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TD-SCDMA base station and PHS base station interference coexistence problem analysis report
1. Introduction
The "TD-SCDMA Scale Network Technology Application Test" conducted by China Mobile, China Telecom, and China Netcom under the Ministry of Information Industry is currently taking place in cities like Xiamen, Baoding, and Qingdao. It is expected that after the trial of this technology, the government will announce clear policies regarding the issuance of 3G licenses. According to industry analysis, it is likely that one or both of the fixed-line operators, China Telecom and China Netcom, will be granted a TD-SCDMA license, leading to the nationwide deployment of an independent TD-SCDMA network. Currently, China Telecom and China Netcom operate a PHS network with over 90 million users, covering almost all densely populated areas across the country. As a result, the two systems may coexist for a long time. Notably, the PHS system operates in the 1900–1920 MHz band, which overlaps with the frequency range of TD-SCDMA. This overlap necessitates the study of interference between the two systems, making it a critical issue.
For TD-SCDMA and PHS systems operating in the same area, the interference can be categorized into four types: TD-SCDMA base station interfering with PHS central station, TD-SCDMA terminal interfering with PHS central station, TD-SCDMA base station interfering with PHS subscriber station, and TD-SCDMA terminal interfering with PHS subscriber station. According to simulation analysis in [1], the existing RF indicators of the two systems meet the requirements for coexistence between base stations and terminals. However, further research is needed on the interference between base stations. This report employs deterministic analysis methods to examine the coexistence of TD-SCDMA base stations and PHS base stations.
2. Deterministic Analysis Method
When System A interferes with System B, the following interference assessment equation can be used [2]:
Pe(Fi) - MCL(Fi) ≤ Imax(Fi) (1)
Where Fi represents the frequency being studied;
Pe(Fi) is the transmit power or stray radiation of the interfering transmitter at frequency Fi;
MCL(Fi) is the minimum coupling loss between the transmitter and receiver at frequency Fi;
Imax(Fi) is the maximum acceptable interference level at frequency Fi.
Based on the above formula, different interference scenarios can be classified as follows:
- **Blocking Interference**: When the useful signal from System A's transmitter (typically with high power) causes interference in System B’s receiving band (excluding adjacent channels), it is known as blocking interference. This primarily assesses the receiver's ability to resist strong signals outside its operating frequency. The threshold for Imax is generally based on the out-of-band blocking characteristics of the receiver.
- **Out-of-Band Interference**: When the out-of-band spurious emissions from System A interfere with the passband of System B, it is called out-of-band interference. This considers the receiver’s sensitivity to withstand maximum interference, with the Imax threshold typically based on the receiver’s sensitivity tolerance.
- **Adjacent Channel Interference**: This occurs when the useful signal from System A’s transmitter interferes with the first adjacent channel of System B, or when the adjacent channel leakage power of System A falls into the passband of System B. In such cases, the Imax threshold is determined by the receiver’s adjacent channel selectivity and sensitivity tolerance.
If interference occurs within the receiver’s passband, the noise level increases, affecting the receiver’s sensitivity. Typically, a sensitivity loss of 0.2–1 dB is considered acceptable. In this study, a base station receiver sensitivity loss of 0.8 dB is assumed, meaning that TD-SCDMA and PHS base stations can tolerate external interference levels of -115 dBm/1.28 MHz and -123 dBm/300 kHz, respectively.
3. Main Results of Interference Analysis
3.1. Analyze the System Parameters Used in the Calculation
According to references [3] and [4], the system parameters such as blocking characteristics, spurious emissions, adjacent channel selectivity, and adjacent channel leakage power of TD-SCDMA and PHS are provided (see Table 1). Based on these values, Equation (1) can be used to calculate the minimum coupling loss (MCL) required for different interference situations. The MCL between base stations includes the transmit antenna gain, receive antenna gain, and isolation loss between the antennas, expressed as:
MCL = IL(dB) - Gain_Tx(dB) - Gain_Rx(dB) (2)
Where Gain_Tx is the transmit antenna gain, and Gain_Rx is the receive antenna gain. IL represents the isolation loss between the two antennas.
Table 1: TD-SCDMA and PHS RF Parameters Used in Deterministic Analysis
[Image: http://i.bosscdn.com/blog/09/24/3M/Y0-0.gif]
The deterministic analysis method focuses on extreme scenarios where small coexistence interference may occur. Assumptions include: for an 8-antenna smart array, the multi-antenna synthesis power factor is 9 dB during transmission, and the beam shaping factor is 7 dB. During reception, only a single beamforming factor of 7 dB is considered. Additionally, it is assumed that the antenna gain remains consistent whether the interference is in-band or out-of-band.
Assuming a TD-SCDMA antenna gain of 11 dBi and a PHS antenna gain of 9 dBi, the calculations for the transmitting and receiving ends are as follows:
- TD-SCDMA Transmit Gain_Tx = 11 + 7 + 9 = 27 dB
- TD-SCDMA Receive Gain_Rx = 11 + 7 = 18 dB
- PHS Transmit Gain_Tx = 9 dB
- PHS Receive Gain_Rx = 9 dB
3.2. TD-SCDMA Base Station Interfering with PHS Base Station
When the TD-SCDMA base station transmits on 2010–2025 MHz or 1880–1900 MHz, it causes blocking interference in the PHS base station receiver. The blocking characteristic of the PHS base station in the TD-SCDMA transmission band is -15 dBm, and the maximum transmit power of the TD-SCDMA base station is 30 dBm. Assuming each user occupies two code channels, the effective transmit power is 21 dBm. Therefore, the minimum coupling loss required to protect the PHS base station is MCL = 21 dBm - (-15 dBm) = 36 dB.
When the TD-SCDMA base station operates at 2010–2025 MHz or 1880–1900 MHz, it generates out-of-band interference to the PHS base station. According to 3GPP specifications, the out-of-band spurious radiation of TD-SCDMA when coexisting with an asynchronous TDD base station at 1900 MHz is -39 dBm/1.28 MHz or -45.3 dBm/300 kHz. The receiving sensitivity of the PHS base station is -123 dBm/300 kHz. Thus, the minimum coupling loss required is MCL = -45.3 dBm - (-123 dBm) = 77.7 dB.
If the working frequencies of the two base stations are close, causing adjacent channel interference, the adjacent channel selectivity (ACS) of the PHS base station is -47 dBm. With a TD-SCDMA base station transmit power of 21 dBm, the minimum coupling loss required is MCL = 21 dBm - (-47 dBm) = 68 dB.
Additionally, the adjacent channel leakage power of TD-SCDMA is -29 dBm/1.28 MHz or -35.3 dBm/300 kHz. Considering the PHS base station’s receiving sensitivity of -123 dBm/300 kHz, the minimum coupling loss required is MCL = -35.3 dBm - (-123 dBm) = 87.7 dB.
3.3. PHS Base Station Interfering with TD-SCDMA Base Station
Using the same analysis method as described earlier, the minimum coupling loss required for PHS base station interference to TD-SCDMA base stations was calculated. The results are summarized in Table 2.
Table 2: Required Isolation Loss Calculations
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From Table 2, it is evident that the interference from PHS to TD-SCDMA is greater than that from TD-SCDMA to PHS, especially in the case of out-of-band interference. This is due to the relatively high out-of-band spurs emitted by the PHS base station.
4. Discussion on Solutions in Project Implementation
From the previous analysis, it is clear that when TD-SCDMA and PHS coexist, the required isolation loss is shown in Table 2. This section discusses how to implement these isolation loss requirements in real-world engineering projects.
4.1. Using Spatial Isolation
The spatial attenuation of the signal is calculated using a free-space propagation model. The formula is:
Lf = 20 log(R) + 38.12 (3)
Where Lf is the free space loss (dB), and R is the distance (in meters) between two base stations.
Table 3: Required Space Distance
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From the calculations, it is clear that achieving the required isolation loss through spatial separation alone would require a maximum distance of 19 km in extreme cases, which is impractical in most network configurations.
4.2. Increasing the Protection Bandwidth
The transmission characteristics of TD-SCDMA consider coexistence with non-synchronous TDD systems. Its adjacent channel leakage power limit is -29 dBm/1.28 MHz in the first and second adjacent channels. If a 1.6 MHz protection bandwidth is used, the second adjacent channel leakage power falls within the PHS receiver passband, requiring a minimum coupling loss of 87.7 dB. If a 3.2 MHz protection bandwidth is used, the adjacent channel interference becomes out-of-band interference, requiring a minimum coupling loss of 77.7 dB.
However, increasing the protection bandwidth beyond 3.2 MHz offers little improvement, as out-of-band interference remains the dominant factor.
4.3. Adding Filters
Given the limitations of spatial isolation and bandwidth protection, adding filters directly to the top of TD-SCDMA and PHS transceivers is another approach. Some key technical indicators of the filters required to achieve the necessary isolation are listed below.
[Image: http://i.bosscdn.com/blog/24/74/21/5-1G22109230A10.png]
It is important to note that the PHS system has larger out-of-band spurs than its adjacent channel leakage power, making the filter requirements more stringent.
4.4. Antenna Installation
If the antennas of the two systems are installed close together, such as within 20 meters, they can be considered as co-site installations. The isolation loss in such cases can be calculated using empirical formulas.
Ih = 22 + 20 log(Dh/Lmd) - (Gt(q) + Gr(q)) (4)
Iv = 28 + 40 log(Dv/Lmd) (5)
Where Ih is horizontal isolation, Iv is vertical isolation, Dh is horizontal distance, Dv is vertical distance, Gt(q) and Gr(q) are the antenna gains, and Lmd is the wavelength.
From the calculation, it is clear that vertical installation provides better isolation than horizontal installation. For example, a vertical distance of 20 meters and a horizontal distance of 1 meter can achieve an isolation of 109.6 dB.
4.5. Summary of Findings
Adding filters to the top of TD-SCDMA and PHS transceivers is the most straightforward method. However, implementing such filters is technically challenging and costly, especially for already deployed PHS base stations. Other methods, such as vertical antenna installation and increasing the distance between the two systems, should also be considered. However, given that PHS networks are already widely deployed, finding suitable locations for TD-SCDMA base stations is difficult.
5. Conclusion
Through the above analysis, it is evident that there is significant interference between TD-SCDMA and PHS systems. The main cause of interference is the lack of strict emission standards for PHS, particularly its large out-of-band spurs, which affect TD-SCDMA base stations. TD-SCDMA also interferes with PHS, especially when operating near the PHS frequency band. Moreover, the effectiveness of increasing the protection bandwidth is limited. Given the extensive deployment of PHS base stations, it is challenging to find suitable locations for TD-SCDMA base stations. Therefore, further research is needed on the coexistence of TD-SCDMA and PHS systems.