WIRELESS BACKHAULA Primer on Microwave and

Satellite Communications

Dr Rowan Gilmore

CEO, EM Solutions

MILCIS November 2015

TUTORIAL OVERVIEW1. The physical layer – the radio air interface

2. Shannon’s equation

3. The air interface– digitising the analog world using modulation

4. Signals in noise

5. The link budget and its components

6. System imperfections

7. Some example link budgets of commercial radios

1. THE PHYSICAL LAYER

Characterise the channel by its bandwidth B and noise power N added to the received signal power S

Diagram courtesy of Agilent Technologies A/N 1298

2. CHANNEL CAPACITY (WITH AVERAGE POWER CONSTRAINT)

• Shannon’s equation predicts the capacity of a communications channel at zero error rate:

• Capacity = B * log 2

(1 +SNR) where

o C = channel capacity = rate of information transmission in bits per second, at zero error rate

o B = channel’s bandwidth

o SNR assumes additive, white, gaussian noise

• Example

o What is the theoretical maximum transmission capacity down a telephone line (B=3.4kHz) for which SNR = 30dB ?

• In practice, sophisticated error detection and coding are required to approach the theoretical Shannon limit of zero errors.

RECEIVER PERFORMANCEBANDWIDTH AND SNR

• SNR and bandwidth can be traded off

• For two equal capacity channels SNR2

= SNR1

(B1/B2)

• If we increase the channel bandwidth, SNR can be lower for the same information transfer rate

o Example: A signal has SNR of 20dB. How much can the SNR decrease if the bandwidth is doubled?

• A relatively small increase in channel bandwidth buys a large advantage in terms of reduced SNR and minimum transmission power

o In spite of a corresponding increase in the noise floor

3. SIGNAL MODULATION

Amplitude and frequency/phase modulation may be expressed

as polar coordinates (magnitude and phase) of the signal

phasor, relative to a constant carrier

Diagram courtesy of Agilent Technologies A/N 1298

QUADRATURE, BI-PHASE, AND 8-PHASE SHIFT KEYED (PSK) MODULATION

QPSK has four allowed

states corresponding to the four

combinations of a pair of bits. All

possible transitions between the

states are permitted.

• What are the differences

between BPSK and 8PSK?

• What are the advantages of

each?

Typical signal constellations for

different modulation schemes

I

Q

Diagram courtesy of Agilent Technologies A/N 1298

QUADRATURE AMPLITUDE MODULATION (QAM)

m-QAM is spectrally efficient as it achieves a lower baud rate for the same bit

rate. It may require more power to achieve greater differentiation between

adjacent states. 256-QAM uses eight bits per symbol, but the symbols are very

close together so distortion and noise must be minimised to avoid bit errors.

QPSK accepts lower SNR than 256-QAM to achieve the same bit error rate. Diagram courtesy of Agilent Technologies A/N 1298

16-QAM CONSTELLATION AT TRANSMITTER AND RECEIVER

-40 -20 0 20 40

Transmitter Constellation

-40

-20

0

20

40

IQ[TP.3,50,1,0]

-1.5 -0.5 0.5 1.5

Receiver Constellation

-1.5

-0.5

0.5

1.5IQ[TP.2,500,1,0]

4. ES/N0 AND S/N (SNR) RATIOS• Symbol Energy Es (Joules) : the notional average energy in each symbol.

• Average signal power P =S : the average power in dBm or dBW. Power is the symbol energy expended per unit time i.e.

• For a single tone, symbol energy and average power have the following relationships:

- Energy per symbol ES = (Power * Symbol period TS) = S/RS where RS = symbol rate

• Now N0=noise spectral density (dBm/Hz)

so ES /N0= S /RS N0

• But the total noise power is just the power density within the noise (channel) bandwidth B i.e. N = noise spectral density * channel bandwidth = N0*B so N0 = N /B

• Therefore (ES /N0)= (S /N) *B /RS = (S /N) * (1+a) where a is the filter roll-off factor

• S /N and thus ES /N0 are both unit-less (measured in dB).

• ES /N0 is a measure of relative signal strength and its lower bound determines the modulation that can be supported. It does not directly depend on the bandwidth.

• A similar expression exists for bit energy, Eb /N0= S /N * BTb (but then you also need to adjust for coding and/or multiplexing of other users)

ordE

P E P dtdt

SYMBOL ERROR RATE VS SNR

2 4 6 8 10 12 14 16 18 20

Es_N0

SER CURVES

.0001

.001

.01

.1

1

SE

R

Simulation Result

16QAM SER

64QAM SER

256QAM SER

Es/No (dB)

THE NEED FOR GOOD ES/N0• ES = S/RS , proportional to S/B = signal power spectral density (PSD)

• Thus lower carrier power S with lower symbol rate maintains the same ES . Two corollaries:

o More complex modulation in same bandwidth, keeping constant power spectral density maintains Es and thus keeps Es/N0 the same

o More complex modulation with same data rate, requires less BW for the same data rate, thus requires less power to maintain Es and keep Es/N0 the same

• BUT! More bits/symbol requires increasingly higher Es/N0 or SNR for error-free detection of a symbol (see previous slide)

• Coding can be used to improve error correction at the expense of data rateo Code Rate is the ratio of data bits to data + check bits (Rate=k/n) e.g. rate ¾ code has

75% data and 25% check bits in each code block

TRADE-OFF BETWEEN MODULATION DENSITY AND SNRTable 13: ES/No performance at Quasi Error Free PER = 10-7 (AWGN channel)

ModeSpectral efficiency

(bits/symbol)Ideal ES/No (dB) for FECFRAME

length = 64 800

QPSK 1/4 0,490243 -2,35

QPSK 1/2 0,988358 1,00

QPSK 2/3 1,322253 3,10

QPSK 3/4 1,487473 4,03

QPSK 9/10 1,788612 6,42

8PSK 2/3 1,980636 6,62

8PSK 3/4 2,228124 7,91

8PSK 9/10 2,679207 10,98

16APSK 2/3 2,637201 8,97

16APSK 3/4 2,966728 10,21

16APSK 9/10 3,567342 13,13

32APSK 3/4 3,703295 12,73

32APSK 4/5 3,951571 13,64

32APSK 5/6 4,119540 14,28

32APSK 9/10 4,453027 16,05

From DVB-S2 ETSI standard EN 302 307 v1.2.1

Symbol Rate (Rs) 83.3 Msps

Filter rolloff factor (α) 1.2

Bandwidth = (1 + α)Rs 100 MHz

Modulation format QPSK 3/4

Required Es/No 4.03 dB

Required SNR 3.24dB

Es/No=SNR(1+α)

Spectral efficiency 1.487 bits/symbol

Data rate 124 Mbps

Shannon's limit 164 Mbps

Example of QPSK 3/4

OR for same data rate use 16APSK ¾ in a 50MHz bandwidth

Use 16APSK ¾ in a 50MHz bandwidth for same data rate Es/No = 10.21 dB

New Bandwidth 50 MHz

New Required Es/No10.21 dB

(approx 6 dB higher)

New noise floor 3 dB lower

New min signal power Approx 3 dB higher

5. LINK BUDGET CALCULATIONNotes

1. Required SNR (e.g. 9.5 dB) for target BER (1x10-11) and given data rate More powerful digital coding= lower required SNR, but more excess latency

2. Implementation loss (e.g.6 dB)includes estimated losses due to phase noise, clock jitter, imperfect equalization, synchronization inaccuracy, nonlinear effects, multipath delay spread, residual diffraction loss

3. Loss due to water vapor and rain fade : calculations use ITU-R models. If rain fade exceeds allowable maximum, required received SNR is not achieved and hop is unavailable

START: transmitter output power

Effective transmitted power

Tx antenna gain

Rx antenna gain

Dry-air path loss

Effective received signal power

Receiver noise power

Implementation loss (2)Theoretical

dry-air receiver

SNR

Maximum allowable rain fade & water vapor loss (3)

Required received SNR for target BER (1)

Measurement point (Rx threshold)

ELEMENTS IN THE LINK BUDGET• If an isotropic antenna radiates a power PT, the beam power

will spread as a sphere in which the antenna is the center. The power flux at a distance “D” from the transmission point is given by the equation.

Flux = PT/4πD2. . . . . (W/m2)

• As the transmit antenna focuses the energy (i.e. has a gain), within the beamwidth of the antenna the equation changes to:

Flux = GTPT/4πD2. . . . . (W/m2)

where GT is known as the transmit antenna gain and GTPT is the Equivalent Isotropically Radiated Power (EIRP)

ELEMENTS IN THE LINK BUDGET• As a receiver antenna 'collects' the signal, the amount of 'collected' signal will

depend on the receiver antenna size Ae. The received power PR will be:

PR = Flux * Ae = [GTPT/4πD2 ]*Ae

• But:

Ae = effective aperture of the receive antenna

= (λ2/4π)*GR

• λ2/4π is the area of a lossless isotropic antenna (which has unity gain).

(Note GR is therefore inversely proportional to λ2 or 1/f2 for a constant antenna size)

• Substituting,

PR = [GTPT/4πD2 ]* (λ2/4π)*GR or

PR = EIRP * GR * (λ/4πD) 2

• In dB

PR (dBW) = EIRP + GR – Lo ** This is the link equation **

ELEMENTS IN THE LINK BUDGET• The expression [4πD/λ]2 is known as the basic free space loss

or spreading loss Lo. The basic free space loss is expressed in decibels as:

Lo = 20log(D) + 20log(f) + 92.5 dBo Where:

D = distance in km between transmitter and receiver

f = frequency in GHz

92.5 dB = 20 log {(4π*109*103)/c}

• For a geostationary satellite, one way loss Lo = 201.5 dB in X-band, 206.5 dB in Ku-band and 213 dB in Ka-band.

• Even though the spreading loss Lo decreases as the square of frequency, GR also decreases by the same amount for a constant antenna size, so GR –Lo is independent of frequency in the link equation, and only GT varies.

THE CHOICE OF ANTENNA SIZE• Antenna gain and beamwidth are inversely related

o GR is proportional to effective antenna area Ae (in fact, [D/l]2)

o Area illuminated by the transmit antenna is proportional to (r * q) 2

where q = beamwidth and r =radius of the “isotropic sphere”, so the transmit antenna gain is proportional to 4pr2/ (r * q) 2

o So the beamwidth is inversely proportional to sqrt (gain) i.e. diameter

• At E-band (73-86GHz) , a 600mm antenna has about 50dBi gain and 0.5 o beamwidth. A 1200 mm antenna has 56 dBi gain and 0.25 o beamwidth

• At Ka-band (20/30GHz), a 600 mm antenna has 40 dB gain and 1.8o beamwidth on receive, and 42.5dBi gain and 1.2o

beamwidth on transmit.

ANTENNA PATTERNS AND ESD

Shadow boundary

THE SYSTEM NOISE FLOOR• System noise floor at input = FkTB= kTeqB

• PR = {EIRP + GR – Lo} must be sufficiently far above the system noise floor to achieve the desired bit error rate

• So (in dB) SNRmargin = PR - kTeqB

= EIRP + [GR –Teq]- Lo - kB

• Therefore, a receiver can be characterised by its G/T ratio

• For satcoms, use S /No = EIRP + [GR –Teq]- Lo –kwhere k is Boltzmann’s constant (-228.6 dBw/K/Hz) and S/No is in dB-Hz

PATH LOSS AND FADE MARGIN• Rain attenuation varies with path length

and frequency e.g.

o Ka-band 30GHz (uplink) 5.5dB/km in 30mm/hr (“heavy”) rain – BNE, SYD

o E-band attenuation is around 12dB/km

• Availability of a link is a complex function of how often rain and multipath cause the Rx signal to drop below the threshold SNR

o Determined by all the terms in the link budget!

6. SYSTEM IMPERFECTIONS

• The antenna receives all signals but must transmit only the desired signal

• Poor linearity (gain compression) and system noise can degrade the BER performance

o Often requires operating at output power backoff from maximum EIRP to prevent distortion

TRANSMITTER IMPAIRMENTS

-1 -0.5 0 0.5 1

RX Constellation

-1

-0.5

0

0.5

1

IQ[TP.RX Constellation,400,1,0]

64QAM System

• All impairments degrade the bit error rate

Spectral re-growth is caused by third-order (+ higher) intermodulation distortion in the transmitter

Constellation showing transmitter PA compression and thermal noise (L), and phase noise (R). Gain compression causes intermodulation distortion and spectral re-growth.

-1.5 -0.5 0.5 1.5

64QAM Constellation

-1.5

-0.5

0.5

1.5DB(IQ(TP.21,1000,1,0))64QAM

RECEIVER IMPAIRMENTS

Actual Oscillator

f0

Ideal Oscillator

Interferer

SignalInterferer

Downconverted

Interferer

Downconverted

Signal

f

f0

f

Downconverted

Interferer

Downconverted

Signal

Signal

(b)

• Linear impairments include thermal noise and intersymbol-interference• Nonlinear impairments include spurious responses (such as to the image

frequency) and reciprocal mixing

7. COMMERCIAL RADIO - THE E10G

MICROWAVE LINK BUDGET

162.3 dB

55.9 dB

E10G LINK BUDGET EXAMPLE

START: transmitter output power

Effective transmitted power

Tx antenna gain

Rx antenna gain

Overall path loss

Effective received signal power

Receiver noise power

Actual receiver

SNR

Maximum allowable rain fade & water vapor loss

Min required receiver SNR for target BER

Rx THRESHOLD

18 dBm(Linear)

55.9 dB

73.9 dBm

162.3 dB

-65.4 dBm

55.9 dB

-32.5 dBm

9.5 dB

23.4 dB

Receiver noise floor = FkTB = 11.6 dB -174 dBm/Hz + 97 dBHz (5GHz BW) = -65.4dBm -88.4 dBm

32.9 dB-55.9 dBm

LINK RESULTS : E10G

Assumes a rain rate of 56 mm/hr for the path

COMMERCIAL SATCOM - COTM

SPECIFICATIONKa-Band

Commercial

Ka-Band

Military

Antenna Size 1m

RF Frequency

Rx 19.2 to 20.2 GHz

Tx 29.0 to 30.0 GHz

Rx 20.2 to 21.2GHz

Tx 30.0 to 31.0GHz

Switchable between Commercial and Military operating bands via

Ethernet User Interface

G/T mid band >20dBK

Antenna GainRx 42dB min

Tx 48dB min

EIRP (linear)60dBW (min)

(with EM Solution 01-360A 25W Ka Multiband Diamond Series BUC)

Polarisation Circular

Sidelobes Mil-Std-188-164

Pointing Error <0.2deg

Height (radome) 1500mm

Base Footprint 850mm diameter

Environmental Tested in accordance to MIL-STD-810G CN1 and IEC 60945:2002

Pedestal Type

3 axis

Az 360o continuous

EL -20o to +110o

XEL ±35o

Tracking TypeMonopulse on Ka-band Beacon or

User Defined Carrier

INU & Gyros Embedded

Modem Support (three modem ports

available)

Integrated Inmarsat GX modem or switchable to Customer modem

Compatible with Viasat EBEM MD-1366 modem or equivalent

Satellite Operator Certifications Inmarsat GX (pending Q1 2016) WGS (pending mid 2016)

Regulatory IEC 60945, IEC 60950 , C tick

SATELLITE LINK BUDGETUplink Transponder Budget: NOTE: Version: 1.3

Uplink

Frequency: 30,000.00 MHz

Parameter: Value: Units: Comments:

Ground Station:

User Uplink Transmitter Power Output: 20.0 wattsThis is the power associated with ONE uplinking user station and ONE channel.

In dBW: 13.0 dBW

Transmission Line Losses: -1.5 dB

Connector, Filter or In-Line Switch Losses: -1.0 dB

Antenna Gain: 48.0 dBiC1m antenna

Ground Station EIRP: 58.5 dBWGround Station Effective Isotropic Radiated Power (EIRP) [EIRP=Pt x Ltl x Ga]

Ground Station Antenna Pointing Loss: -0.2 dB

Uplink Path:

Antenna Polarization Losses: -0.75 dB

Path Loss: -213.5 dB

Atmospheric (Gaseous) Losses: -3.1 dBUse Value Appropriate for Elevation Angle Selected in Orbit Performance W/S. See Ippolito.

Ionospheric Losses: -0.2 dB

Rain (Ice Fog) Losses: 0.0 dB

Isotropic Signal Level at the Spacecraft: -159.2 dBW

Spacecraft:

Spacecraft Rcvr Antenna Pointing Loss: -0.4 dB

Spacecraft Rcvr Antenna Gain: 38.3 dBiC

Spacecraft Transmission Line Losses: -2.0 dB

Spacecraft LNA Noise Temperature: 250 K

Spacecraft Sky Temperature: 250 K

Spacecraft Effective Noise Temperature: 520 K

Spacecraft Figure of Merrit (G/T): 8.1 dB/K

S/C Signal-to-Noise Power Density (S/No): 77.1 dBHz Boltzman's Constant: k= -228.6 dBW/K/Hz

Transponder IF Bandwidth: 36000.0 kHz

Transponder Uplink Input Noise Power -125.9 dBWPn = kTB; Additive White Gaussian Noise (AWGN); The satellite receiver's White Noise.

Single User Uplink S/N in Transponder Bandwidth: 1.6 dBThis is the S/N for ONE user seen at the S/C Rcvr IF, measured after the BPF, in the bandwidth determined by that filter.

Single User S(N+I) in Transponder Bandwidth: 1.5 dBThis is the uplink performance measured in the ENTIRE transponder bandwidth (NOTE: This could be a negative number)

Single User Signal Bandwidth: 10000.0 kHz

Single User Uplink S(N+I) in User Terminal Bandwidth: 7.04 dBTHE BOTTOM LINE FOR THE UPLINK (NOTE: This is the average S/(N+I), not the peak value).

SATCOM SYSTEM LINK SUMMARYNOTE:

Developed by: Jan A. King, W3GEY/VK4GEY Version: 1.3

EMS On the Move Terminal 2.70 =Loss(dB) Southern Hemisphere Gateway Station

I/F Filter B.W.= 36.00 MHz

2.0 =Loss(dB) End-to-End Gain = 136.3 dB 1.0 =Loss(dB)

1.5 =Loss (dB)

15.00

15.0 Mbps 1 =Supported Users @ 15.00 Mbps each

4.3 = Each User Eb/No (dB) at Full Supported Data Rate

Spacecraft Current Altitude: 36,000.00 km

Uplink User Slant Range: 37,627.45 km

HPA Sig. Power = 11.57 Watts Downlink User Slant Range: 37,557.85 km

48.0 = Gain (dBi) 38.3 = Gain (dBi) 44.5 =Gain(dBi)

Uplink Frequency: 30,000.00 MHz

HPA Power = 20.0 Watts 520.0 K=System Noise Temp. 46.1 =Gain (dBi) 326.7 K=System Noise Temp. Downlink Frequency: 20,000.00 MHz

58.5 = EIRP (dBW) 8.1 dB/K=G/T 56.4 = Signal EIRP (dBW) 17.8 dB/K=G/T

UPLINK DOWNLINK

S/(No+Io)= 77.0 dB-Hz S/(N+I) = 1.55 dB S/(No+Io)= 86.0 dB-Hz S/(N+I) = 15.98 dB

In Xpdr B.W.= 36.00 MHz In Xpdr B.W.= 10.00 MHz

S/(N+I) = 7.04 dB

In Chan B.W. = 10.00 MHz

20.0 Watts

200.0 Watts

180.0 Watts S/(N+I) = 6.52 dB

0

SYSTEM LINK SUMMARY

Data

Transponder Power Characteristics:

Therm. Dissipation =

=Supported Data Rate (Mbps)

LINK BUDGET

RF Power Output =

UPLINK + DOWNLINKDC Power Input =

UPLINK DOWNLINK

UserXmit

TerminalHPA LNA HPA

TransponderConverters &IF Amplifiers

LNA

UserRcvr

Terminal

User Terminal Spacecraft User Terminal

DIGITAL LINK SUMMARY

GENERAL COMPARISON OF TERRESTRIAL AND SATELLITE COMMUNICATIONS

MICROWAVE SATELLITE

Elevation is horizontal Elevation low up to Vertical

1-50 km per link ~ 36,000 km per link

To/fro frequencies typically close (Up/Down) frequencies typically well separated

Typical band plans (6GHz, 11GHz, 18GHz, 80GHz) 4/6GHz, 7/8GHz, 12/14GHz, 20/30GHz

Low power, path limited High power, power limited

Fade margins 20-50dB Margins (3-5dB typical)

Single hop paths Two hop path

Symmetrical data rates typical Asymmetrical data rates typical

Point to point Point - point or point to multi - point

Fixed terminals Fixed or mobile terminals

Small to medium antennas (0.5 to 2m) Small to very large antennas (0.5 to 20m)

Low noise not critical Low noise is critical

High power not important High power is critical

Short propagation delay (10-200us) Long delay (~250ms round trip)

Regulated bands: low interference Regulated bands: moderate interference

Unregulated bands – possible interference No unregulated bands

Low terminal costs, low to high spectrum costs Medium terminal costs, high spectrum costs

QUESTIONS?

• Rowan.gilmore@emsolutions.com.au