) + (4 1.6 s) + (4 T 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 interms of 64 kbit/s speech channels?

Answer: 768 (64 kbit/s) voice channels

(b) How many channels can each earthstation transmit?

Answer: 192 (64 kbit/s) voice channels


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TDMA EXAMPLE - 6 What happens in the previous example if weuse an INTELSAT 2 ms frame length?

2 ms = 2,000 s = 4 4 + 4 1.6 + 4 T

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 - 7 With 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/access

2 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 controllingearth station

Distance to satellite varies continuouslyEarth station must monitor position of itsburst within the frame AT ALL TIMES


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TDMA SUMMARY - 1 ADVANTAGES

No intermodulation products (if the fulltransponder is occupied)Saturated transponder operation possibleGood for data

With a flexible Burst Time Plan it willoptimize capacity per connection


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TDMA SUMMARY - 2DISADVANTAGES

ComplexHigh burst rateMust stay in synchronization


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CDMA


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CDMA - 1SHARE TIME AND FREQUENCY

SEPARATION OF SIGNALS IS THROUGHTHE 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 - 2SYSTEM OPERATOR - OR INDIVIDUALPAIRS OF USERS - ASSIGN UNIQUE

SPREADING OR HOPPING CODES TOEACH DUPLEX LINK CDMA IS A SOLUTION FOR SEVERE

INTERFERENCE ENVIRONMENTS,USUALLY AT A CAPACITY LOSSCOMPARED 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 THESAME TIME AND FREQUENCY SIGNALS ARE SEPARATED BY USING A UNIQUE CODE

Codes must be orthogonal so that User

A does not respond to a code intended forUser B Codes are usually very long : PNsequence, Gold, or Kasami codes


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CDMA - 1CDMA CAN BE ONE OF THREE TYPES

Direct 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 FrequencyHoppingWe will concentrate on Direct Sequence


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DIRECT SEQUENCE CDMA - 1

Multiply the information stream (thedata) by a high speed PN codeUse two codes: one for a 1 and onefor 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 isessentially the code

rate from the PNsequence generator


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Narrow-band data Narrow- band data spreadover the full bandwidth

Other spread signals added,filling up the channel with

many noise-like signals

De-spreading process brings thewanted channel out of the noise


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Figure 6.16 in the text

Each incoming bitis multiplied bythe PN sequence

SpreadingSequence


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De-spreadingSequence

Figure 6.18 in the text

Incoming bit-stream multipliedby a synchronized

copy of the PN

sequence


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CDMA SPECTRUM

FLAT - usually below the noise

Code must be compressed (de-spread) toraise the signal above the noiseReceiver must synchronize to a codesequence which is below the noise Requires 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 APPLICATIONSMILITARY

Anti-Jam (AJ)Low Probability of Intercept (LPI)

COMMERCIAL

VSATs (due to wide beams)GPSMicrowave Cellular Systems


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On Board Processing


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SATELLITE REQUIREMENTS - 1

LEO SYSTEM Adapt to rapid movement of the satellitewhich causes

rapid change in pathlength (time of arrival andpower balancing problems)

rapid change in look-angle (multi-path andblockage environment problems)

rapid change in Doppler spread (spectrumbroadening)


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SATELLITE REQUIREMENTS - 2

GEO SYSTEM Adapt to long path length to satellite whichcauses

Large path loss (low EIRP and/or capacityproblem)

Long delay (protocol problem requiring an

emulation or spoofing procedure) Large satellite antenna footprint (frequency re-use problem)

Both GEO and LEO systems now make useof extensive OBP technological approaches


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OBB APPROACHESReceive aggregate uplink channel(s)Detect each (narrow-band) uplink signalProcess each uplink signal so that

errors removedaddress read

Repackage signals into a large TDMstreamTransmit TDM stream

MF- TDMA approach is emerging as the way to go


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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 ADVANTAGESRelatively narrow-band uplink Detection 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 captureof desired return signal at the terminal

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Acces Techniques Unit5