The Rationale for Tower-Top Amplifiers

1. Chapter 7 The Rationale for Tower-Top Amplifiers

2. The Battle with Noise • At the end of every radio link, there is a struggle to hear the signal • if the signal is sufficiently stronger than the noise, it can be amplified and processed successfully • if the signal is too weak, noise covers it and it cannot be successfully demodulated • analog signals sound scratchy, unintelligible • digital signals have high bit error rates and are unintelligible • Noise is insidious and ever-present • ordinary heat in the environment generates radio noise even in innocent environments like cables and other circuitry • The true measure of usability of a signal is not just its raw strength, but rather its Signal-to-Noise Ratio

3. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - The Nature of Noise Current is the flow of electrons past a point in a conductor. Wire What is a signal? A current, deliberately caused, flowing past a point along a conductor such as a wire. The current contains information and we desire to amplify it and extract the information. Wire What is noise? Noise is an unintended, incidental current flowing past a point along a conductor such as a wire. The current is caused by the random motion of electrons in reaction to thermal agitation or some other unintended mechanism. The noise current contains no useful information, and may completely mask and cover up any weaker signals which are competing with it in the same wire. Thermal noise is proportional to absolute temperature (degrees Kelvin), which is its energy source.

4. Types of Noise in Receivers • Thermal Noise ("White Noise") (the main noise in wireless receivers) • Caused by the random thermal agitation of electrons • Called "white" because it covers the whole spectrum, with a specific amount of energy per hertz of bandwidth • Noise Power = K T B • K = Boltzman's Constant, 1.38 x 10-23 watts/Hertz • T = temperature, degrees Kelvin • B = bandwidth, hertz • Shot Noise (not commonly a problem in wireless systems) • Apparent when currents are so small that the passage of individual electrons is noticeable • Analogy: 10,000 people clapping make a steady roar, but when just two people are clapping, the individual claps are noticeable • Partition Noise (not commonly a problem in wireless systems) • noise resulting fluctuations in current flow within electronic devices

5. Conventional System Tower-Top Amplifier System • Tower-top amplifiers "lock-in" the signal-to-noise at its best value, before it is degraded by transmission line loss. • Tower-top amplifiers often have better noise figures than the base-station front-end amplifiers they feed, resulting in even better system S/N enhancement • Tower-top amplifiers often have better dynamic range and intermodulation performance than their downstairs counterparts Gain 15 db Signal level -110 dbm Signal level -110 dbm Thermal Noise -129.2 dbm Thermal Noise -129.2 dbm Signal level -95 dbm Thermal Noise -114.2 dbm -3 db line loss -3 db line loss Signal level -113 dbm Signal level -98 dbm S/N ratio degraded to 16.2 db and further amplification cannot improve it Noise level -117.2 dbm Noise re-established at -129.2 dbm S/N R 19.2 db Conventional vs. Tower-Top AmplifiersIn Mobile Communication Facilities

6. Evaluating Benefits of Tower-Top Amplifiers

7. -50 -60 -70 -80 RSSI, dBm -90 -100 -110 -120 0 3 6 9 12 15 18 21 24 27 30 33 Distance from Cell Site, km What is the Relationship betweenGain and Coverage? • A gain improvement from tower-top amplifiers translates into an improvement in reverse-link signal strength from any mobile location • The net improvement in range is statistical, since the path loss from mobile locations is statistical • The graph at right shows the statistical fluctuation of real-world signal strength as a function of distance from the base station, with the effects of a modest increase in reverse link gain shown as red (after) and green (before) • The table at right shows the actual coverage distance increase in areas of specified path loss slopes 10 Log (PathLoss) = K (Slope) Log (Distance)

8. 34 25 What is the Relationship between Gain and Number of Cells Required to Cover an Area? • The table below shows the effects of a 3 db gain improvement on the number of required cells in several typical path loss slopes • The cell-grid overlays at right illustrate graphically a typical reduction in number of cells for a real system due to typical gain improvement from tower-top amplifiers

9. Special Considerations When Using Tower-Top Amplifiers • Physical Integrity • commercial units for wireless applications have rugged housings designed to withstand years of environmental effects • Lightning and Transients • commercial units combine well-designed isolation with advanced surge suppression and decoupling techniques to achieve immunity to even direct lightning transients • Dynamic Range and Intermodulation Production • Modern units have dynamic range and intermod intercept points even higher than the base stations they feed • System Levels - Handoff and Traffic Considerations • Proper attention to system parameters settings is required to achieve improved system performance from the improved reverse link gain • Forward/Reverse Link Balance • After installation of tower-top amplifiers, actual drive tests should be completed to determine if adjustments to the forward link may also be beneficial