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Satellite Communications

Multiple Access (Ch.6)

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Two way communicationInbound or forward link: The communication link

from the subscriber to the service provider via satellite.Outbound or reverse link: The communication linkfrom the service provider to the subscriber via satellite.

----- inbound link

Out bound link

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Multiple Access ?Multiple access is a technique whereby avariable number of users can access acommon resource for the purpose of

communications. Or Share the TransmissionResource i.e. Radio Spectrum

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Satellite Multiple AccessThe ability of the satellite to carry many signals at the same time isknown as multiple access.It is also called multiple destination because transmissions from eachearth station are received by all the other earth stations in the systemMultiple access allows the communication capacity of the satellite tobe shared among a large number of earth stations, and toaccommodate the different mixes (voice, video, data, facsimile) ofcommunication traffic that are transmitted by earth stationSuch signals can be sent through the same satellite using multipleaccess and multiplexing techniquesMultiplexing is the process of combining a number of signalsinto a single signal at one location, so that it can processedby a single amplifier or transmitted over a single radiochannel

Multiplexing is part of multiple access capability of all satellite systems

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Where do you find multiple access used?

Multiple access is employed in most wireless

systems, particularly in satellite systems and cellularsystems.The users interface with the common resource (i.e.,the satellite transponder) via an air interface at thephysical layerSimilar to multiplexing in a wire-line system

Aim is to maximize system capacity thru dynamicresource allocation and spectrum reuse

Multiple Access 1

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Why do you need multiple access?

Multiple access has many advantages Increases efficiency for provider Reduces costs to user

Enhances network control Enables more flexible designs

Multiple Access 2

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Design ImportanceThe designer of a satellite communication system must make decisionsabout the form of multiple access to be usedThe multiple access technique used will influence:

the system capacity the system flexibility the system costs

the ability to earn revenueBasic problem in any multiple access system is how to permit achanging group of earth stations to share a satellite such that

Capacity is maximized Bandwidth is used efficiently Flexibility is maintained Cost to user is minimized Revenue to operator is maximized Should allow for changing patterns of traffic over satellite life

time(10-15 years)

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How do you achieve multiple access?

We must be able to separate users from eachother inside the common resourceThree primary techniques Use a unique frequency (FDMA) flexible

and simple Use a unique time slot (TDMA) popular Use a unique code (CDMA) highly secure

Multiple Access 3

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FDMA Band pass filter extracts signal in the correct

frequency slot (band)

TDMA De- multiplexer captures signal in the correct

time slot

CDMA Direct sequence (DS) or frequency hopped (FH) De-spreader or de-hopper extracts signal with

the correct code

Multiple Access 4

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Frequency Division Multiple Access(FDMA)

Satellite frequency is already broken into bands, and is broken in tosmaller channels in Frequency Division Multiple Access (FDMA).Each user transmits all of the time, but on a different frequency Overall bandwidth within a frequency band is increased due to frequencyreuse (a frequency is used by two carriers with orthogonal polarization)

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Time Division Multiple Access(TDMA)

Each user transmits at the same frequency, but not at the sametime

TDMA breaks a transmission into multiple time slots, each onededicated to a different transmitter

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Code Division Multiple Access(CDMA)

In CDMA, Each user use the same carrier frequency and

may transmits simultaneously (i.e. at the and time)

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Code Division Multiple Access(CDMA)-3D View

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More MATsFDMA can be performed in two ways:If the resources (frequency, time, code) are allocated in advance, it ispre-assigned multiple access or fixed assignment multiple access(FAMA) OR The sub-channel assignments are of a fixedallotment. Ideal for broadcast satellite communication.In case of Pre-Assigned System, a given number of available voice-band channels from each earth station are assigned to a dedicateddestination .Some-times wastage of Precious BW Resource If the resources are allocated in response to changing trafficconditions in a dynamic manner it is demand assigned multipleaccess (DAMA) or The sub-channel allotment based ondemand. Ideal for point to point communication.In case of Demand-Assigned System, Resources allocation is on needbasis, versatile and efficient usages of Radio Spectrum, but aComplex Mechanism is required at all Earth Stations/Users

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Difference between multiplexing &multiple Access

Multiplexing is sharing of resources on links inside the network i.e. corenetwork. (The links between the network elements (NEs) of serviceprovider or between two service providers). Multiple Access is sharingof resources on the access part of the network.

The main difference between TDM and TDMA (also FDM/FDMA, etc) isthat with TDM (also FDM, etc.) the signals multiplexed (i.e. sharing aresource) come from the same node, whereas for TDMA (also FDMA,etc.) the signals multiplexed come from different sources/transmitters.

MULTIPLEXING (dial up internet) Multiple Access (satellite internet)

http://en.wikipedia.org/wiki/File:Multipexing_demultiplexing_scheme_en.svg

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FDMA (1)Satellite frequency is already broken into bands, and is broken in tosmaller channels in Frequency Division Multiple Access (FDMA).Users share the bandwidth (i.e. the frequency available) within thecommon resource

Time is common to all signals

Overall bandwidth within a frequency band is increased due tofrequency reuse (a frequency is used by two carriers with orthogonalpolarization). Need to develop a frequency plan , either from userrequests or from market forecasts.The number of sub-channels is limited by three factors:

Thermal noise (too weak a signal will be effected by backgroundnoise).Intermodulation noise (too strong a signal will cause noise).Crosstalk (cause by excessive frequency reusing).

The transponder in the satellite requires a loading plan to minimizeintermodulation (IM) product

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FDMA (2): Transponder LoadingPlan

Available transponder bandwidthtypically 27 to 72 MHz

Four medium-sizedFM signals

One large and foursmall digital signals

Important to Calculate IntermodulationProducts

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Intermodulation ?It is an unwanted amplitude modulation of signalscontaining two or more different frequencies in asystem with non-linearities.The intermodulation between each frequencycomponent will form additional signals atfrequencies that are often at sum and differencefrequencies of the original frequencies.Sometimes filtering can remove the IM products,but if they are within the bandwidth of thetransponder they cannot be filtered out.

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Intermodulation Products (1)

Intermodulation (IM) When two, or more, signals are present in a channel,

the signals can mix together to form someunwanted products

With three signals, 1, 2 and 3, present in achannel, IM products can be second-order, third-order, fourth-order, etc.

E 1a1 cos

1t

E 2a1

cos 2t Non-linearsystem

V i(t ) V o(t )

Linear summation

gain a1Small-signal

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IM Product Order

First-order is 1, 2, 3, and 4 Second-order is 1 + 2, 1 + 3, 1 + 4, 2 + 3, Third-order is 1 + 2 + 3, 1 + 2 - 3 , 2 1 - 2, 2 2 - 1..Terms falling inside the amplifier bandwidth are important

Usually, only the odd-order IM products fall within thepassband of the channel. First order terms are the desiredsignal.

Amplitude reduces as order risesMost important are third-order IM (3IM) products , such

as

IM terms are:

i + j - k , 2 i - j With two carriers

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IM Product Order

Third-order IM is important because thirdorder IM products have frequencies closeto the signals that generate the inter-modulation, and are within thetransponder bandwidth

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Intermodulation Distortion(Mathematical)

Intermodulation distortion is relevant to devices that handlemultiple frequencies.Consider an input signal with two frequencies 1 and 2:

vi = A cos 1t + B cos 2t

Non-linearity in the device function is represented byvo = a 0 + a 1 v i + a 2 v i2 + a 3 v i3 neglecting higher order terms

In this model, a 1 represents the linear component of the system, which wouldnormally be dominant, a 2 represents the square-law component and so on.Considering the system input to consist of two carriers of frequencies 1 and 2 and of amplitudes E 1 /a 1 and E 2 /a 1.

Therefore, device output isvo = a 0 + a 1 (A cos 1t + B cos 2t) DC and fundamental

+ a 2 (A cos 1t + B cos 2t) 2 2nd order terms+ a 3 (A cos 1t + B cos 2t) 3 3 rd order terms

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Problems to SolveDerive the following:

vo = a 0 + a 1 (A cos 1t + B cos 2t) + a 2 [ A 2 (1+cos 1t)/2 + AB cos ( 1+ 2)t +

AB cos ( 1 2)t + B 2 (1+cos 2t)/2 ] + a3 (A cos 1t + B cos 2t) 3

Hint: Use the identity: cos cos = [cos( + ) + cos( )] / 2Simplify a 3 (A cos 1t + B cos 2t) 3

linear component (a 1) gives rise to just two output signals at

frequencies 1 and 2; the square-law component (a 2) givesrise to four output signals at frequencies 2 1, 2 2 and 1 2; the cube-law component (a 3) gives rise to six outputsignals at frequencies 3 1, 3 2, 2 1 2 and 1 2 2 andso on.

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Two-Tone Distortion ProductsOrder for distortion product mf

1 nf

2 is |m| + |n|

Nunber of distortion products Frequencies

Order Harmonic Intermod. Total Harmonic Intrmodulation

2 2 2 4 2f 1 , 2f 2 f 1 + f 2 , f 2 f 1

3 2 4 6 3f 1 , 3f 2 2f 1 f 2 , 2f 2 f 1

4 2 6 8 4f 1 , 4f 2 2f 1 2f 2 , 2f 2 2f 1 , 3f 1 f 2 , 3f 2 f 1

5 2 8 10 5f 1 , 5f 2 3f 1 2f 2 , 3f 2 2f 1 , 4f 1 f 2 , 4f 2 f 1

6 2 10 12 6f 1 , 6f 2 3f 1 3f 2 , 3f 2 3f 1 , 5f 1 f 2 , 5f 2 f 1 ,4f 1 2f 2 , 4f 2 2f 1

7 2 12 14 7f 1 , 7f 2 4f 1 3f 2 , 4f 2 3f 1 , 5f 1 2f 2 , 5f 2 2f 1 ,6f 1 f 2 , 6f 2 f 1

N 2 2N 2 2N Nf 1 , Nf 2 . . . . .

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Intermodulation Products (2):Example

Amplifier bandwidth is from 10 to 11 GHzTwo carriers exist in the amplifier, one at 10.5GHz and the other at 10.6 GHz

At what frequency will the third order inter-modulation product appear at? Answer:

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Intermodulation Products (3):Example (cont.)

The two third-order IM products are 10.4 and 10.7GHz We therefore have:

Why is this a problem?

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IM can cause in-band interferenceIf a new user now accesses the amplifier at 10.4 or 10.7 GHz,the new signals fall right on top of the intermodulation signalsSo the intermodulation signals will interfere with them & producecross talk.

Intermodulation Products (4):Example (cont.)

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Intermodulation Products (5) Extremely important to calculate where intermodulationproducts will fall

Avoid regularly spaced carrier signalsSatellite systems have very complicated software to calculateIM productsIM products become more severe as an amplifier becomesnon-linear/output of transponder (amplifier) increases towardssaturationTo achieve linearity, amplifiers should not be run at their

maximum saturation (rated) powerReducing the output power increases life. Process is calledbacking off an amplifierThe output power of an operating transponder is related

to its saturated output power by output backoff

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IM ExampleSuppose 3 carriers are carrying three different signals inFDMA mode & pass through HPA of transponder. One carrieris at 1 MHz, second at 2 MHz & third at 3 MHz. Due to non-linear properties of HPA the 1 & 2 MHz frequencies will

produce two intermodulation products i.e. 2-1 = 1 MHz & 2+1= 3 MHz . As there are valid signals already present at 1 &3MHz so the intermodulation signals will interfere with them &produce cross talk.To avoid inter modulation products the carrier power of eachsignal in FDMA must be reduced before passing through HPA.This is called back off.

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Amplifiers (1)

In the transponder(amplifier),the output power level is controlled by the input powerWhen the transponder is operated at output back-off,t he input power is reduced by theinput back-off

The non-linearity of the transponder causes the input and output back-off values to beunequal

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Saturated output power Manufacturers usually specifySmall signal gain

This is the gain of the amplifier when it isoperating in its linear region

1 dB compression point

This is the point 1 dB down from the linear gainline

Amplifiers (2)

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Amplifiers (3)

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As in the previous example, care must betaken when interpreting the statedcharacteristics

Summary: For 55 W maximum power read 46 W For linear gain read 35 W maximum output

power If there is more than one carrier, power is

divided up as ratio of occupied bandwidth

Amplifiers (4)

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Amplifiers (5): Multi-CarrierExample

Amplifier has linear output power of 50 W,and bandwidth of 50 MHzTwo signals, occupying 20 MHz and theother 30 MHzFind: Output power of each of the signals?

Answer: The 30 MHz signal uses 30 W The 20 MHz signal uses 20 W

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FDMA Advantages

Simplest to implementGenerally less supervisory control required

Can achieve lowest bandwidth and powerrequirementsCheapest

Quickest customer acceptance

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FDMA LimitationsIM can cause C/N to fallBack-off is needed to reduce IMParts of band cannot be used because of IM

Transponder power is shared amongst carriersPower balancing must be done carefullyFrequencies get tied to routesRequires guard bands b/w the frequency bandsto reduce adjacent channel interference.

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TDMA (1)With TDMA, only one carrier uses the transponder at any onetime, and therefore, IM products, which result from the nonlinearamplification of multiple carriers, are absent.TDMA (Time Division Multiple Access) breaks a transmission intomultiple time slots, each one dedicated to a different transmitter.Users share the time i.e. users to access the whole channelbandwidth for a fraction of the time, called slot , on a periodicbasis

Frequency is common to all signals

Because the signal information is transmitted in bursts traffic(variable bit rate) , TDMA is only suited to digital signals. Digitaldata can be assembled into burst format for transmission and re-assembled from the received bursts through the use of digitalbuffer memories

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TDMA concept Basic TDMA concept: stations transmit bursts in sequence. Burst

synchronization is required, one station is assigned solely for thepurpose of transmitting reference bursts to which the others canbe synchronized.

TDMA using a reference station for burst synchronization

The time interval from the startof one reference burst to thenext is termed a frame . A framecontains the reference burst Rand the bursts from the otherearth stations, these beingshown as A, B, and C.

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TDMA (2)

Develop a burst traffic time plan from usercapacity requestsLarge system burst traffic time plans canbe complicated and difficult to change

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TDMA (3): Burst Traffic Time Plan

TDMA plans first organize the users into acommon frame structureFrame is fixed length in time

Receiver information required to detect start ofthe frame

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TDMA (4): Burst Traffic Time Plan(cont.)

Frame is not completely filled (as it is inTDM)Guard bands are left between eachpayload element and traffic is organizedinto a set sequence the burst time planEvery user must be synchronized in timeTiming is crucial

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Users occupy a set portion of the frame according tothe burst time planNote:

Guard band times between bursts Length of burst bandwidth allocated

TDMA (5): Burst Traffic Time Plan(cont.)

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TDMA (6): Schematic

Frame consists of four payload pulses

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TDMA (7)

Frame header contains control informationPreamble in each traffic burst providessynchronization information , signaling information,and data

Minimum frame length is 125 s 125 s 1 voice channel sampled at 8 kHzMaximum frame length is determined by trafficand system requirements

Examples 2 ms for INTELSAT, 120 Mbps TDMA 90 ms for Iridium

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TDMA (8): Synchronization

Best if range between user and commonresource is accurately known at all timesDistance between users and commonresource varies continuouslyUser and common resource must monitorposition of burst within the frame at alltimes

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TDMA (9): ExampleTransponder bandwidth = 36 MHzBit rate (QPSK) 60 Mbit/s = 60 bits/ sFour stations share transponder in TDMA using 125

s framesPreamble = 240 bitsGuard time = 1.6 sFind:

What is the transponder capacity in terms of 64kbit/s speech channels? How many channels can each earth station

transmit?

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TDMA (10): Example (cont.)

240 bits /60bits per s =4 s

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TDMA (11): Example (cont.)Solution There are four earth stations transmitting within the 125 s frame,

so we have125 s frame gives125 = (4 4 s) + (4 1.6 s) + (4 T s)

T = (125 - 16 - 6.4)/4 = 25.65 s is the period of data transmissionfor each earth station per frame60 Mbit/s 60 bits/ s, thus 25.65 s = 1539 bits per earth stationper frame.1 frame is sent every 125 s, i.e., 8000 frames are sent per second.

Voice channels: 8000 words per second, 8 bits per wordHence channels/earth station = 1539/8 = 192words/frame/earth station= 192 voice channels/earth station

8 bits/word for a voice channel

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TDMA (12): Example (cont.)(a)What is the transponder capacity in terms of 64

kbit/s speech channels?

Answer: 4x 192 = 768 (64 kbit/s) voice channels

(b)How many channels can each earth stationtransmit?

Answer: 192 (64 kbit/s) voice channels (users perframe)

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What happens in the previous example if we usean 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), wehave T 29,664 bits

Remember we have 128 bits for a satellite channel

TDMA (13): Example (cont.)

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With 128 bits for a satellite channel we haveNumber of channels/access = 29,664/128

= 231Capacity has increased due to less overhead

125 s frame 192 channels/access

2 ms frame 231 channels/access

TDMA (13): Example (cont.)

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TDMA Advantages

No intermodulation products (if full bandwidth ofthe common resource is occupied) meansincreased efficiency Saturated transponder operation possible

A flexible burst traffic time (variable bit rate) planoptimizes capacity per connection No guard bands required for wideband system.There are advantages in digital transmission

techniques. Ex: error correctionTDMA is more amenable to digital transmission (storage, processing, rate-conversion etc.) thanFDMA

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ComplexEach user must transmit at a commonburst rate that is much higher than usersrequired rateMust stay in synchronizationRequires complicated channel equalizationin mobile systems

TDMA Disadvantages

CDMA (1)

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CDMA (1)In CDMA, All user use the same carrier frequency and may

transmits simultaneously (i.e. at the and time) i.e. shareboth time and frequency Separation of signals is through the use of unique codes

(technically, they are mutually orthogonal)

Many users can simultaneously use the same bandwidthwithout significantly interfering with one anotherEach user is assigned a spreading code

Station 1 code 1

Station 2 code 2

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CDMA (2):Orthogonal Codes

( )

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CDMA (3)CDMA is a wireless communications technology that uses theprinciple of spread spectrum communication.Spread spectrum system uses transmission bandwidth (BSS )larger than the signal bandwidth (B). i.e. (BSS >> B) In CDMA, user data is multiplied with a pseudo-noise (PN)sequence, called the spreading signal.This causes the spectrum to be spread to several orders ofmagnitude greater than minimum signal bandwidth. Code rate (or chip rate) >> data rateThe Chip Rate is essentially the code rate from the PN

sequence generator

CDMA (4) PN S

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CDMA (4):PN SequencePN sequence is a binary sequence that appears randombut can be reproduced in a deterministic manner byintended receiver. i.e. The information signal isdemodulated at the R r by cross correlation with a locallygenerated replica of the users PN sequence. (Cross-correlation with PN sequence of other users results in a

very small noise at the R r

)The rate of PN code called the chip rate , must be muchhigher than the rate of the information signal. Code rate(or chip rate) >> data rate

CDMA (5)

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CDMA (5) The signal is de-spread and is restored to the originalsignal

Spreading factor = Processing Gain (G) = Chip Rate/Date Rate = f c/f i

Where f c is the chipping frequency (the bit rate of the PN code), f c isInformation Frequency (the bit rate of the digital data).The higher G batter system performance with lower interference.

G2 indicates the number of possible codes.Not all of the codes are orthogonal.

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There are two ways to spread the bandwidth of thesignal:Direct sequence-CDMA (DS-CDMA): Occupies fullbandwidth all the time

Frequency hopped-CDMA, FH-CDMA: A pair offrequencies (one for 1 and one for 0) hopover the full bandwidth randomly.

CDMA (6)

DS CDMA

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DS-CDMA

DS CDMA

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DS-CDMA

DC CDMA S l O i

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DC-CDMA: Spectral Occupation The spectrum of the carrier c(t), of power C and

frequency f c is given by:

Spectrum is broadenedby the spreading ratioR c/R b . This is the resultof combining themessage with the chipsequence.

DS-CDMA: Realization of Multiple

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DS-CDMA: Realization of Multiple Access

Received signal at the ES is the sum of wanted carriertogether and all other carriers c i(t) of the (N-1) otherusers (i = 1, 2, .. (N-1))

If the codes have low cross-correlation function, then thesecond term (which is like noise) will be very small and

can be neglected.

CDMA (7): DSSS Generation

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CDMA (7): DSSS GenerationPN sequence +1, +1, +1, -1, +1, -1, -1 is used to spread theincoming bits -1 and +1.

+1 in the original bit stream would be transmitted by the chipstream: +1, +1, +1, -1, +1, -1, -1 and -1 in the original stream istransmitted by the chip stream: -1, -1, -1, +1, -1, +1, +1

CDMA (8): DSSS Receiver

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CDMA (8): DSSS ReceiverThe original bit stream can be recovered at the receiver if we multiplythe received stream by a synchronized copy of the PN (Pseudo-

random) sequence, which was used at the transmitter.

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CDMA (9): DSSS Spectrum

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CDMA (10): DSSS Spectrum (cont.)

Flat - usually below the noiseCode must be compressed (de-spread) toraise the signal above the noiseReceiver must synchronize to a codesequence which is below the noiseRequires the use of a generator andcorrelator

DS CDMA

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DS-CDMAThe (S/N) output in the spread spectrum receiver after the correlator

(S/N) out = (C/N) SS + 10 log 10 (G)

In DS-CDMA, number of CDMA signals present at the input of eachreceiver, it is usually the unwanted (interfering) CDMA signal as noise.If the receiver has an input containing Q input signals, each at a powerlevel C watts, and R r thermal noise power is N t, the (C/N) in for thewanted signal,

(C/N) in = 10 log 10 [C/(N t + (Q-1) x C)] dB

where (N t + (Q-1) x C) watt is the total noise at R rinputTerm (Q-1) x C = I watt is power of (Q-1) interfering CDMA signals

(S/N) out = 10 log 10 [C/( Nt + (Q-1) x C)] + 10 log 10 (G) dBIf Q is a large number, [C/( Nt + (Q-1) x C )] (Q-1) x C) watts(S/N) out = 10 log 10 [C/( Nt + (Q-1) x C)] + 10 log 10 (G) = 10log 10 [M/(Q-1)] dB

If Q is also large such that M>>1 (S/N) out = 10 log 10 [M/Q] dB

The bit rate of each signal isR b = R c/N = B/ [N x (1+ )]

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FH CDMA

Coherent detection at the receiver will result in:

Second term is eliminated by the low pass filter.Spectral Occupation:

Three types of systems can be considered:One frequency hop per information bit R H = R bSeveral frequency hops per information bit R H R bOne frequency hop covers several bits R H R b

FH CDMA

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FH CDMA e.g. R H R b, R H is the hop rate

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FH CDMA

The various network carriers follow differenttrajectories on the grid. Only the carrier whosetrajectory coincides with that regenerated by thelocal synthesizer will be demodulated.

at the output of the low pass filter -------- m(t) plusnoise caused by ci(t)= c which has a smallprobability.The spectrum spread factor is large and is equal to(B/b).

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FH-CDMAFH-CDMA is based on FDM system in which anindividual users transmission is spread out over anumber of channels over timeChannel choice is varied in a PN fashion. If the carrier ischanged every symbol than it is referred as a fast FH

system , if it is changed every few symbols it is slowFH system .

Hopping means thefrequencies that aresent hop randomlyamongst a large setof frequencies=>

spread signal

FH-Spread Spectrum

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p pSuppose we have decided to haveeight hopping frequencies. In thiscase, M is 8 and k is 3. The PN codegenerator will create eight different3-bit patterns. These are mapped toeight different frequencies in thefrequency table.

The pattern for this station is 101,111,...,100. The pattern ispseudorandom it is repeated aftereight hoppings. This means that athopping period 1, the pattern is101. The frequency selected is700 kHz; the source signalmodulates this carrier frequency

The IInd k -bit pattern selected is 111, which selects the 900-kHz carrier; the eighth pattern is 100, the frequency is 600 kHz.

After eight hoppings, the pattern repeats, starting from 101

again.

TH = Hop Period

FH Spread Spectrum

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FH-Spread Spectrum Hop rate = R H

N is the number of possible carrier frequencies

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DS-CDMA Capacity

The capacity of a system is approximated by :

is the maximum number of simultaneous calls

is the processing gain

is the total signal to noise ratio per bitis the inter-cell interference factor.

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CDMA (5):DSSS/FHSS Hybrid

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CDMA (11): Applications

Military Anti-jam (AJ) Low probability of intercept (LPI)Commercial

VSATs (due to wide beams) GPS Microwave cellular systems (IS-95, IS-

95B, IS-2000, WCDMA)

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