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In practice, Shannon's limits in He above table are not reached because: I) He mode] assumes only additive "white" noise and an overwise perfect channel over than a bandwidth limit, and 2) because Shannon's theory only states "an arbitrary small error rate" and does not quantitatively identify BER as a function of SNR and over impairments. In practice, this relationship is usualRy modeled and verified by measurement. In He real world, many additional channel impairments exist: I. -Channel distortions of bow amplitude and lime delay as a function of frequency; 2. Impulse noise (i.e., short term spikes); 3. MulUpa~ in wireless and echo in wire; 4. Periodic interferences (e.g., 60 Hz power line); 5. Frequency offset and phase jitter in modulation techniques; 6. 7. Attenuation (frequency independent); and Nonlineanties in amplifiers and data acquisition converters (A/D, D/A converters). Communication system designers and equipment designers have developed many techniques to accommodate these impairments and achieve reliable communications over various communication mediums. As each communication medium, or channel, has unique impairment charactenstics and combinations, the details of He specific solution are medium dependent and accommodated by equipment manufacturer designs and specifications. For ITS system-level design, He important parameters are supportable bit rate, bandwidth, and resulting bit error rate (}3ER). A.2.3 RepeaterIess Link Distances: Link Budgets A principle determinant of cost in a communication system is the number of field cabinets, nodes, or hubs required. In ITS systems, cost savings can be achieved by minimizing numbers required Thus, it is desirable to maximize He supportable link distances so Hat intermediate equipment locations exclusively for repeater functions are minimized. The link budget is the principle determinant of a supportable link distance. ~:\NC~Phase~p~\ NCHRP 3-51 · Phase 2 Final Report A2-9
Although the details and exact procedures for design of communication links vanes for each communication medium, the general concept of linlc budget is applicable to all types of wire, wireless, and fiber mediums. Therefore, we wiD present a generic overview of He concept. Transmitter Receiver Receive ~ Figure Ae2~3~1 Link Budget Concept Receiver Sensitivity: Minimum Power for specified BER (digital) or SNR (analog) Figure A.23-1 illustrates the general link budget concept. The transmitter generates a signal that represents the input and converts it into an electrical (wire), optical (fiber), or radio frequency (wireless) signal that is suitable for insertion onto Be medium for transmission to Be receiver. The receiver extracts the signal from Be medium and converts it into an electrical signal for processing by the receiver. A link budget establishes that Be transmitter power, transmission attenuation, and received signal power meet the foDour~ng: PT PL = PR >= PSEN where: PT PA PR PSEN (Equation A.2.41) is the transmit power is the transmission loss or attenuation is the receive power is He minimum received Power for the specified Bit Error Rate (BER) essence, receivers for all mediums require that received signal power be greater Han the minimum receiver power requires} for achieving the specified receiver Bit Error Rate HER) for digital or Signal-to-Noise (SNR) for analog. This minimum receiver power level is referred to as the receiver's sensitivity. The receiver sensitivity should specify receive power In dBm and an achievable BER(digital) or SNR(analog). ~\NC~h~pt\ NCHRP 3-51 · Phase 2 Fmal Report A2-10
Transmission Link Loss components vary according to medium and ~nstaBation, but all are usually dominated by attenuation as a function of distance along the medium. It is typically specified in dB per meter or kilometer (dB/m or dB/km) for wire and fiber. RF propagation loss is an exponential function (not linear) of distance as presented in the wireless propagation Section A.~.3.~. It should be noted that the link budget attenuation, while usually the dominant factor, is not the only impairment that determines repeateriess link distance. It is, however, a required design consideration and is usually adequate for link distance planning (and cost estimating) with fine tuning for other factors In the detailed communication system design. As an example, consider Be following representative SMFO fiber link budget calculation: I. Transmitter launch power: 2. Receiver sensitivity: 3. Allowable medium loss: 4. Power gain margin: 5. Design medium loss: Pax = 0 dBm PSEN = -20 dbm LA Plx PSEN = 0 dBm - (-20dB) = 20 dB PMARG~ 6 dB (design cntena) PDESIG~ PLA - PGA~ = 20 dB - 6 dB = 14 dB Thus, the goal is to have no individual link with a loss greater Man 14 dB. The Gain Mark is a design buffer Mat typically accounts for reasonable worst case operational conditions over the life of the installation. For example, there is statistical variation of equipment, components, and medium. Furthermore, performance often degrades as Me installation ages and as a function of temperature. Design gain margin buffers can vary for each medium, by application requirements, and even by designer preferences. Continuing Me example, the medium loss typically consists of several components. For SMFO, Be typical loss components consist of fiber loss (dBlkm), connector loss (dB/connector), and splice loss (dB/splice). The following table illustrates loss calculations for representative rRs inks: t;\NCH'Wba~\ NCHRP ~51 · Phase 2 final Report A2-11
/ Table A.2.3~1 Loss Calculations for Representative ITS Links , _ Link ~LmR ~ Loss LosslU nit number of Totalnumber of Total Component Units Component Units Component Loss Loss Link Length .35 DB/krn 8 hen 2.8 DB 28 km 9.8 DB (5 Miles) 1~) _ Connector ~.5DB ~ 4 Connector ~2.0 DB Splice Losses .3 DB 4 Splices 1.2 DB 6 Splices 1.8 DB Total Link Loss (okay if less than 14 DB) 6.0 DB 13.6 DB Table A.2.3-2 presents representative link budgets and parameters for popular llS communication mediums and includes typical I) t~sm~tter launch powers, 2) Transmission Loss Components, 3) Receiver Sensitivity power levels and expected BERs, and 4) representative repeateriess link distance. / L;\NCHRP`Ph~2rp ~NC~3-51 · IF A2-12
Table A.2.3~2 Representative Link Budgets and Parameters for Popular tTS Communication Mediums Me~ _ Modems/rWP Single Mode Fiber Optics SMFO Multi Mode Fiber Optics MMFO -16 dBm (LED) Spread Spectrurn/lSM Band Typical Transmitter Launch Power, dBm +5, -15 dBm (private network) -9 dBm (Public) O dBm, 1 to -3 dBm (laser) Typical T· - ransmlsslon Loss dB/krn, {dB/mile) 1.75 dB/km, (2.8 dB/mi) Typical Receiver Sensitivity, dBm 1200 bps or less - 0,-35 to -50 dBm 2400 or greater dBm _ ~ Representative Repeaterless Link Distance (km) 16 hen (10 mi) 12.5 km (8 mi) .19 dB/lon (.3 dB/mi) -30 dBm 80 to 160 km (50 to 100 Mi) 30 dBm (1 Watt) 4 db/km (850nm) 1.5 db/km (1300nm) 1 20 dB2 -148dB2 -30 dBm -90 dBm 3 km (850 nm) .8-10km(1300 nm) . 26 km (915 MHz) 9.9 hen (2.4 G Hz) 4.1 km(5.8GHz) - 5 miles2 Analog Cellular (800 MHz) 7 watts 38dBm -1 10 dBm 1-200 miles3 Analog AM Radio (540 - 1600 kHz) 10,000 watts 70 dBm -160 dB2 -157dB2 _ -80 dBm -90 dBm Analog FM Radio 50,000 watts (88 - 108 MHz) 77 dBm _ -80 dB2 50- 100 miles3 1 Less than 1 mile3 Analog AM Radio HARrrIS 540 kHz 100 MW (Part 15) 10 dBm 50 watts (Part 90) 16 dBm . -90 dBm -1 16 dB2 1RF transmission loss = -92.4 - 20 log (fGH~) - 20 log (d~n) [free space] 2Cellular typicaBy lim~ted by adjacent cell (co-channel) ~nterference 3Actual expenence, not free space 5-10 miles3 ~,:~NCHR~t\ NCHRP3-51 · Phase2FmalReport A2-13
