communications
L-3 Communications System has been providing specialized applications unique state-of-the-art wide band communication systems to all DoD users for over three decades. These equipments have bridged the applications of two-way (duplex) communication from surface-to-air, surface-to-satellite, air-to-air and air-to-satellite. L-3 Communications has maintained it's market share by continuously advancing the state-of-the-art through an aggressive internal IRAD activity focused toward increased functionality and performance versus cost. The technology of communication systems has matured significantly over the past decade primarily in the areas of component development with an attendant increase in communications/data link performance and functionality. L-3 Communication's technical staff maintains currency with both the applications and technology of communications. Our operating tenet is that - only in this way can effective solutions to the multifaceted communication user needs be devised. The "Link Budget" is the communication system designers primary tool for deriving the solution from the user requirements. The inputs to the "Link Budget" represents the users needs and requirements; for example, the allocated operating frequency, the required throughput rates, the line-ofsight (LOS) range and logically the terminal(s) SWAP (size, weight and power) constraints. The resulting "Link Budget" will virtually describe the communications solution and it's generation process entails a broad set of intuitive trade-offs which an experienced communication designer possesses and encompasses a broad domain knowledge in areas of antennas, transmitter power and efficiencies, noise figures, processing capabilities, etc. The military communications user may also impose additional communication requirements by specifying reliable communications in the presence of intentional interference (jamming) and/or covert communications. Covert communications has a figure-of-merit described by low probability of detection (LPD) and/or low probability of intercept (LPI). L-3 Communications possesses the domain knowledge and the hardware designs to address your specific data link requirements including AJ/LPI/LPD and will provide design solutions to your specific requirements.
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Data Link Basics: The Link Budget We presume you have started to read this brochure because you have at least an interest and, perhaps, considerable knowledge of data links and their applications. If you are a potential user of a data link, desiring a better understanding of the data link function, we invite you to continue with us as we explore the application and utility of a fundamental tool-the link budgetemployed in the system design of a data link. We propose to discuss this tool in sufficient detail herein that the data link novice, with a specific application, may subsequently develop his own link budget for the application. The link budget is directly applicable to the one-way, point-to-point data link. More complex linkage requires application of the link budget to each pointto-point section of the multisection link. The one way, point-to-point link called. SIMPLEX. The two-way, point-to-point data link is called FULL DUPLEX (simultaneously two way) or HALF DUPLEX (sequentially two way). The link budget is highly analogous to the more familiar financial budget. Both are concerned with a balancing of assets and liabilities. Both use projected or predicted entries, at least in part. Both provide a preview of the net balance and suggest areas where adjustments may be made. The link budget balances gains and losses within the data link to achieve a necessary net gain, or signal margin. Consequently, the link budget permits one to establish the feasibility of a desired data link before proceeding with design and development.
The Link Budget The link budget is a systems design tool first exercised when the prospective data link is in the conceptual or formative stage. This early application involves tentative data entries to establish feasibility. As planning matures these tentative data become link requirements. The link budget is properly maintained current throughout the development process, including verification testing. In this way, inevitable changes occurring to system parameters during development are assessed for impact on projected system performance. Finally, using the link budget tool during verification testing of the data link permits validation of the tool and its usage. A properly maintained link budget will predict data link performance in close agreement with that demonstrated by verification testing. Transferring information from one point in space to another by electromagnetic wave propagation requires a transfer of energy. One may express this energy transfer as an equation, where each data link operation contributes a gain or loss. This equation and the associated link budget will quantify the essentials of data link operation. The operative data link equation may be expressed as equation 1 where all symbols will subsequently be defined. First, it is convenient to express the receiving area of the receiving antenna, AR, in more readily available terms. This is related to, but may differ markedly from, the physical aperture area.
Equation 1
Si =
PT GT LT LP AR LR 4pR 2
Ni = kTBE SI PT GT LT LP AR LR = Ni 4pR 2 (kTBF )
1
Typically, the receiving antenna is characterized by its gain GR, and the "effective" aperture is related to gain as shown in equation 2, where l is the free space wavelength of the carrier signal. Now one may express Si and Ni as illustrated in equation 3.
Equation 2
Units of measure for the various parameters may, of course, be taken from any self-consistent system. Typically, the link equation is found expressed in metric units.
Equation 3
The data link equation expresses the "available" signal to noise ratio at the receiver in terms of measurable link parameters. (Equation 4) By taking ten times the common logarithm of both sides of this equation, parameters are expressed in decibels, for convenient tabulation (Equation 5) 2
1 æ l ö The term ç geometric ÷ is a product composite of 4pR 2 è 4pR ø æ l2 ö ÷÷ the effective dispersion of electromagnetic wave propagation, and çç è 4p ø
aperture of an isotropic receiving antenna. The resultant is called the "freespace propagation loss" since it represents the loss that would be exhibited in a system of two isotropic antennas, separated by distance R and operating at wavelength k. Given these definitions of terms, the link equation may conveniently be expressed in tabular format. A suggested tabular format is exhibited in Figure 4. Completing the individual entries is a systematic way of making data link implementation decisions. All such decisions are initially tentative. Ultimately, a proper balance between gains and losses must be achieved. Many parameters are available for adjustment to achieve a desired signal margin. Discussion of the link budget will follow an explanation/example format. Consequently, the tabular link budget, figure 4 includes example entries for all line items. (This example link budget is also provided as a foldout for reading convenience, as the last page of this pamphlet.) Each line item is first explained, in general terms, followed by justification of the specific value used in the example. This procedure may then be paralleled by the data link novice in applying this tool to his application. A physical example data link is depicted in Figure 1. The implications of the physical scenario are quantified in the notes at the bottom of the example link budget. The notes include other parametric data externally imposed but not obvious from the physical scenario. Occasionally, some of these conditions may require a best estimate to substitute for established requirements. Typically, the first consideration in any data link application is the type of data (analog or digital) and the quality or fidelity of transfer. Such consideration will constrain the choice of modulation, determine the receiver structure and establish a value of "required" SNR. Specific discussion of the choice of modulation will, however, be postponed in deference to discussing the body of the link budget. 'The "required" SNR (required to achieve a specified fidelity of information transfer) is a lower bound for the "available" SNR that must result when the gains and losses of the link budget are totaled. Hence, it is convenient to begin the link budget discussion with this item in the SUMMARY section of the Data Link Budget, Figure 4. 2
AR =
Si =
G R l2 4p
PT GLTL PG RlLR 4pR 2 Rp
æ l ö = PT GT LT LP GR LR ç ÷ è 4pR ø
2
Ni = kTBF SiNiPTGTLT-
signal power available at the data link receiver noise power corrupting the received signal is the transmitter power, is the transmitting antenna again, is the total signal loss from the transmitter through the antenna, LP- is the total absorptive propagation loss, GR- is the receiving antenna gain, LR- is the total receiving signal loss from the antenna to the RF amplifier, l- is the free-space wavelength of the RF carrier signal, R- is the range separating the transmitting and the receiving terminals, k- is the Boltzmann's constant (1.38054 E-23 Joule/º K), T- is the ambient temperature (taken as standard temperature of 290ºK), B- is the effective noise bandwidth of the receiving process, F- is the noise factor.
Equation 4 PG L L G L æ l ö Si = T T T P R Rç ÷ kTBF Ni è 4pR ø
2
Equation 5 SNR(dB) = 10 log10 (Si/Ni) SNR(dB) = PT(dBm) + GT(dB) + LT(dB) +LP(dB) + GR(dB) + LR(dB) + 20 log10 æç
l ö ÷ è 4pR ø
-10 log10 (kT) -10 log10 (B in Hz) - NF(dB) where generally XR (dB) = 10 log10 (XR) ì - 114 dBM / MHz 10 log10 (kT) = í î - 174 dBM / Hz
10 log10(B in Hz) = BW (dB 1Hz) NF (dB) = 10 log10(F)
Required SNR (or Eb/No), (item 19) For high quality data transfer, the required SNR will typically be in the range of 6 to 12 dB. The required SNR will be directly related to the ultimate measure of transfer quality, such as bit error rate (BER), word error rate (WER) or output SNR. The incorporation of data coding (not treated herein) may have a major influence on this requirement. Representative data for some digital and analog modulations follows. (Figure 2 and Figure 3). For digital modulations, the relationship given is required SNR to BER while for analog modulations, the input and output SNRs are related.
Additional SNR for Implementation Loss (item 20) Receiver implementation never quite achieves the theoretically ideal performance. This loss accounts for the deviation from ideal performance that will be obtained in the detection process. Typically losses in the range of 1 to 3 dB will prevail, depending on the care (and cost) expended in detector implementation.
Required SNR in the example link budget is 10.6 dB. This value is taken from the curve for coherent PSK (digital) modulation required to yield a BER <10-6 BER <10-6 was a system requirement stated in note 6 at the bottom of the example link budget.
The "additional SNR for implementation losses" in the example link budget is set at 0.8 dB. A knowledge of the specific hardware implementation is required to properly establish this value.
Figure 1
Possible Scenario Military Data Link
Net Signal Margin (item 21) Signal margin is a contingency guard or safety factor in the data link design and, as such, is seldom a fixed requirement. Signal margin provides continued link operation when the nominal design conditions are exceeded. For example, a torrential rain may occur in the path of propagation or multipath propagation may occasionally exist. Specific explanation or justification is frequently required for signal margins less than 10 dB. (10:1 safety margin) or greater than 20 dB. (100:1 safety margin). Clearly, signal margin is a judgment factor that may sometimes be related to probabilistic measure of "acceptable" link outage (or required link availability).
Example Scenario - Artist's rendering of a physical situation that could lead to the requirements stated for the example data link.
The example link budget has a signal margin of 8.5 dB. This is less than the desired 10 dB, but recall that the subject budget includes 6.5 dB, for rain losses. Heavy rain or fading due to multipath propagation would presumably cause the example data link to "drop out" occasionally.
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Available SNR (item 18) The algebraic total of items 19, 20, and 21 will establish a minimum value for available SNR. Recall that available SNR is the resultant of evaluating the link equation. Hence, the various link parameters must be suitably appropriated to produce an available SNR compatible with the data link quality required (required SNR). The available SNR, of course, results from totaling the gains and losses of items 1 through 17 in the tabulation.
Tx Power Pt (item 1) Transmitter power may be adjusted over a broad range to satisfy link requirements. A foremost constraint on higher transmitter power is higher implementation cost. Other constraints are technology, available prime power, heat dissipation and personnel safety. The severity of each of these constraints is linked to the frequency of RF carrier operation. Frequently, transmitter power is one of the more flexible parameters in achieving the necessary available SNR. Figure 2
Figure 3
Representative Digital Performance
Comparisons of SSB FM and PCM-FS Systems with The Minimum-power Criterion
The example link budget produces an available SNR of 19.9 dB. As indicated, this value satisfies the required SNR and implementation losses and provides an 8.5 dB signal margin.
The example Tx Power is 70 watts or 48.5 dBm. This power level is readily achievable at the link carrier frequency. A "one for one" increase in signal margin may be achieved by increasing transmitter power.
Representative Digital Performance 1 Coherent PSK Uncoded 2 Coherent PSK Differentially Encoded 3 Diff. Coherent PSK 4 FSK Coherent 5 FSK Non-Coherent Representative Analog Performance 6 FM (Band-Dividing) 7 PCM-FS (n=1) 8 PCM-FS (n=2) 9 FM (Conventional) (No Modulation) 10 SSB (AM)
Fig. 3 Reprinted by permission from National Bureau of Standards Tech Note 167 Digital Detection - Performance curves for several popular types of digital modulation. These curves represent ideal performance and adjustments are required for implementation imperfection.
Analog Detection - Performance curves for several popular types of analog modulation. These curves represent ideal performance and adjustments are required for implementation imperfection.
Tx Component, Line Losses, Lt, (item 2) This loss is present to some extent in any implementation since it includes any losses incurred between the transmitter and transmitting antenna. Typical losses components in this path are transmission filters, transmission line and rotary joints. Minimization of these losses are especially important since each may represent significant heat dissipation in addition to a link liability. Note that the term Lt from the link equation is subdivided in the tabulation into several commonly occurring contributors. 4
An example value of 1.0 dB is included. In practice, a knowledge of transmission line hardware would determine this value.
Data Link Budget
I.D. - BPSK Down Link
Plus
Minus
Units
Transmit (Tx) 1. Tx power, Pt
48.5
2. Tx Component, Line Losses Lt 3. Tx Antenna Gain (Peak), Gt
12.0
-
dBm
1.0
dB
-
dBi
4. Tx Pointing Loss, Lt2
-
0.8
dB.
5. Tx Radome Loss, Lt3
-
0.5
dB.
6. Free Space Loss, (l/4pR)2
-
160.4
dB.
7. Atmospheric Absorption, Lp1
-
2.7
dB.
8. Precipitation Absorption, Lp2
-
6.5
dB.
-
dBi.
EIRP = 58.2 dBm. Propagation
Total Prop. Loss = 169.6 dB. Receive (Rx) 9. Rx Antenna Gain (Peak), Gr
29.0
10. Rx Polarization Loss, Lr1
-
0.5
dB.
11. Rx Pointing Loss, Lr2
-
0.0
Db.
12. Rx Radome Loss, Lr3
-
0.0
dB.
13. Rx Component Line Losses, Lr4
-
2.0
dB.
14. Spreading Implementation Loss, Lr5
-
1.2
dB.
Eff. Carrier Power = -86.1 dBm. Noise 15. Thermal Noise Density, kT
174.0
-
dBm/Hz.
16. Rx Noise Bandwidth, BW (1 MHz)
-
60.0
dB.
17. Rx Noise Figure, NF
-
8.0
dBHz.
Eff. Noise Power= -106.0 dBm. Summary
263.5
18. Available SNR
243.6
19.9
-
dB.
19. Required SNR (or Eb/No)
-
10.6
dB.
20. Additional SNR for Imp, Losses
-
0.8
dB.
21. Net Signal Margin
8.5
dB.
Notes: 1. Geometry
AIR TO GROUND, 60 K FT. q EI>0.60º
2. Frequency
10 GHZ NOMINAL
3. Range
250 KM MAY
Figure 4
4. Weather
LIGHT RAIN
Data Link Budget - Tabular form of the link equation convenient to accounting for data link gains and losses. See foldout of same at end of document.
5. Miscellaneous
CIRCULAR POLARIZATION
6. RD = 1 MB/SEC, BER< 10-6 BPSK COHERENT 7. GROUND ANTENNA PRECISION TRACKER, NO RADOME
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Tx Antenna Gain (peak) Gt (item 3) The transmitter antenna gain is a measure of the degree of focusing or directing of the available signal power to the intended receiver. For a single intended receiver, much benefit accrues from directing, rather than scattering available power. The peak gain is used here to cover the ideal pointing situation. Frequently antenna size and gain are physically constrained by external factors.
Tx Pointing Loss, Lt2 (item 4) Both tracking (closed loop) and pointing (open loop) antenna subsystems have accuracy limitations and may direct less than the peak gain of the antenna at the intended receiver. Multiple sample tracking schemes, like conical scan, require some misdirection of the beam to derive an error signal. The Lt2 term accounts for the antenna misdirection loss arising from whatever source. Figure 5A
Figure 5B
Antenna Half-Power Beam Widths vs. Aperture Width for Circular or Elliptical Apertures
Antenna Gain vs Half-Power Beam Width [qÆ] (approximate) G = 27000 qÆ
The example data link uses an airborne antenna exhibiting 12.0 dB gain. Graphical data given below (Figure 5) allows relating this gain value to associated parameters of interest Suppose this 12 dB gain antenna exhibits circular symmetry. The graphical data then determines the beamwidth to be approximately 40 degrees and the aperture to be approximately 3 wavelengths in diameter. The example airborne antenna is assumed to be pointed by open-loop computation and, consequently, unable to always direct the peak gain at the ground station. The example pointing loss is taken as 0.8 dB. Antenna Relationships - These curves provide several useful antenna parameter relationships. (A) Beamwidth vs aperture size for circular or elliptical aperture (B) Gain vs beamwidth for antenna types noted. Figure 5A
1 2 3 4 5 6
Illumination Taper (Cosine)4 (Cosine)3 (Cosine)2 (Cosine) Average Antenna Uniform
Theoretical First Side Lobe Level 44.6 dB 39.0 dB 32.8 dB 25.8 dB 20.0 dB
Bearnwidth Constant HPBW x D l 112 100 90 70 65
17.6 dB
70
Figure 5B Notes: 1. Subtract 1½ dB for CSC2 Shaped Beam Antennas 2. Add 1½ dB for Two Dimensional Arrays 3. Add ½ dB for Collinear-Array-Fed Parabolic Cylinders
Tx Radome Loss Lt3 (item 5) The transmitter terminal antenna may require protection of the antenna with a dielectric covering called a radome. A radome may be required for protection from weather, winds, including slipstream, or physical observation. In any event, the radome almost universally either reflects and/or absorbs RF energy. The term Lt3 accounts for any losses attributable to the radome. In most cases of interest this loss is small (0.5 to 1.0 dB). Transmitter hardware losses were separated into three distinct categories above, partially as a reminder that such sources of loss typically do exist. Subtotaling the first five link budget items leads to a value for Effective Isotropic Radiated Power (EIRP). This term represents the transmitter power that would be required if the antenna were an ideal isotropic radiator connected in a lossless manner to the transmitter. EIRP is a figure of merit for the transmitting terminal and denotes the power density or field intensity directed at the intended receiver. Transmitter terminal parameters may be allocated with impunity to data link operation, if the EIRP remains fixed.
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Airborne antennas almost always require a radome for physical protection. The example radome is assumed to add 0.5dB loss to the antenna.
The example transmitter exhibits an EIRP of 58.2 dBm or 661 watts effective radiated power. This value exemplifies the value of directional antennas
Figure 6
Range/Elevation Angle Relationship Tropospheric Propagation - This exhibit relates altitudes, elevation angle and range in the earths refractive lower atmosphere. Calculated for atmospheric refractive index model: n(h) = 1 + .000313e -.04385h (h in thousands of feet)
Free-Space Loss (l/4pR)2, (item 6) This term was discussed previously in connection with the link equation. l=c/f (speed of light/frequency) is the most overt appearance of link carrier frequency in the link budget. Frequency is also an important factor in antenna gain, propagation and hardware losses. Hence, the data link is heavily influenced by operating frequency, but in ways less obvious than that of free-space loss.
The example data link utilizes a carrier frequency of 10 GHz (l =.03 meters) and operates to 250 km resulting in a free-space loss of 160.4 dB as 20 log10 (l/4pR).
Atmospheric Absorption, Lp1, (item 7) Water vapor and biatomic oxygen (O2) are atmospheric constituents that absorb RF emissions to a significant degree. The concentration of water vapor is dependent on relative humidity and altitude while O2 is dependent only on altitude. Obviously, Lp1 losses will depend on the portion of the propagation path lying within the "lower" atmosphere, which will depend on the data link terminal altitudes. Graphical data permitting crude estimates of propagation loss due to water vapor and O2, follows. A considerable body of theoretical and experimental literature exists on the subject of propagation losses. NBS Technical Note 101 (Revised), "Transmission Loss Predictions for Tropospheric Communications Circuits," is perhaps a valid introduction to the literature for those so inclined. Data links operating entirely above the atmosphere escape this loss term entirely. Data links operating at the lowest microwave frequencies (<1 GHz) may experience negligible loss from this source.
The example link budget includes a loss attributed to atmospheric absorption of 2.7 dB. This value is extracted from the graphical data provided. First, the elevation angle of propagation may be deduced from "Range, Elevation Angle Relationship" (Figure 6) for maximum range (250 km) and airborne terminal altitude (20K ft) as qEL> 0.6 degrees. Second, the elevation angle enables use of "Water Vapor plus O2, Propagation Loss" (Figure 7) to determine the loss of approximately 2.65 dB for qEL> = 0.01 radian and range of 250 km.
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Figure 7
Water Vapor Plus O2 Propagation Loss
Atmospheric Propagation Loss -These curves provide some estimates of losses due to O2 and water vapor (average humidity) for slant propagation in the earth's lower atmosphere. Notes: Curves are asympotic because one terminal is above lower (absorptive) atmosphere at greater ranges.
Precipitation Absorption, Lp2, (item 8) Precipitation (rain primarily, snow secondarily) within the propagation path absorbs a portion of the incident radiation. Precipitation also reflects incident RF energy but such is negligible relative to absorption. The mechanism of absorption is a very complex (and not fully characterized) process depending on drop size and distribution, temperature and frequency. Generally, the absorption of energy increases with increasing precipitation rate and frequency of operation. The geometry of precipitation absorption is also very complex since the propagation path may be moving (mobile terminals) while the precipitation cell is moving, changing size, and changing rate. All of these factors are inherently unpredictable, in any precise sense, at the time of data link design. Typically, the design procedure is to assume a moderate rain cell (say 80 Km.) a moderate rainfall rate (say 5-7.5 mm/hr) and design to operate successfully in this environment. More inclement weather than assumed (losses in excess of signal margin) will cause link outage (or substandard performance) for the period of severe weather. Designing data links for the worst case weather conditions is not cost effective, and possibly not feasible, since hurricane type rain conditions (intense and widespread) would entail absorptive losses of several tens of dB. Conversely, it is unduly optimistic to assume a data link will always operate in clear weather, thus suffering no precipitation absorption. (Figure 8 exhibits graphical data that may be helpful in estimating precipitation losses, within the confines discussed above.) Totaling all sources of propagation loss again produces a useful figure of merit for the data link. Within this total, various trade-offs of interest become apparent, reduced range with increased absorption, reduced frequency with increased absorption. Also, the impact of inclement weather versus clear weather is apparent in this compilation.
Rx Antenna Gain (peak), Gr (item 9) The description and commentary for Item 3 above applies here. Antennas exhibiting directivity, concentrate available energies where required and generally relax the requirements on other system parameters. 8
The example data link includes a loss of 6.5 dB for rain absorption. The rain condition postulated for the example is 'light rain' (note 4 of Data Link Budget) taken here as approximately 2.5 mm/hour. Rainfall loss curves are provided for elevation angles of 0.01 radian and 0-05 radian. The elevation angle for the example is approximately 0.01 radian (0.57 deg.) and the corresponding loss from the curve for 10 GHz, 2.5 mm/hour rainfall is 6.5 dB at 250 Km range.
The example employs a focal feed parabolic dish exhibiting 29.0 dB gain at the ground terminal. Again, the graph relating antenna parameters serves to determine that this antenna would exhibit a half-power beamwidth of approximately 6º and an aperture approximately 15 wavelengths in diameter.
Figure 8A
Incidence Angle = 0.01 RAD (0.573 deg.) Homogeneous Rain Distribution
Figure 8B
Incidence Angle = 0.05 RAD (2.865 deg.) Homogeneous Rain Distribution
Propagation Loss of Rain - Curves for developing estimates of propagation loss in homogenous rain. This curve applies for an elevation angle of 0.01 radian (.573 deg).
Propagation Loss in Rain - Curves for developing estimates of propagation loss in homogeneous rain. This curve applies for an elevation angle of 0.05 radian (2.865 deg).
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Figure 9A
Polarization Loss - These cirves represent maximum (B) and minimum (A) polarization loss between two eleptically polarized antennas having the axial ratios indicated.
Fig 9A Reprinted by permission from Microwave Journal
Rx Polarization Loss Lr1, (item 10) Polarization loss may occur for both linear and circular polarization applications. In linear polarization applications, loss results when the field vectors of the two antennas are misaligned. Misalignment is common and may become quite severe in situations where one of the terminals is highly mobile in attitude. Polarization loss increases as the cosine of the misalignment angle for linear polarization. Circular polarization is commonly incorporated to avoid the severe losses resulting from attitude variation with linear polarization. Unfortunately, most antennas designed for circular polarization actually produce elliptic polarization (non-unity axial ratio). When both antennas exhibit some ellipticity, misalignment of the major axes will again cause some polarization loss, Hence, circular polarization alleviates the polarization problem but seldom solves it entirely. In any event, polarization losses anticipated should be accounted for in the data link budget. Graphical data (Figure 9) for estimating this loss is provided.
The example data link allocates 0.5 dB to polarization loss. Reference to the graphical data for polarization losses below, reveals that maximum polarization loss of 0. 5 dB implies both circularly polarized antennas should exhibit less than 3 dB axial ratio. Given these antenna designs, no restrictions would be imposed on the attitude of the airborne vehicle.
The example antenna, at the ground terminal, tracks emissions from the airborne terminal and therefore directs its peak gain at the emission source. Consequently, the Rx Antenna will require no loss allocation for misdirection.
Rx Pointing Loss, Lr2 (item 11) The earlier discussion of Tx Pointing Loss (item 4) applies here.
Rx Radome Loss, Lr3 (item 12) The earlier discussion of Tx Radome Loss (item 5) applies here. 10
The example ground antenna does not employ a radome. Therefore, the loss allocation is 0 dB.
Figure 9B
Maximum Polarization Loss , Same Sense Polarization Loss (Same Sense) (b=90º)
Fig 9B Reprinted by permission from Microwave Journal
Rx Component Line Loss, Lr4 (item 13) The earlier discussion of Tx Component Line Loss C item 2) applies here.
Spreading Implementation Loss, Lr5 (item 14)
The example data link allocates 2.0 dB of loss to this source. Transmission line losses increase with the increase of operating frequency.
"Spread Spectrum (SS)" data links are employed in many applications. Spreading involves applying a second modulation for the sole purpose of distributing the signal energy over a much wider frequency band than would be required to contain the information being transferred. Phase (direct sequence) or frequency (hopping) modulation are commonly used to achieve the subject spreading. Frequently, the "spreading" signal is a pseudo noise signal to insure that the resulting spectrum is devoid of energy concentrations in the form of "line" spectra. Spreading complicates unauthorized detection of the emissions, provides processing gain against several types of interference and generally reduces spectral energy densities. Theoretically, the process of spreading is a superposed operation that is transparent to the underlying transfer of information. Practically, the implementation of spreading is never ideal and the "spread" data link will suffer some performance disadvantage vis-a-vis the "non spread" data link. Hence, an implementation loss is incurred and the loss requires quantification and inclusion in the link budget. An implementation loss of 1-2 dB for spreading is typical for well designed data links. The "effective carrier power" available for the information detection process may now be derived from the gains and losses tabulated. Effective carrier power is the transmitter EIRP less the total propagation loss plus (or minus) the net gain of the receive function. Subsequently, the effective carrier power will be compared with the effective noise power to establish the available SNR, so crucial to the information detection process.
The example data link is assumed to exhibit 1.2 dB detection loss of efficiency as a result of employing spread spectrum. Nominally, the derivation of this loss for an application requires either detailed receiver analysis or experimental evaluation or both.
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Thermal Noise Density, (kT) (item 15)
Equation 6
Thermal noise arises from molecular vibrations of the atmosphere and receiver hardware. The molecular vibrations are directly proportional to the absolute temperature (degrees Kelvin) of the physical elements involved. Artificial cooling of the receiving equipment can be applied to reduce this noise contribution. Marked improvement, however, requires cooling to temperatures in the vicinity of absolute zero and the complexity of so doing is prohibitive, except in extreme cases.
kT = (1.3054E-23 Joules/ºK) (290ºK) = 4.004 E-21 Joules (watt-sec) = 4.004 E-18 milliwatt-sec. = 4.004 E-18 milliwatt/Hz. and 10 log10 (kT) = 10 log10 (4.004E-18 mw/Hz) = -174.0 dBm/Hz. = -114.0 dBm/MHz.
Thermal noise density is quantified as the product of Boltzmann's constant (1.3805 E-23 Joules/ºK) and absolute temperature (ºK). Nominally, the temperature is simply taken as the "standard" or earth average temperature of 290ºK (17ºC, 62.6ºF). See equation 6. This value is the thermal noise baseline that ultimately limits the detection of "small" signals. Note that the data link transmitter characteristics were not a factor in establishing the noise baseline. A reasonably designed data link transmitter will provide an emitted carrier-to-noise ratio of at least 50 dB. Minimum signal detection will result for carrier to-noise ratios of the order of 10 dB. Hence, the transmitted noise is present at the receiver but negligibly small, relative to receiver noise. Under very strong signal conditions the achievable carrier-to-noise ratio will be limited to that transmitted.
Rx Noise Bandwidth, BW (item 16) The matched filter will produce an effective noise bandwidth that is the reciprocal of the data interval. That is, B (Hz) = 1/Tb (sec/bit) =Rb (bits/sec) is the ideal receiver bandwidth and BW (dB) = 10 log10 (B/1 Hz) is the value for inclusion in the link budget. Cases where the detector filtering is grossly "mismatched" to the signal spectra must be treated on an individual basis and the resulting effective noise bandwidth included in the link budget. Otherwise, item 20 in the link budget is included as a coverall for, light imperfections in the detection process. The typical datalink receiver presents several "bandwidths" to an incoming signal and determining the "bandwidth" for detection calculations can be somewhat involved. For example, receivers commonly employ RF front end bandpass filters for rejecting out-of-band interference, This filter may be many times the receiver's detection bandwidth, especially in an application employing spectral spreading or tunability. The receivers intermediate frequency section will establish yet another bandwidth predicated upon technology or interference analysis or dynamic range. The IF bandwidth may or may not be influential in establishing the detection bandwidth. Finally, the incoming signal arrives at the detector section of the receiver. Typically, the implementation of the detection process will define the narrowest bandwidth and consequently the dominant filtering of the incoming signal. The function of receiver filtering is, of course, to maximize the SNR for detection which, in turn, maximizes the fidelity or accuracy of the detector. The celebrated synthesis work leading to the "matched filter" achieves the desired maximum SNR. That is, the matched filter is matched to the modulating signal waveform (or spectra) in such a way that SNR is maximized (in the least square sense) in the presence of white Gaussian noise (WGN). Thus, the matched filter is the goal to be achieved when the receiver is implemented. 12
Typically, the coherent receiver (phase tracking, correlation detection with integrator) is capable of near ideal implementation of the matched filter (perhaps within 1 dB). The noncoherent receiver (no phase synchronization is dependent on a combination of pre and post detection filtering and stray further from the ideal matched filter (perhaps 3 or 4 dB). The structure and effective noise bandwidth of the candidate data link receiver is an important parameter worthy of considerable effort in evaluation.
Rx Noise Figure NF (item 17) The receiver noise figure establishes the noise floor and the "sensitivity" of the receiver. By convention, NF (dB) =10 log10 (F), where F is called the 'noise factor.' These terms refer to the noise generated in the receiver in excess of ambient thermal noise, kT. That is, a receiver noise factor of F implies the receiver absolute noise level is F kT (all references are standardized to T = 290ºK). The noise figure (or factor) of an existing receiver may be measured directly or the receiver noise figure may be calculated using the contributions of the receiver components. To wit, (see equation 7) where subscripts refer sequentially to component noise factors and gains from the front end onward to the detector. Obviously from the equation, the noise factors of latter components are masked by preceding gain and, consequently, become less influential. Note that the entire noise evaluation (items 15, 16, and 17) is based on thermal noise. Interference, natural or man-made, intentional or unintentional, is not explicitly included in the subject link budget. Some external noise sources may be modeled as thermal noise and simply included by adjustment and creation of an artificial kT. Generally, special consideration must be given to each of the many possible sources of interference.
The example data link is configured for 1 MB/sec data transmission, the matched filter detector for the data rate will exhibit an effective noise bandwidth of 1 MHz or 60.0 dB-Hz. The coherent detection process of this example can achieve, approximately, the ideal bandwidth.
Equation 7
F(rec.) = F1 +
(F2 - 1) + (F3 - 1) + (F4 - 1) + - G1
G1G2
G1G2G3
The example data link allocates 8.0 dB for receiver noise figure. This value represent good, not state of the art, performance at 10 GHz.
Available SNR (item 18) The ratio of Effective Carrier Power/Effective Noise Power is the Available SNR. The validity of link parameters is checked by comparing the Available SNR to the Required SNR plus the Additional SNR (Items 19 and 20 discussed above). Item 18 must exceed the sum of Items 19, 20, and 21 to provide both satisfactory operation and a desired "safety factor." If the Available SNR is sufficiently large, the proposed data link design and implementation may proceed. If the Available SNR is insufficient, then one or more of the several link parameters must be adjusted to achieve the desired balance of gains and losses. Perhaps greater transmitter power is feasible or one of the terminal antennas can be increased in size and gain, Perhaps the range of operation required is not as great is initially stated or the weather assumptions were overly severe. The data link budget was devised to include all contributions to link gains and losses. Consequently, the data link budget becomes a very valuable working paper for only predicting performance, but tracking influential changes throughout the design and development process. Further, the data link budget is sufficiently simple and straightforward that the potential data link user, as well as the designer, can apply this tool.
The example data link available SNR is 19.9 dB. In this case, this value covers the required SNR and provides an 8.5 dB signal margin. Typically, the available SNR will be an inappropriate value on the "first pass." One then proceeds to make adjustments in the several link budget entries to achieve the desired available SNR
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The Impact of an ECM Environment! ECM is an acronym for Electronic Counter Measures. Thus, ECM are those efforts by an adversary to electronically defeat the operation of, in this case, a data link. ECCM is an acronym for Electronic Counter Counter Measures. Thus, ECCM are those techniques incorporated in the data link to render same invulnerable or more resistant to ECM. Generally, the requirement for working in an ECM environment complicates the fundamental data link task. Most ECCM techniques incorporated in the data link design will be detrimental to data link performance. However, the alternative to incorporation of ECCM may be total link outage due to effective ECM. The scope of the present discussion does not include a thorough discourse on ECM/ECCM. Mention of the more prominent factors will, however, serve to better outline the whole problem of data link design. Antenna patterns are very important factors in the ECM/ECCM scenario, This is also the one area where ECCM enhancements can also result in data transfer enhancements, Narrow-beam antennas concentrate signal energies in those spatial regions most useful to data link operation. Necessarily, they scatter/accept less energy in spatial regions of no interest. Normally, the potential jammer or intercept receiver does not have the luxury of positioning himself in the main beam of the data link antenna but must accomplish his nefarious deeds through the sidelobes. Therefore, narrow beam (high gain) antennas present a minimum target to the ECM source while exhibiting maximum efficiency in transferring energy from one point in space to another. Optical frequencies, and the resultant very narrow beamwidths, are favorably considered for some specialized applications because of their superior spatial discrimination. Frequency or spectral spreading is incorporated in many data links for the sole purpose of providing a countermeasure to ECM. Spreading complicates the clandestine detection of data link emissions and provides significant discrimination against the "brute force" jammer. Unfortunately, this technique also complicates the data link design. With the added complexity real gain against ECM can, however, be realized. Certain pulse jammers have the capability of emitting very high power levels within short pulses. Specialized forward error correction codes, in conjunction with bit interleaving, can tolerate short bursts (pulses) of effective jamming by detecting and correcting a limited number of forced errors. Forward error correction codes function by imposing a minimum distance (maximum distinguishability) constraint on transmitted messages. Sit interleaving "shuffles" the data, such that long bursts of jamming are broken up and scattered into a near uniform distribution of errors, prior to decoding. This commentary is expected only to convey that these techniques are being applied to data link design, and quite effectively for some applications. Error detection and correction coding is, of course, a field of active research and study in its own right, only alluded to herein. Finally, encryption of data is occasionally invoked for the sole purpose of preventing unauthorized deciphering of sensitive messages. Encryption neither prevents energy detection nor increases jamming resistance. Encryption is effective in its intended purpose. 14
Figure 10
Generic Data Link Terminal Functional Signal Flow
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Data Link Budget
I.D. - BPSK Down Link
Plus
Minus
Units
Transmit (Tx) 1. Tx power, Pt
dBm
2. Tx Component, Line Losses Lt
dB
3. Tx Antenna Gain (Peak), Gt
dBi
4. Tx Pointing Loss, Lt2
dB.
5. Tx Radome Loss, Lt3
dB.
EIRP = dBm. Propagation 6. Free Space Loss, (l/4pR)2
dB.
7. Atmospheric Absorption, Lp1
dB.
8. Precipitation Absorption, Lp2
dB.
Total Prop. Loss = dB. Receive (Rx) 9. Rx Antenna Gain (Peak), Gr
dBi.
10. Rx Polarization Loss, Lr1
dB.
11. Rx Pointing Loss, Lr2
Db.
12. Rx Radome Loss, Lr3
dB.
13. Rx Component Line Losses, Lr4
dB.
14. Spreading Implementation Loss, Lr5
dB.
Eff. Carrier Power = dBm. Noise 15. Thermal Noise Density, kT 16. Rx Noise Bandwidth, BW (1 MHz) 17. Rx Noise Figure, NF
dBm/Hz. dB. dBHz.
Eff. Noise Power= dBm. Summary 18. Available SNR
dB.
19. Required SNR (or Eb/No)
dB.
20. Additional SNR for Imp, Losses
dB.
21. Net Signal Margin
Notes: 1. Geometry 2. Frequency 3. Range 4. Weather 5. Miscellaneous 6. 7.
16
dB.
Conclusion Data link usage and the link budget have been discussed at some length. Implementation of the data link was beyond the scope of this discussion and only alluded to occasionally. However, some idea of the functional configuration of a data link terminal would be helpful in completing the concept. The functional organization of a generic, duplex data link terminal is illustrated in figure 10. This terminal includes many of the "bells and whistles" associated with ECCM techniques. In particular both the transmitted and received data link terminals include spectral spreading, coding and interleaving. Ranging is also included as a data link function since the configuration is duplex with high chip rate spreading on both links. Encryption is not included in this configuration but it could be. If included, the encryption function would reside between the operations of multiplexing (MUX) and encoding. This functional block diagram is equally applicable to either terminal of a point-to-point duplex data link. A relay data link terminal would be significantly more complex since the same is, in effect, two such terminals. The foregoing discussion has endeavored to simplify data link concepts and elaborate on elementary principles. The object of so doing was to introduce the potential data link user to the thinking process of the communication systems designer. Further, it is hoped that the potential data link user will make use of the material provided herein to "rough out" a data link budget for his/her particular application. A blank data link budget is included for your convenience to rough out your specific communications solution. In addition, it is noteworthy to add that the data link design process is significantly more complex, with interdependent relationships, than might be inferred from the foregoing discussion.
Postscript This document has attempted to provide military planners and designers an introduction to some of the basic concepts of data link characteristics. As a leader in this area, L-3 Communications has, over the years, assembled an unrivaled team of design and application specialists to assist users of data link systems. We are available to provide technical planning/support to any potential user of data link equipment. Such support may easily be tailored to your specific requirement by contacting:
The Link Budget provides the foundation for developing the data link system requirements. The Link Budget solution is driven by the user in the form of the mission requirements; such as, scenario, threats, interoperability, multiple access, etc. in addition to the system constraints; such as, the operating frequency band, data rates, signal margin, lineof-sight range and terminal size, weight and power requirements. L-3 Communications is the solution to your data link requirements.
L-3 Communications Communication Sytems - West 640 North 2200 West P.O. Box 16850 Salt Lake City, Utah 84116-0850 Tel: 801-594-2242 FAX: 801-594-2908
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