This paper provides an overview of the role and operational mechanism of the GSM base station coverage extension system. It focuses on analyzing the uplink and downlink signal balance after the installation of the system, while also highlighting important considerations for its deployment.
I. Introduction
As the number of mobile users continues to grow, their activities are expanding into more diverse environments, including remote areas such as mountainous roads and rural villages. To meet this demand, mobile networks must continuously expand and deepen their coverage. However, traditional solutions like building new base stations or using repeaters often face practical challenges in these regions, making them costly and inefficient in terms of return on investment.
It is well known that increasing a base station’s transmission power can significantly extend its coverage. Using the free space attenuation formula: Ls = 32.45 + 20*log(f)MHz + 20*log(d)Km, it can be shown that increasing the transmit power by 6 dBm doubles the coverage distance. While this enhances the received signal strength at the mobile device, the maximum output power of a standard mobile phone is only 33 dBm. If the phone is too far from the base station, the uplink signal may not be strong enough to be properly received, leading to unbalanced uplink and downlink performance. This imbalance can result in one-way communication, poor call quality, and dropped calls.
The base station coverage extension system addresses these issues by improving both the downlink and uplink signals, offering an effective solution for better signal coverage and deeper penetration into challenging areas.
II. Base Station Coverage Extension System
The system primarily consists of two components: a base station amplifier (referred to as the "base") and a tower amplifier (referred to as the "tower"). The base is installed in the base station room to increase the transmit power and expand the downlink coverage. The tower, located at the antenna port, acts as a low-noise amplifier that boosts the uplink signal strength and improves the base station's receiving sensitivity.
The working principle of the system is illustrated in Figure 1. Normally, a base station operates with a transmit power of 43 dBm per carrier. With the use of a 200W base amplifier, the output power can reach 53 dBm per carrier, resulting in a 10 dB enhancement in the downlink signal. Similarly, the system can improve the uplink signal by about 10 dB, effectively extending the overall coverage area.
III. Impact of Installing the Base Station Coverage Extension on Uplink and Downlink Coverage
1. Noise Figure Analysis After Tower Installation
The noise figure (NF) is a key parameter used to measure the degradation of the signal-to-noise ratio in an amplifier. A lower noise figure indicates better signal quality. In a multi-stage amplification system, the total noise figure is dominated by the first stage, as shown by the formula:
$$ NF = F_1 + \frac{F_2 - 1}{G_1} + \frac{F_3 - 1}{G_1 G_2} + \dots $$
Where $ F_1, F_2, F_3 $ are the noise figures of each stage, and $ G_1, G_2 $ are the gains between stages. Passive components have a noise figure equal to their loss value.
The tower is designed to enhance the base station’s receiving performance by adding a low-noise amplifier right at the antenna, thus reducing the impact of noise introduced by long feeder cables.
2. Uplink and Downlink Balance Analysis
2.1 Definition of Uplink and Downlink Balance
For proper communication within the coverage area, both uplink and downlink signals must be strong enough to allow successful demodulation. This ensures that both the base station and the mobile device can establish and maintain a stable two-way connection.
2.2 Theoretical Calculation of Original Base Station System Balance
Assuming ideal conditions, the uplink receiving sensitivity of the base station is around -110 dBm, with a diversity gain of 4 dB. The mobile phone’s receiving sensitivity is -104 dBm, and its maximum transmit power is 33 dBm. Based on these values, the system achieves an ideal balance when the base station’s transmit power is set to 43 dBm, which corresponds to 20 W per carrier.
2.3 Adjustments After Installing the Base Station Coverage Extension
2.3.1 Tower Gain Selection
Without the tower, the system’s noise figure is mainly determined by the base station’s own noise figure and the feeder loss. When the tower is added, the noise figure becomes heavily influenced by the tower’s noise figure. A higher tower gain reduces the overall system noise but may also increase white noise levels, affecting signal reception.
To maintain acceptable uplink quality, the tower gain should typically be limited to around 10 dB, depending on the feeder loss.
2.3.2 Base Power Adjustment
When the base station’s transmit power is set to 43 dBm, the system achieves a balanced state where both uplink and downlink signals are strong enough for reliable communication. By adjusting the tower gain and ensuring optimal power levels, the system can maintain this balance even in challenging environments.
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