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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 being implemented in cities such as Xiamen, Baoding, and Qingdao. It is expected that after this trial period, the government will announce a clear policy regarding the issuance of 3G licenses. Based on current 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. This would lead to the deployment of an independent TD-SCDMA network across the country. Currently, China Telecom and China Netcom operate PHS networks with over 90 million users, covering nearly all densely populated areas in towns and villages. As a result, TD-SCDMA and PHS systems may coexist for a long time. Notably, the PHS system currently uses the 1900–1920 MHz band, which overlaps with TD-SCDMA’s frequency range. Therefore, studying the interference between these two systems is essential and urgent.
When TD-SCDMA and PHS systems operate in the same area, their mutual 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 performance of the two systems meets the requirements for base station-to-terminal, terminal-to-base station, and terminal-to-terminal coexistence. However, the interference between base stations requires further study. This report applies deterministic analysis methods to evaluate the coexistence of TD-SCDMA and PHS base stations.
2. Deterministic Analysis Method
To analyze the interference from System A to System B, the following equation is used [2]:
Pe(Fi) - MCL(Fi) ≤ Imax(Fi) (1)
Where:
- Fi is the frequency of interest;
- 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 this equation, different interference scenarios can be classified. For instance, if the useful signal from System A interferes with System B's receiving band (excluding adjacent channels), it is called blocking interference. The receiver’s ability to resist out-of-band strong signals determines the acceptable Imax threshold.
If the out-of-band spurious emissions from System A interfere with System B’s passband, it is referred to as out-of-band interference. In this case, the receiver’s sensitivity to external interference defines the Imax threshold.
For adjacent channel interference, the useful signal from System A may interfere with the first adjacent channel of System B, or the leakage power from System A may fall into the passband of System B. These situations require considering the receiver’s adjacent channel selectivity and sensitivity tolerance.
In general, the sensitivity loss due to interference is within 0.2 dB to 1 dB. In this study, a sensitivity loss of 0.8 dB is assumed. This means that TD-SCDMA and PHS base stations can tolerate maximum external interference levels of -115 dBm/1.28 MHz and -123 dBm/300 kHz, respectively.
3. Main Results of Interference Analysis
3.1. 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 (as shown in Table 1). Using these parameters, Equation (1) can be applied to calculate the required minimum coupling loss (MCL) for different interference scenarios.
The MCL between base stations includes factors like transmit antenna gain, receive antenna gain, and isolation loss between antennas. It can be expressed as:
MCL = IL(dB) - Gain_Tx(dB) - Gain_Rx(dB) (2)
Where:
- Gain_Tx is the transmit antenna gain;
- Gain_Rx is the receive antenna gain;
- IL is the isolation loss between the two antennas.
Table 1 shows the RF parameters of TD-SCDMA and PHS used in deterministic analysis.
During the deterministic analysis, the focus was on extreme situations where small interference could cause significant issues. Assumptions were made for the smart antenna of the 8-antenna array. On the traffic channel, the multi-antenna synthesis power factor was 9 dB when transmitting, and the beam shaping factor was 7 dB. When receiving, only one beamforming factor of 7 dB was considered. Additionally, it was assumed that the antenna gain remained consistent regardless of whether the interference was 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 were made as follows:
- TD-SCDMA transmit end: Gain_Tx = 11 + 7 + 9 = 27 dB
- TD-SCDMA receive end: Gain_Rx = 11 + 7 = 18 dB
- PHS transmit end: Gain_Tx = 9 dB
- PHS receive end: Gain_Rx = 9 dB
3.2. TD-SCDMA Base Station Interfering with PHS Base Station
When the TD-SCDMA base station transmits at 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, while the TD-SCDMA base station transmits at 21 dBm (maximum is 30 dBm, with each user occupying two code channels). Thus, the minimum coupling loss required is MCL = 21 dBm - (-15 dBm) = 36 dB.
When operating at 2010–2025 MHz or 1880–1900 MHz, the TD-SCDMA base station also generates out-of-band interference. According to 3GPP specifications, the out-of-band spurious radiation requirement is -39 dBm/1.28 MHz or -45.3 dBm/300 kHz. With a PHS base station receiving sensitivity of -123 dBm/300 kHz, the minimum coupling loss needed is MCL = -45.3 dBm - (-123 dBm) = 77.7 dB.
If the frequencies are close, such as at 1900 MHz, adjacent channel interference occurs. The PHS base station has an adjacent channel selectivity (ACS) of -47 dBm. With a TD-SCDMA base station transmit power of 21 dBm, the required MCL 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 method, the minimum coupling loss required for interference from the PHS base station to the TD-SCDMA base station is calculated. The results are summarized in Table 2.
From Table 2, it is evident that PHS interference on TD-SCDMA is more severe than the reverse, especially in out-of-band interference cases. This is because the out-of-band spurs of the PHS base station are relatively high.
4. Discussion on Solutions in Project Implementation
From the above analysis, it is clear that to prevent interference between the two base stations, specific isolation losses are required, as shown in Table 2. This section discusses how to implement these isolation requirements in practical engineering.
4.1. Using Spatial Isolation
Spatial attenuation 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 the two base stations.
Table 3 provides the required space distances. From the calculation, it is found that achieving the required isolation through spatial separation alone is impractical, as the maximum distance required in extreme cases can reach 19 km.
4.2. Increasing the Protection Bandwidth
TD-SCDMA’s transmission characteristics consider coexistence with non-synchronous TDD systems. Its adjacent channel leakage power limit is -29 dBm/1.28 MHz. If a 1.6 MHz protection bandwidth is used, its second adjacent channel leakage power falls into the PHS receiver passband. The required MCL for adjacent channel interference is 87.7 dB. If the protection bandwidth is increased to 3.2 MHz, the adjacent channel interference becomes out-of-band interference, and the required MCL is 77.7 dB. However, increasing the protection bandwidth beyond 3.2 MHz does not significantly improve performance, as out-of-band interference remains the main issue.
Similarly, for PHS interference on TD-SCDMA, increasing the protection bandwidth has little effect. The out-of-band spurs of PHS are large, and even with a 1.6 MHz protection bandwidth, the required MCL is still 95.1 dB. Thus, increasing the protection bandwidth is not an effective solution. At this stage, since TD-SCDMA operates at 2010–2025 MHz and PHS at 1900–1920 MHz, there is no need to increase the protection bandwidth.
4.3. Adding Filters
Since spatial isolation and protection bandwidth are not effective, adding filters directly to the top of the TD-SCDMA and PHS transceivers is considered. The filter requirements for satisfying the isolation are listed in Table 4. Due to the high out-of-band spurs of PHS, the filter requirements are more stringent than those for the 1880–1900 MHz band.
4.4. Antenna Installation
If the antennas of the two systems are installed close together (within 20 meters), they can be considered co-sited. The antenna installation isolation can be calculated using empirical formulas. Vertical installation provides better isolation than horizontal. For example, with a vertical distance of 20 meters and a horizontal distance of 1 meter, the isolation is 109.6 dB, whereas with a vertical distance of 1 meter and a horizontal distance of 20 meters, the isolation is only 63.8 dB.
4.5. Summary of Findings
Adding filters is the most straightforward method, but it is technically challenging and costly, especially for existing PHS base stations. Other methods, such as vertical installation and increasing the distance between the two systems, can help achieve the required isolation loss. However, due to the widespread coverage of PHS, finding suitable locations for TD-SCDMA base stations is difficult.
5. Conclusion
Through the analysis, it is clear that TD-SCDMA and PHS systems interfere with each other. The main reason is the lack of strict emission standards for PHS, particularly its large out-of-band spurs, which affect TD-SCDMA. TD-SCDMA also interferes with PHS, especially when operating near the same frequency. Measures to increase the protection bandwidth have limited effectiveness. Given the extensive deployment of PHS, finding suitable locations for TD-SCDMA base stations is challenging. Therefore, further research on the coexistence of TD-SCDMA and PHS is necessary.