A brief description of the GSM base station coverage extension system

This paper provides an overview of the role and operational principle of the GSM base station coverage extension system. It focuses on analyzing the uplink and downlink balance after installing such a system and highlights important considerations for implementation. I. Introduction With the growing number of mobile users, their activities have expanded to more remote areas, requiring mobile networks to extend both the scope and depth of their coverage. However, traditional methods like adding new base stations or using repeaters are often impractical in challenging terrains such as mountainous roads and remote villages due to high costs and low return on investment. It is well known that increasing a base station’s transmit power can extend its coverage. According to the free space attenuation formula: Ls = 32.45 + 20*log(f)MHz + 20*log(d)Km, a 6dBm increase in transmit power can double the coverage distance. While this enhances the signal strength received by mobile devices, most phones have a maximum transmit power of only 33dBm. If a phone is too far from the base station, its uplink signal may not be properly received, leading to imbalanced coverage—resulting in one-way communication, poor call quality, and dropped calls. The base station coverage extension system addresses this issue by improving both uplink and downlink signals, offering an effective solution for better signal coverage and deeper penetration. II. Base Station Coverage Extension System The system typically consists of two main 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 boost the downlink signal strength, while the tower is placed at the antenna port to enhance the uplink signal from the mobile device. This dual-direction enhancement ensures more balanced and reliable communication. Usually, a base station operates with a transmit power of 43dBm per carrier. With a 200W base amplifier, the output can reach 53dBm per carrier, providing a 10dB improvement in downlink signal strength. Similarly, the system also boosts the uplink signal by about 10dB, effectively extending the coverage area of the base station. III. Impact of Installing the Base Station Coverage Extension on Uplink and Downlink Balance 1. Noise Figure Analysis After Installing the Tower The noise figure (NF) measures how much an amplifier degrades the signal-to-noise ratio. A lower NF means less degradation. In a multi-stage amplification system, the overall noise figure is dominated by the first stage. By placing a low-noise amplifier at the front end of the base station's receiving system, the tower significantly improves the base station’s reception performance. 2. Uplink and Downlink Balance Analysis 2.1 Definition of Uplink and Downlink Balance For proper communication, both uplink and downlink links must function correctly. The base station must receive and demodulate signals from the mobile device, and the mobile device must receive and process signals from the base station. 2.2 Theoretical Calculation of Uplink and Downlink Balance In theory, the base station’s uplink receiving sensitivity is -110dBm, with a diversity gain of about 4dB. The mobile phone’s receiving sensitivity is -104dBm, and its maximum transmit power is 33dBm. Assuming a 4dB loss in the feeder and a certain antenna gain, the system achieves balance when the uplink and downlink losses are equal. When the base station’s single-carrier transmit power is 43dBm, the system is ideally balanced. 2.3 Uplink and Downlink Balance After Installing the Base 2.3.1 Tower Gain Selection The system’s noise figure without the tower is calculated based on the base station’s own noise figure and feeder loss. After installing the tower, the noise figure becomes mainly dependent on the tower’s noise figure. The tower gain should be carefully chosen to avoid excessive noise levels. Typically, the tower gain is limited to around 10dB to maintain good signal quality. 2.3.2 Base Power Selection Under normal conditions, when the base station’s transmit power is 43dBm, the system achieves ideal balance. After installing the tower, adjustments may be needed to ensure continued uplink and downlink stability. Factors such as the tower’s noise figure, feeder loss, and gain all play a critical role in maintaining this balance.

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