WIRELESS BACKHAUL. A Primer on Microwave and Satellite Communications. Dr Rowan Gilmore CEO, EM Solutions MILCIS November 2015

Transcription

1 WIRELESS BACKHAUL A Primer on Microwave and Satellite Communications Dr Rowan Gilmore CEO, EM Solutions MILCIS November 2015

2 TUTORIAL OVERVIEW 1. 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

3 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

4 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 o o C = channel capacity = rate of information transmission in bits per second, at zero error rate B = channel s bandwidth 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.

5 RECEIVER PERFORMANCE BANDWIDTH AND SNR SNR and bandwidth can be traded off For two equal capacity channels SNR 2 = SNR 1 (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

6 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

7 QUADRATURE, BI-PHASE, AND 8-PHASE SHIFT KEYED (PSK) MODULATION Q I 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 Diagram courtesy of Agilent Technologies A/N 1298

8 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

9 16-QAM CONSTELLATION AT TRANSMITTER AND RECEIVER 40 Transmitter Constellation IQ[TP.3,50,1,0] 1.5 Receiver Constellation IQ[TP.2,500,1,0]

10 4. E S /N 0 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. de P or E P dt dt For a single tone, symbol energy and average power have the following relationships: - Energy per symbol E S = (Power * Symbol period T S ) = S/R S where R S = symbol rate Now N 0 =noise spectral density (dbm/hz) so E S /N 0 = S /R S N 0 But the total noise power is just the power density within the noise (channel) bandwidth B i.e. N = noise spectral density * channel bandwidth = N 0 *B so N 0 = N /B Therefore (E S /N 0 )= (S /N) *B /R S = (S /N) * (1+a) where a is the filter roll-off factor S /N and thus E S /N 0 are both unit-less (measured in db). E S /N 0 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, E b /N 0 = S /N * BT b (but then you also need to adjust for coding and/or multiplexing of other users)

11 SYMBOL ERROR RATE VS SNR 1 SER CURVES.1 SER Simulation Result 16QAM SER 64QAM SER 256QAM SER Es_N0 Es/No (db)

12 THE NEED FOR GOOD E S /N 0 E S = S/R S, proportional to S/B = signal power spectral density (PSD) Thus lower carrier power S with lower symbol rate maintains the same E S. Two corollaries: o o More complex modulation in same bandwidth, keeping constant power spectral density maintains E s and thus keeps E s /N 0 the same More complex modulation with same data rate, requires less BW for the same data rate, thus requires less power to maintain E s and keep E s /N 0 the same BUT! More bits/symbol requires increasingly higher E s /N 0 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 rate o 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

13 TRADE-OFF BETWEEN MODULATION DENSITY AND SNR Table 13: E S /No performance at Quasi Error Free PER = 10-7 (AWGN channel) Mode Spectral efficiency (bits/symbol) Ideal E S /No (db) for FECFRAME length = QPSK 1/4 0, ,35 QPSK 1/2 0, ,00 QPSK 2/3 1, ,10 QPSK 3/4 1, ,03 QPSK 9/10 1, ,42 8PSK 2/3 1, ,62 8PSK 3/4 2, ,91 8PSK 9/10 2, ,98 16APSK 2/3 2, ,97 16APSK 3/4 2, ,21 16APSK 9/10 3, ,13 32APSK 3/4 3, ,73 32APSK 4/5 3, ,64 32APSK 5/6 4, ,28 32APSK 9/10 4, ,05 From DVB-S2 ETSI standard EN v1.2.1 Example of QPSK 3/4 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 bits/symbol Data rate 124 Mbps Shannon's limit 164 Mbps OR for same data rate use 16APSK ¾ in a 50MHz bandwidth Use 16APSK ¾ in a 50MHz bandwidth for same data rate New Bandwidth New Required Es/No New noise floor New min signal power Es/No = db 50 MHz db (approx 6 db higher) 3 db lower Approx 3 db higher

14 5. LINK BUDGET CALCULATION Effective transmitted power Tx antenna gain START: transmitter output power Effective received signal power Rx antenna gain Receiver noise power Dry-air path loss Theoretical dry-air receiver SNR Maximum allowable rain fade & water vapor loss (3) Implementation loss (2) Measurement point (Rx threshold) Required received SNR for target BER (1) Notes 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

15 ELEMENTS IN THE LINK BUDGET If an isotropic antenna radiates a power P T, 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 = P T /4πD (W/m 2 ) As the transmit antenna focuses the energy (i.e. has a gain), within the beamwidth of the antenna the equation changes to: Flux = G T P T /4πD (W/m 2 ) where G T is known as the transmit antenna gain and G T P T is the Equivalent Isotropically Radiated Power (EIRP)

16 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 P R will be: But: P R = Flux * Ae = [G T P T /4πD 2 ]*Ae Ae = effective aperture of the receive antenna = (λ 2 /4π)*G R λ 2 /4π is the area of a lossless isotropic antenna (which has unity gain). (Note G R is therefore inversely proportional to λ 2 Substituting, P R = [G T P T /4πD 2 ]* (λ 2 /4π)*G R or In db P R = EIRP * G R * (λ/4πd) 2 or 1/f 2 for a constant antenna size) P R (dbw) = EIRP + G R Lo ** This is the link equation **

17 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: o Lo = 20log(D) + 20log(f) db Where: D = distance in km between transmitter and receiver f = frequency in GHz 92.5 db = 20 log {(4π*10 9 *10 3 )/c} For a geostationary satellite, one way loss Lo = db in X-band, db in Ku-band and 213 db in Ka-band. Even though the spreading loss Lo decreases as the square of frequency, G R also decreases by the same amount for a constant antenna size, so G R Lo is independent of frequency in the link equation, and only G T varies.

18 THE CHOICE OF ANTENNA SIZE Antenna gain and beamwidth are inversely related o G R is proportional to effective antenna area A e (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 4pr 2 / (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 1.8 o beamwidth on receive, and 42.5dBi gain and 1.2 o beamwidth on transmit.

19 ANTENNA PATTERNS AND ESD Shadow boundary

20 THE SYSTEM NOISE FLOOR System noise floor at input = FkTB= kt eq B P R = {EIRP + G R Lo} must be sufficiently far above the system noise floor to achieve the desired bit error rate So (in db) SNR margin = P R - kt eq B = EIRP + [G R T eq ]- Lo - kb Therefore, a receiver can be characterised by its G/T ratio For satcoms, use S /No = EIRP + [G R T eq ]- Lo k where k is Boltzmann s constant ( dbw/k/hz) and S/No is in db-hz

21 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!

22 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

23 TRANSMITTER IMPAIRMENTS 1 RX Constellation QAM Constellation DB(IQ(TP.21,1000,1,0)) 64QAM IQ[TP.RX Constellation,400,1,0] 64QAM System 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 regrowth. All impairments degrade the bit error rate

24 RECEIVER IMPAIRMENTS Linear impairments include thermal noise and intersymbol-interference Nonlinear impairments include spurious responses (such as to the image frequency) and reciprocal mixing Downconverted Signal Ideal Oscillator Actual Oscillator Downconverted Interferer Signal Interferer Downconverted Signal Interferer Downconverted Interferer Signal f f f 0 f 0 (b)

25 7. COMMERCIAL RADIO - THE E10G

26

27 MICROWAVE LINK BUDGET 55.9 db db

28 E10G LINK BUDGET EXAMPLE 73.9 dbm Effective transmitted power 55.9 db Tx antenna gain 18 dbm (Linear) START: transmitter output power Overall path loss db dbm Effective received signal power Receiver noise floor = FkTB = 11.6 db -174 dbm/hz + 97 dbhz (5GHz BW) dbm 55.9 db Rx antenna gain Receiver noise power = -65.4dBm dbm Actual receiver SNR 32.9 db Maximum allowable rain fade & water vapor loss Min required receiver SNR for target BER Rx THRESHOLD dbm 9.5 db 23.4 db

29 LINK RESULTS : E10G Assumes a rain rate of 56 mm/hr for the path

30 COMMERCIAL SATCOM - COTM

31 SPECIFICATION Antenna Size RF Frequency G/T mid band Antenna Gain EIRP (linear) Ka-Band Ka-Band Commercial Military 1m Rx 19.2 to 20.2 GHz Rx 20.2 to 21.2GHz Tx 29.0 to 30.0 GHz Tx 30.0 to 31.0GHz Switchable between Commercial and Military operating bands via Ethernet User Interface >20dBK Rx 42dB min Tx 48dB min 60dBW (min) (with EM Solution A 25W Ka Multiband Diamond Series BUC) Polarisation Sidelobes Pointing Error Height (radome) Base Footprint Circular Mil-Std <0.2deg 1500mm 850mm diameter Environmental Tested in accordance to MIL-STD-810G CN1 and IEC 60945:2002 Pedestal Type Tracking Type INU & Gyros Modem Support (three modem ports available) Integrated Inmarsat GX modem or switchable to Customer modem 3 axis Az 360 o continuous EL -20 o to +110 o XEL ±35 o Monopulse on Ka-band Beacon or User Defined Carrier Embedded Compatible with Viasat EBEM MD modem or equivalent Satellite Operator Certifications Inmarsat GX (pending Q1 2016) WGS (pending mid 2016) Regulatory IEC 60945, IEC 60950, C tick

32 SATELLITE LINK BUDGET Uplink Transponder Budget: NOTE: Version: 1.3 Uplink Frequency: 30, 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: db Path Loss: 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: 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= dbw/k/hz Transponder IF Bandwidth: khz Transponder Uplink Input Noise Power 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: 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).

33 SATCOM SYSTEM LINK SUMMARY 0 LINK BUDGET SYSTEM LINK SUMMARY Developed by: Jan A. King, W3GEY/VK4GEY Version: 1.3 NOTE: User Terminal Spacecraft User Terminal EMS On the Move Terminal 2.70 =Loss(dB) Southern Hemisphere Gateway Station I/F Filter B.W.= MHz 2.0 =Loss(dB) End-to-End Gain = db 1.0 =Loss(dB) 1.5 =Loss (db) DIGITAL LINK SUMMARY UPLINK DOWNLINK =Supported Data Rate (Mbps) User Transponder User 15.0 Mbps Xmit 1 =Supported Mbps each HPA Data LNA Converters & HPA LNA Rcvr Terminal IF Amplifiers Terminal 4.3 = Each User Eb/No (db) at Full Supported Data Rate Spacecraft Current Altitude: 36, km Uplink User Slant Range: 37, km HPA Sig. Power = Watts Downlink User Slant Range: 37, km 48.0 = Gain (dbi) 38.3 = Gain (dbi) 44.5 =Gain(dBi) Uplink Frequency: 30, MHz HPA Power = 20.0 Watts K=System Noise Temp =Gain (dbi) K=System Noise Temp. Downlink Frequency: 20, 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) = db In Xpdr B.W.= MHz In Xpdr B.W MHz S/(N+I) = 7.04 db In Chan B.W. = MHz Transponder Power Characteristics: RF Power Output = 20.0 Watts DC Power Input = Watts UPLINK + DOWNLINK Therm. Dissipation = Watts S/(N+I) = 6.52 db

34 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 High power not important Short propagation delay (10-200us) Regulated bands: low interference Unregulated bands possible interference Low terminal costs, low to high spectrum costs Low noise is critical High power is critical Long delay (~250ms round trip) Regulated bands: moderate interference No unregulated bands Medium terminal costs, high spectrum costs

35 QUESTIONS?