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OCR for page 353
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
OCR for page 354
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
OCR for page 355
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
OCR for page 356
/
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
OCR for page 357
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
Representative terms from entire chapter:
link distance
