Anti-jamming technology in mobile communication

Summarize

Interference is the twin of mobile communication. Since the birth of mobile communication, people have been fighting with interference. Civil mobile communication has been through four generations, various methods to deal with interference have their own strengths, we take this opportunity to take a general inventory.

Let's first look at the concept of interference tolerance: when the system is still working, the maximum interference ratio (the ratio of interference to useful signals) allowed by the receiver, which reflects the system's tolerance to interference in the interference environment.

The conditions for the normal operation of the communication system are:

Therefore, from the general direction, we can improve the anti-interference ability of the system from the two aspects of reducing the input interference ratio and improving the system interference tolerance, and several generations of mobile communications are also doing so.

Reduces input interference ratio

The communication interference equation expressed in terms of interference ratio is as follows:

Therefore, the way to reduce the input interference ratio can be divided into three parts: reducing the interference signal, improving the useful signal, and increasing the time-frequency domain coincidence loss between the useful signal and the interference.

1. Reduce interference signals

For mobile communication, interference is divided into network interference and outside interference, outside the network interference in addition to the frequency sweep investigation interference signal source, we can not arbitrarily change PTj, GTj, Lj, GRj.

As for the control of interference in the network, various standard mobile communication systems take basically the same means, there are the following means:

1.Reduce GTj/ GRj: Use directional antennas to sector the cell and align the sidelobes to the area not desired to be covered, which is equivalent to reducing the gain in the interfered/interfered direction; TDSCDMA and TDD-LTE systems also use smart antennas (beamforming) for better results.

2.Reduce PTj: Use power control and DTX discontinuous transmission.

Power control is one of the most important means to control interference in the network. For GSM system, power control command is issued through SACCH, and the control period is 3 measurement reports, about 1.5 seconds. 3G and 4G power control is similar, divided into open loop power control and closed loop power control two kinds, simply put, open loop power control is no feedback power control, generally used in the initial access stage, and closed loop power control according to the type of feedback value and feedback unit, is divided into inner ring and outer ring. The power control speed of different systems is different, the power control speed of WCDMA is 1500HZ, the power control speed of CDMA2000 is 800HZ, and the power control speed of LTE is 200HZ.

It should be noted that due to the existence of the near and far effect, the uplink is more susceptible to interference, so the power control in mobile communication mainly refers to the uplink power control.

2. Boost useful signals

There are several ways to improve useful signals:

1) Increase the transmission power PTs

The transmission power is limited by the hardware equipment, and for mobile communications, each user is not only their own signal source, but also other users of the interference source, so simply increase the transmission power in improving the communication effect of their own side at the same time, will increase the interference of other users in the network, the overall point of view is not necessarily good. Therefore, the means of power control is used in mobile communication to adjust the power to ensure that the power of each user is just enough.

2) Diversity reception improves receiving power Psi

The so-called diversity reception refers to the method by which the receiving end merges a number of independent (carrying the same information) fading characteristic signals received by it to reduce the signal level fluctuation. It includes two parts: receiving and merging processing.

There are three common receiving modes: spatial diversity, polarization diversity and time diversity.

Spatial diversity: The use of spatially relatively independent overpay receiving antennas to receive signals, and then merge, in order to ensure the irrelevance of the received signal, which requires that the distance between the antennas is large enough, the purpose of doing so is to ensure that the received multipath signal fading characteristics are different, the distance between the receiving antennas is at least more than 10 wavelengths. Is one of the most commonly used diversity methods.

Polarization diversity: Overpaying receiving antennas with different polarization modes are used to receive signals and then combine them. The common antenna in mobile communication is the 45-degree polarization antenna.

Time diversity: Time diversity is represented by Rake receiving technology. RAKE receiving technology is an important technology in CDMA mobile communication system, which can distinguish the subtle multipath signals in time, and make the weighted adjustment of these resolved multipath signals to make them compound into enhanced signals.

There are three types of merger: maximum ratio merger, selective merger and equal gain merger. The most commonly used scheme is maximum ratio merging, which is simple and easy to implement by linear processing of the received signal at the receiving end. Multiple diversity branches are formed at the receiving end, and after phase adjustment, they are added in phase according to the appropriate gain coefficient, and then sent to the detector for detection. The gain generated by merging is proportional to the number of diversity branches N.

In addition to a few single-polarized antennas left over from early engineering construction, all standard mobile communications use polarization diversity and spatial diversity, while Rake reception is only used for CDMA systems.

3. Increase Lf/Lp/Lt

The principles of these three methods are:

Lf: The interference and useful signals are staggered from the frequency domain, because the frequency band of civil mobile communication can not be determined independently, so the use of this anti-interference method is limited.

Lp: It is isolated from interference in the polarization direction, but because the polarization direction of radio waves changes frequently in the propagation process of mobile communication, it is impossible to reduce interference by increasing Lp.

Lt: Isolation of interference in the time domain, generally used in military, such as burst transmission technology, the data is compressed in a burst pulse transmission, so that the enemy can not interfere.

In addition, in a sense, the multiple access technology of each system is also such anti-interference technology, such as the time division multiple access of GSM, which is actually to isolate the signal of each user from time to avoid mutual interference.