[Technical Science] | A brief analysis of radio interference monitoring and troubleshooting methods in 5G and other TDD networks

This article combines the principles of mobile communication TDD technology.

Briefly analyze the common interference types in TDD networks.

Briefly sort out the TDD network

Interference troubleshooting methods,

For on-site monitoring engineers to observe

and locate interfering signal reference.

Through time division multiplexing (TDM) technology, multiple signals can be transmitted through different time slots in the same frequency channel. Time division duplex (TDD) technology is an application of TDM technology, and its reception and transmission use different time slots of the same frequency as the carrier of two-way services. The TDD signal of mobile communication usually appears as continuous occupancy in the frequency domain, but in the time domain it can be clearly found that it is transmitted frame by frame in time, as shown in Figure 3.

Figure 3 Frequency domain and time domain examples of TDD signals

Frequency Division Duplex (FDD) technology is another major mode in modern mobile communications, which often uses two independent channels to transmit information to the terminal and the base station respectively. FDD technology allocates spectrum resources independently for uplink and downlink signals, and uplink and downlink signals are allowed to occupy continuously in time, as shown in Figure 4. Therefore, FDD technology usually requires more spectrum resources, often twice that of TDD technology. In addition, appropriate spectrum isolation must be performed between the transmit channel and the receive channel. This so-called "guard interval" will not be used, resulting in a waste of spectrum resources.

Figure 4 Frequency domain and time domain examples of TDD signals

In the past, FDD technology has been widely used in the field of mobile communications due to its advantages of not requiring clock synchronization and having a wider coverage area. Especially in early mobile communication systems, FDD technology played an important role.

In FDD technology, uplink and downlink signals are transmitted using different frequency bands, which means that clock synchronization is not required between uplink and downlink signals. This brings great advantages in practical applications, because the implementation of clock synchronization often requires additional hardware and software, which increases the complexity and cost of the system. In addition, in some special scenarios, such as high-speed rail and other environments with fast movement speeds, clock synchronization will be more difficult or even impossible to achieve. The clock synchronization-free feature of FDD technology solves this problem well, making it widely used in the field of mobile communications.

In addition, in FDD technology, the uplink and downlink signal frequency bands are separated, which means that the uplink and downlink signals will not interfere with each other. This allows the base station to transmit at a higher power, thereby expanding the signal coverage. In contrast, in TDD (time division duplex) technology, the uplink and downlink signals use the same frequency band, and only one of the signals can be transmitted in the same time period, which limits the power output of the base station, resulting in a relatively small coverage area.

However, as the new generation of communication technology requires wider bandwidth to meet the growing demand for data transmission, spectrum resources are becoming increasingly scarce. FDD technology faces irreconcilable contradictions in dealing with spectrum tension, the demand for large bandwidth from new communication technology, and the efficient use of asymmetric services (such as downlink data is much larger than uplink). These contradictions constitute the "impossible triangle" of FDD technology, that is, it is difficult to meet the requirements of large bandwidth and high resource utilization at the same time under limited spectrum resources. TDD technology can improve spectrum utilization by flexibly allocating uplink and downlink time slot ratios, and can increase data rates by increasing the number of time slots. Therefore, with its characteristics of saving spectrum resources and meeting asymmetric services, TDD technology has gradually been widely used in mobile communication systems in recent years. At present, according to the 3GPP standard and the Ministry of Industry and Information Technology's planning for 5GNR frequencies, most of the 5GNR frequency bands use TDD technology. Although FDD technology is still used, from the overall situation, as the current spectrum resources tend to be tight, it is a general trend to use TDD technology in future spectrum allocation and re-cultivation. Accordingly, field monitoring engineers will also face more and more radio interference in TDD networks, and will need to observe interference signals in the TDD network and track and locate them.

In TDD networks, uplink and downlink signals use exactly the same frequency band. If you use traditional monitoring methods to observe the spectrum, you will find that the stronger downlink signal from the base station completely covers up the uplink signal and other existing signals, such as weak interference signals. Secondly, the TDD signal bandwidth in mobile communications is relatively wide. The TDD signal bandwidth of 4G is usually up to 20MHz, and 5GNR FR1 can be as high as 100MHz, as shown in Figure 5.

Figure 5: 5GNR FR1 TDD signal bandwidth can be up to 100MHz

In TDD networks, the uplink signal power of mobile terminals is usually low and easily interfered by external signals, resulting in the inability of the terminal to establish a stable connection with the base station. At the same time, the downlink signal of the base station is often high in power and often drowns out the uplink signal of the terminal and other interference signals in the spectrum. For field monitoring engineers, it is difficult to effectively identify interference signals in the wider TDD frequency band using traditional radio monitoring methods.

Judging from the interference situation in recent years, the common broadcasting MMDS and satellite station co-frequency interference in the early stage of 5G network construction has gradually decreased. At present, the common interference sources in TDD networks are usually repeaters, wireless WLAN transmission equipment (such as wireless bridges), interference shields, leakage of industrial, scientific and medical equipment, etc.

From the perspective of the spectral time dimension characteristics of interference signals, they are mainly divided into continuous signals and discontinuous signals. Continuous interference signals mainly include glitches or "wandering" sweep-frequency signals caused by leakage of industrial, scientific and medical (ISM) equipment. The spectral characteristics of such signals usually occupy a narrow range, have weak power, and are difficult to monitor and locate. In addition, interference jammers are also common continuous signals. Interference jammers transmit interference signals at high power to block the communication between base stations and terminals. They are mainly deployed in government departments, colleges, and mortgage parking lots. However, interference jammer signals often occupy a wider frequency band, have a significantly raised noise floor, have strong power, and are easier to identify.

Discontinuous interference signals mainly come from wireless WLAN devices, which often occur in China Mobile's N41 frequency band (2515MHz-2615MHz), such as wireless bridges and wireless routers for video surveillance, which are all illegal frequency usage. In order to improve transmission efficiency, some applications such as building, community security, elevators, and tower crane cameras privately use adjustable frequency wireless transmission equipment to monopolize the frequency band for transmission, resulting in overlap with the N41 frequency band, thereby generating co-frequency interference. Since WIFI also uses the TDD duplex mode, if its signal is hidden in the normal TDD network, it is difficult to identify.

Colorful spectrum technology is a display overlay technology that can assist field monitoring engineers in distinguishing two or more co-frequency signals on the same spectrum (at least one must be a time-varying signal). The difficulty in separating co-frequency signals is that they appear in the same frequency band at the same time and may have similar power. In order to cope with such complex co-frequency signal scenarios, instruments with colorful spectrum function store and accumulate a large amount of trace data collected at high speed, and display the accumulation of traces over a period of time in a color-coded manner, so as to analyze the frequency of different co-frequency signals occurring in the same period of time.

By default, signals shown in red (such as the noise floor) appear more frequently than signals shown in green or blue. The multi-color spectrum is very useful for separating signals of the same frequency, such as seeing a Bluetooth signal superimposed on a WiFi signal.

This technology is also applicable to detecting continuous interference signals in TDD networks. When using the multi-color spectrum function to search, it is recommended to slowly rotate the directional antenna so that the instrument can accumulate enough trace data and update the color display.

In the following animation, the "wandering" leakage of industrial, scientific and medical equipment is simulated by the micro-power sweep signal of the signal source. When the multi-color spectrum function of the R&S PR200 monitoring receiver is turned on, only the accumulated granularity (100% time, such as 5-10ms) can be fine-tuned to obtain a better interference visualization effect, so that the "wandering" interference signal can be found in the TDD network.

Animation 1: The colorful spectrum function clearly identifies the "wandering" interference signal in the TDD network

监测仪表以每秒钟超过百万次的采样速率完成对模拟射频信号的采样，所有的采样数据基于特定的测量时间间隔进行采集，并由检波器进行合并后处理。测量时间间隔典型时长是毫秒级或微秒级。当测量时间结束，检测器会在每个像素点下生成一个电平值，这些电平值组成迹线显示在仪表的界面上。当一个测量时间间隔结束，检波器输出结果后，缓存会清空，并开始下一个测量时间间隔。因此在整个采样计算显示过程中，除了检波器，测量时间也会显著影响数据从获取、处理并最终显示这一过程。

For continuous interference signals in TDD networks, the measurement time can be greatly shortened to less than the duration of a subframe (1ms), thereby increasing the spectrum refresh rate to visualize continuous interference in time-varying networks. The high-speed scanning method can be used in intermediate frequency mode or in panoramic scanning mode. When the interference signal occurs in a larger spectrum span, or the interference signal cannot be roughly determined by the operator to affect the frequency band, it is recommended to use the panoramic scanning mode to perform a high-speed scan of the entire frequency band. As shown in Figure 3, when the R&S PR200 monitoring receiver gradually shortens the measurement time, gradually increases the scanning speed, and adjusts the detector to negative peak detection, the "wandering" continuous interference signal can be clearly observed from the time-varying TDD network.

Animation 3: Shorten the measurement time and find the interference signal

If the instrument can support zero span and gated trigger spectrum functions, the frame structure of the TDD signal can be effectively viewed in the time domain and the FFT spectrum corresponding to a specific time slot can be observed. The wireless frame (duration 10ms) in the TDD network (including 3G, 4G, 5GNR, private network, etc.) is divided into 10 subframes (each subframe is 1ms long), and the uplink and downlink subframe ratio is flexibly allocated according to business needs. For example, a common ratio structure is 2 subframes for mobile terminal uplink and 8 subframes for base station downlink. By setting the appropriate demodulation bandwidth (analysis bandwidth) and scanning period in zero span mode (time domain), on-site monitoring engineers can clearly view the uplink and downlink subframes of the TDD signal, as shown in Figure 7.

Figure 7: The uplink and downlink subframes of the TDD signal can be clearly seen in the zero span mode (time domain).

At this point, the on-site monitoring engineer can continue to use the gated trigger function to separate the uplink and downlink signals and eliminate the impact of the base station downlink signal. In this mode, it is necessary to define the time domain trigger position (gated delay) and set the appropriate trigger duration (gated duration). For TDD networks, the gated trigger position can be moved to an idle uplink subframe and a certain trigger duration (for example, 0.5-1ms) can be set. Since the uplink signal in the TDD network is often weak and occupies less, the uplink spectrum may be displayed as a noise floor. Therefore, if there is an interference signal, it will be clearly present in the gated spectrum on the screen and will no longer be overwhelmed by the continuous downlink signal. By viewing the spectrum of the uplink signal in this way, the instrument will no longer be affected by the strong downlink signal, and the interference signal can be discovered.

Next, we use the signal source to simulate the more difficult co-channel interference of a wireless bridge. Taking the R&S PR200 monitoring receiver as an example, we use a relatively idle uplink time slot to set the gated trigger function and traverse the entire frequency band. We can find the OFDM waveform of a typical WIFI signal, as shown in Figure 4.

Figure 4: Activating the gated spectrum through the uplink time slot and traversing it to find the WiFi signal

The default settings of the monitoring receiver are suitable for monitoring applications in most signal scenarios. On-site monitoring engineers can also adjust according to the scenario and try richer and more flexible configurations. By adjusting various modes, detectors and measurement time, the most suitable combination can be configured or verified for various complex signals, so as to maximize the performance of the instrument and complete various difficult monitoring tasks.

In TDD networks, since uplink and downlink signals overlap and reuse in the spectrum, it is a difficult task to use conventional monitoring methods (such as spectrum + waterfall diagram) to try to distinguish interference signals. Continuous interference signals can be detected by using colorful spectrum, adjusting detectors and measuring time, but it takes some time to try and fine-tune parameters such as 100% time and measurement time. The gated spectrum function can solve the TDD network interference problem with the help of uplink time slots. It can not only deal with continuous interference signals, but also identify non-continuous WIFI signals that are also time-varying signals.

This article discusses several methods for observing interference signals in TDD networks through monitoring receivers. When the trace and time-frequency characteristics of the suspected signal can be clearly identified, it will no longer be difficult for field monitoring engineers to search and locate interference in TDD networks by connecting a handheld directional antenna and using level amplitude or single-tone tones.