Selecting Tower Mounted Amplifiers (Kaelus white paper)

Selecting Tower Mounted Amplifiers – 1v0

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Network Coverage at its best Tower Mounted Amplifiers can help!

Table of Contents 1. Overview 1

2. Introduction 2

3. BTS Cell Coverage Fundamentals 2

4. BTS Cell Coverage in Practice 3

5. BTS Receiver Sensitivity 4

6. Site Installation Influence on BTS Sensitivity 4

7. Benefit of Installing a MHA 5

8. TMA Influence on BTS System Parameters 6

9. Choosing the Right TMA 7

1. Overview:

As cellular telephone networks continue to expand, operators keep finding cost effective ways to improve network performance. Improvements that provide increased capacity and better coverage. Achieving maximum coverage is often as easy as boosting the uplink signal from the network users cell phone, at the BTS (base station), and should not be confused with increasing the possible coverage from a site by boosting the downlink as well (increasing the link budget). Appropriately installed low noise amplifiers (LNA)s in the BTS uplink (receive channel) will significantly improve receiver system sensitivity when installed as close as possible to the receive antenna (particularly where cable losses are significant). LNAs located in this way are referred to as Tower Mounted Amplifiers (TMA). This paper provides a generalized overview of BTS receiver system sensitivity, and the benefits of installing a quality TMA.



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2. Introduction:

The following discussion attempts to simplify the technical issues relevant to improving BTS cell coverage. The aim being to provide a broad yet substantial understanding of the subject for all network stakeholders. The issues dealt with include:

• BTS cell coverage fundamentals • BTS cell coverage in practice • BTS Receiver sensitivity • Site installation influence on BTS sensitivity • Benefit of installing a TMA • TMA influence on BTS system parameters • Choosing the right TMA

3. BTS Cell Coverage Fundamentals:

The two primary components of any cell are the operators BTS installation and the customer handset or cell phone. These two items must be able to communicate with each other. Installed at a fixed location, the BTS is the major component of the cell. While there are many geographic and environmental issues to be considered, this is where all network coverage improvements are to be made.

The customer’s cell phone is powered from a battery, it has low transmit (TX) power and a small antenna, and is therefore often the weak link in the communications chain. The amount of “talk time” available is a function of the rate that stored power is consumed from the battery. The less power that the cell phone needs to output the longer the battery will last. TX power from the handset is controlled by a command from the BTS. Here the quality of the received (RX) signal from the cell phone is continually monitored. The result is a bit error rate (BER) value which the BTS attempts to minimize by instructing the cell phone to increase its TX power.



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Generally, a BTS installation will support a number of cells or sectors from a common antenna tower. Each of these sectors has a theoretical foot print within which cell phones will operate. This is the BTS coverage referred to in this paper. BTS equipment is designed to have enough TX power for its signals to reach the outer limits of a cell and matching Rx sensitivity to detect signals from cell phones transmitting at this outer limit. Both are generally measured in dB and are referred to as the link budgets. The signal strength relationship between the BTS downlink (TX) and uplink (BTS RX or cell phone TX) is referred to as the link budget balance. In theory, improving the uplink sensitivity (budget) should be matched by the same improvement to the down link Tx power in order to maintain link balance. 4. BTS Cell Coverage in Practice:

Cell coverage is also a function of the gain and positioning of the BTS antenna. Regardless of design choices made while considering these factors, the following remains true. In practice, network operators have found, through field tests, the BTS uplink is the cause of less than expected theoretical coverage. The BTS downlink signal strength at the outer limit of a cell is generally well within specification but the received signal from the cell phone is either marginal or not possible to be detected by the BTS receiver at all. This has been found to be caused by an uplink shortfall (imbalance) of typically 3 to 5dB.

BTS sensitivity here is -105dB showing that a uplink carrier signal from

a cell phone being barely detected.

Figure 2 The above conceptual drawing (figure 2), attempts to present the situation where the sensitivity of the BTS uplink is found to be -105dBm and is therefore unable to detect a cell phone signal at -104dBm. With this BTS uplink sensitivity, the cell phone signal would need to be at -98dBm to be detected with a BER of 1%(7dB C/N). The following model shows the same problem in a different way. Here the downlink arrow shows sufficient power to comprehensively reach the cell phone while the uplink arrow shows a short fall in uplink signal of 3 to 5dB to balance the link.



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Figure 3

5. BTS Receiver Sensitivity:

Receiver sensitivity is a function of the sum of three fundamental factors.

• Ambient Noise Power (NP) - Ambient Noise Power is a measure of the noise in nature. Noise being the random electromagnetic signals resulting from the movement of atomic particles of all matter. In the band width of a GSM carrier, this is approximately -120dBm.

• Carrier to Noise ratio (C/N) - C/N is a measure of the relative strength that the received signal must be above the noise floor to ensure satisfactory detection. In a GSM BTS, a BER of 1% is achieved when the received carrier signal from the cell phone is 7dB above the background noise of the receiver.

• Noise Figure (NF) - Noise Figure is a measure of extra noise caused by the receiver circuitry. For a GSM receiver this is typically 3.5dB.

The maximum sensitivity of a GSM receiver channel is:

Sensitivity = Noise Power + Noise Figure + Carrier to Noise ratio Sensitivity = -120dB + 3.5dB + 7dB

= -109.5dBm. Being a function of the GSM carrier bandwidth and circuit design, both Noise Power and Carrier to Noise ratio are fixed as far as the network operator is concerned. The only component effecting receiver sensitivity that may be improved by the operator is the system Noise Figure, NF. 6. Site Installation Influence on BTS Sensitivity:

In any given installation, the BTS equipment must be connected to antennas. These are generally mounted at the top of a tower, some distance from the BTS equipment, and connected via coaxial cable. Depending on site configuration, filters and duplexer may also be used in the signal path. These components introduce a loss in the signal path which is added directly to the BTS receiver NF. Typically these additional losses can be as much as 4.5dB. The example of the GSM Rx sensitivity, given in the previous section (BTS Receiver sensitivity), now changes as follows:

BTS NF = 3.5 dB + 4.5dB (3.5dB is BTS NF, 4.5dB Feeder losses) = 8dB

Sensitivity = Noise Power + Noise Figure + Carrier to Noise ratio Sensitivity = -120dB + 8dB + 7dB

= -105dBm Refer back to Figure 2.



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7. Benefit of Installing a MHA:

A TMA is used to reduce the system NF and therefore increase sensitivity. As previously mentioned a TMA is a LNA mounted as close as practical to the sector Rx antenna. In this way, the cable losses are negligible and do not significantly affect system noise figure. System NF is calculated as follows:

NFs = 10 log(Fs)

Fs = F1 + [(F2 -1)/G1]

Where:

NFs = System Noise Figure Fs = System Noise Factor F1 = Noise factor of the TMA low noise amplifier F2 = Noise factor of the BTS Rx (includes system losses) G1 = Gain of the TMA (multiplier not dB) Note: Noise factor is a multiplier not dB. In theory cascading LNAs will reduce the NF toward zero. A NF of between 1.5 and 2dB is the practical limit of the BTS system. The gain choice of gain in an LNA is as crucial as its NF. Typically the range is 8dB to 16dB. Less than 8dB gain and the NF improvement reduces significantly and with more than 16dB you will be only be amplifying the noise floor and incurring excessive BTS dynamic range compression. The following table gives an example of the typical improvements that are possible using TMAs with either a Picocell or a Macrocell.

Base Station Configuration

With 12dB Gain TMA NF = 1.6dB

With 16dB Gain TMA NF = 1.6

Picocell 6dB NF – Base Station 2dB Feeder Loss 8dB total System NF

The system performance is improved to 2.5dB NF. More than 5.5dB better.

The system performance is improved to 2.0dB NF. More than 6dB better.

Macrocell 3dB NF – Base Station 2dB Feeder Loss 5dB total System NF

The system performance is improved to 2.0dB NF. More than 3.0dB better.

The system Performance is improved to 1.76dB NF. More than 3.2dB better.

Table 1

Here the Picocell system has its NF improved to as much as 2.0db and the Macrocell to 1.76db, a system sensitivity improvement of 6 and 3.2 dB respectively. Referring back to the example given in figure 2 where the BTS uplink sensitivity was calculated to be -105dB, the following diagram provides a conceptual view of the benefit of using a TMA.



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Figure 4

Typically an low noise TMA with a gain of 12dB would be used to achieve the above results. These improvements in uplink sensitivity simply allow the BTS to work at its best. In return, as much as a 40% improvement in cell phone coverage can be achieved.

Expected increase in cellular phone coverage by installing Tower Mounted Amplifiers

Figure 5 This increase in coverage is a direct result of improving the BTS uplink sensitivity. The benefits are considerable:

• More content network users, lower BER therefore less dropped calls • Lower Tx power from the user cell phone therefore longer battery life • Improved network performance, leading to a better investment returns • Happy customers, good coverage, no dropped calls

8. TMA Influence on BTS System Parameters:

BTS manufacturers design their equipment so that the dynamic range of the BTS receiver is sufficient to ensure un-distorted detection of the closest transmitting cell phone to those on the outer perimeter of the cell.



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While a TMA will improve the BTS uplink sensitivity it also introduces additional gain to the RX system. This has the effect of reducing the dynamic range of the receiver making strong signals even stronger. Operator feedback suggests, for TMAs with gains in the order of 12dB, the detrimental effect on signal detection is negligible. Generally, no BTS parameter changes are needed when fitting a TMA. Within limits, the signal strength of in band receive signals will determine the output power of the relevant cell phone, therefore introducing a TMA should require no significant system parameter changes on this account. The possibility of additional “just” out of band interfering signals becoming a problem is relevant. The likelihood of a weak in band Rx signal being blocked by these signals should be considered when selecting the TMA. Well designed TMAs will include highly selective band pass filtering prior to the LNA stage. For well designed BTS installations, the out of band rejection characteristic of these filters will neutralize potential RX blocking problems and possibly even improve sector performance. 9. Choosing the Right TMA:

Adding TMAs increases the investment in the network. Selecting the right product is paramount to achieving greater returns without increasing maintenance costs. The TMA should be selected, installed and forgotten. It should be as reliable as the coaxial cable it is connected to. Many companies can supply TMAs with very good Gain and NF specifications, however, to be able to “install and forget” requires the selection of a product designed and manufactured to survive often hostile installation environments. The TMA must:

• Be easily added to a site installation • Include excellent out of band frequency rejection • Incorporate Antenna Interface Standards Group (AISG) recommended specifications

for digital remote control and monitoring • Use heavily de-rated electronic components and robust moisture proof coaxial

cable connectors • Be housed in a strong weather proof enclosure made from materials that are

corrosion resistant. • Be lightning strike protected • Be physically tested for the following at time of design and manufacture:

o Sound design and construction by vibration testing, o Thermal reliability and stability by temperature testing o Pressure tested to ensure the design continues to work over a range of

different installation altitudes.



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These last three tests can only be achieved using the right laboratory equipment:

(a) (b) (c)

Figure 6

Shown above is:

a. Thermal oven with a testing range of -40 to +65 Deg C. b. Pressure testing apparatus with altitude simulation to >4000m c. Vibration table with a capability of 5-3000Hz and up to 100g

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Rev_B_8.31.11

Technology

Selecting Tower Mounted Amplifiers (Kaelus white paper)