5 Basic Propagation Models
5.7 LINK BUDGETS
A calculation of signal powers, noise powers and/or signal-to-noise ratios for a complete communication link is a link budget, and it is a useful approach to the basic design of a complete communication system.2Such calculations are usually fairly simple, but they can give very revealing information as to the system performance, provided appropriately accurate assumptions are made in calculating the individual elements of the link budget.
Essentially, the link budget is simply an application of the principles already explained in this chapter. The maximum acceptable path loss is usually split into two components, one of which is given by the distance-dependent path loss model (such as the free space or plane earth models of Sections 5.5 and 5.6) plus a fade margin, which is included to allow the system some resilience against the practical effects of signal fading beyond the value predicted by the model. Thus
Maximum acceptable ¼ Predicted þ Fade
propagation loss½dB loss margin ð5:37Þ
2Note that the process of calculating the maximum acceptable path loss resembles that of calculating a bank balance to determine the money available to spend, justifying the term link budget. A system where the desired range causes the loss to exceed the MAPL is thus analogous to an overdrawn account.
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The greater the fade margin, the greater the reliability and quality of the system, but this will constrain the maximum system range. Later chapters will give details of how the fade margin may be chosen.
A sample link budget for the downlink (base station to mobile) of an imaginary, but representative, terrestrial mobile system is shown in Table 5.2. The output in this case is the maximum acceptable propagation loss, but it may be that in practice the designer starts with the propagation loss, knowing how this corresponds to the desired range of the system, and then uses it to calculate some other parameter such as the effective isotropic transmit power or antenna height. Notice that how the units initially used to specify the parameters are rarely the most convenient to work with in practice, and how the units have been converted to consistent units [dBi, dBW, dB] in the last two columns.
In the case of the uplink, the mobile transmit power would typically be only 1 W or less, which would reduce the maximum acceptable propagation loss and limit the range of the system to less than that calculated in Table 5.2. The system would then be uplink limited.
Table 5.2: Sample downlink budget for terrestrial mobile communications
Quantity Value Units Value Units
(a) Base station transmit power 10 W 10 dBW
(b) Base station feeder loss 10 dB 10 dB
(c) Base station antenna gain 6 dBd 8.2 dBi
(d) Effective isotropic transmit 8.2 dBW
powerða b þ cÞ
(e) Maximum acceptable L dB
propagation loss
(f) Mobile antenna gain 1 dBd 1.2 dBi
(g) Body and matching loss 6 dB 6 dB
(h) Signal power at receiver 3.4 L dBW
terminalsðd e þ f gÞ
(i) Receiver noise Bandwidth 200 kHz 53 dBHz
(j) Receiver noise figure 7 dB 7 dB
(k) 10 log10(kT) with 204 dBW Hz1
k¼ 1:38 1023W Hz1K1, T ¼ 290 K
(l) Receiver noise power 144 dBW
referred to input (iþ j þ k)
(m) Required signal-to- 9 dB 9 dB
noise ratio
(n) Required input signal 135 dBW
power (lþ m)
(o) Maximum acceptable 138.4 dB
propagation loss (by solving h¼ n)
(p) Fade margin 15 dB
(q) Predicted cell radius 5.8 km
(using plane earth loss with hm¼ 1:5 m and hb ¼ 15 m)
102 Antennas and Propagation for Wireless Communication Systems
The two links are usually balanced in practice by improvements in the base station receiver, such as using a lower noise figure, or by using some diversity gain (Chapter 16).
5.8 CONCLUSION
This chapter has defined the important steps in the prediction of the useful range of a wireless communication system.
Construction of a link budget, which identifies the maximum acceptable propagation loss of the system, with due regards to the key system parameters, including signal and noise powers and by selecting an appropriate fade margin.
Use of a propagation model to predict the corresponding maximum range.
In the following chapters, more sophisticated propagation models will be used to estimate the system range and to predict the fade margin.
REFERENCE
[Connor, 82] F. R. Connor, Noise, 2nd edn, Edward Arnold, London, ISBN 0-7131-3459-3, 1982.
PROBLEMS
5.1 A base station antenna with a 9 dBd gain is supplied with 10 W of input power.
What is the effective isotropic radiated power?
5.2 Demonstrate that free space loss increases by 6 dB each time the distance between transmitter and receiver is doubled.
5.3 The speech quality for a particular mobile communication system is just accep-table when the received power at the terminals of the mobile receiver is
104 dBm. Find the maximum acceptable propagation loss for the system, given that the transmit power at the base station is 30 W, base station feeder losses are 15 dB, the base station antenna gain is 6 dBi, the mobile antenna gain is 0 dBi and the mobile feeder losses are 2 dB. Express the field strength at the receiver antenna in dBmV m1.
5.4 See Problem 4.12 (Chapter 4): Calculate the received power (in dBm) at antenna B if antenna A is transmitting at 60 W.
5.5 Calculate the maximum range of the system described in 5.3 using (a) the free space loss and (b) the plane earth loss, assuming a frequency of 2 GHz and antenna heights of 15 m and 1.5 m.
5.6 A 1 V RMS sinusoidal source is applied across the terminals of a half-wave dipole, with source and load impedances perfectly matched. An identical dipole is placed 10 m from the first. What is the maximum power available at the terminals of the receive antenna and under what conditions is this power produced? State any assumptions made in the calculation.
5.7 A satellite is operated at C-band (6 GHz in the uplink and 4 GHz in the downlink) for video broadcasting. Calculate the free-space loss experienced at this frequency, if the satellite is in geostationary orbit (GEO), located at 36 000 km above ground
Basic Propagation Models 103
level. What is the minimum EIRP needed to provide adequate reception, assuming a receiver sensitivity of120 dBm and effective antenna gain of 20 dBi?
5.8 A one-way microwave link operating at 10 GHz has these parameters:
Transmitter power 13 dBW Transmitter feeder loss 5 dB Transmitter antenna gain 18 dBd Receiver antenna gain 10 dBi Receiver feeder loss 3 dB
Receiver noise bandwidth 500 kHz Receiver noise figure 3 dB
The receiver operates satisfactorily when the signal-to-noise ratio at its input is at least 10 dB. Calculate the maximum acceptable path loss.
5.9 A transmit antenna produces an EIRP of 1 W and is received by an antenna with an effective aperture of 1 m2at a distance of 1 km. The frequency is 100 MHz.
Calculate the received power and the path loss. Assuming this is the maximum acceptable path loss for the system, how does the maximum system range change at 1 GHz and 10 GHz, assuming all other parameters remain constant?
104 Antennas and Propagation for Wireless Communication Systems