Video Surveillance Transmission Wireless Technology Analysis

It is an inevitable trend that the video surveillance access method is changed from wired to wireless. The wireless video image transmission breaks the disadvantage of traditional coaxial cable and optical fiber image monitoring, which is subject to the hardware connection. It has greater flexibility and convenience, and a video monitoring system based on wireless network has emerged. The development of wireless video transmission technology has had a profound impact on the architecture and protocols of wireless mobile networks. However, due to the limited wireless channel bandwidth resources, many interference factors, and the large amount of video signal data, high real-time requirements and other issues.

Development status of wireless video transmission technology

With the development of the information society, people's requirements for security monitoring are becoming higher and higher. In addition to centralized party and government agencies, enterprises and institutions, such as offshore, mountainous areas, mines, basements and other complex environments, it is impossible to achieve wired network installation. local. All need to realize security video surveillance, which requires the use of wireless video transmission technology.

At present, most of the wireless video transmission technologies on the market use GPRS and CDMA technologies. The GPRS transmission bandwidth is insufficient, the transmission video is only a few frames per second, and breakpoints and wireless reception blind spots are prone to occur in the event of an emergency. CDMA transmission also has such defects, its downstream bandwidth is 153 kb / s, and its upstream bandwidth is 70-80 kb / s, so it is basically impossible to achieve smooth video transmission. Since the image is only a few frames, it is transmitted in the form of a screenshot and has a small screen size. Obviously, this cannot meet the real-time application requirements of the video surveillance system. For microwave (digital microwave, spread spectrum microwave), wireless local area network (WLAN, 802.11 (ab, g)) and other higher wireless transmission schemes, the video coding is based on MPEG-2 / 4, H.264, etc. the Lord. However, most of them have the same problem, that is, they can only achieve cross-view transmission and directional transmission, and it is difficult to support mobile transmission, which limits the application of video surveillance systems.

The monitoring system generally uses a low transmission frame rate to ensure the clarity of the transmission, because only the image clarity above the CIF of MPEG-4 can meet the needs of investigation and evidence collection. Therefore, the wireless transmission technology should fully exert its absolute advantages in the monitoring system. It should meet: can be applied in non-visual and obstructive environments; suitable for high-speed mobile wireless transmission of real-time images; suitable for transmission of high bandwidth, high Streaming, high-quality audio and video; has excellent anti-interference and anti-fading capabilities. In practice, there are two main concepts in real-time wireless video transmission: one is mobile transmission; the other is broadband transmission. Therefore, to develop a stable unlimited video transmission system capable of performing high-definition video with a wide frequency band, the optimization of the data transmission mechanism is one of the key issues to be solved. The bandwidth resources of the unlimited link are limited, and this limitation is passed on to the mass video transmission The embodiment is particularly obvious. Here, the corresponding research on the fault tolerance of the data transmission mechanism in the wireless video transmission system is carried out, aiming to solve the contradiction between the limited bandwidth resources and the large amount of video data in the wireless video transmission. Bandwidth resources are wasted due to unnecessary retransmissions. The wireless video transmission system is evaluated by using the concept of bandwidth-distortion cost function. On this basis, a partial retransmission error control mechanism based on the bandwidth-distortion cost minimization criterion is further given to improve the bandwidth utilization rate, and the corresponding experimental analysis is carried out.

Wireless video transmission mechanism analysis and fault-tolerant transmission technology

The purpose of signal transmission research on reliable channels is to make full use of the channel's bandwidth resources; for unreliable channels, the focus of research in transmission is to make full use of bandwidth resources to achieve reliable transmission, that is, fault-tolerant transmission technology. The video transmission mechanism on the wireless channel is discussed here, and its main research point is fault-tolerant transmission control. The fault-tolerant transmission control technology can be divided into three categories according to the different control methods: forward error control, feedback-based ARQ, and source channel joint coding. Forward Error Control (FEC) includes channel error correction coding technology, interleaved packaging technology, and optimized packet scheduling mechanism. The feedback-based ARQ technology includes a reference frame selection (Reference Picture SelecTIon, RPS) mechanism using a multi-frame reference mechanism, a hybrid ARQ (Hybrid, HARQ) mechanism, and an ARQ-based feedback error tracking technology. Since the ARQ-based fault-tolerant transmission control technology has excellent performance, we will focus on the ARQ-related transmission control technology and discuss the shortcomings of the existing video fault-tolerant transmission mechanism.

Forward error control uses forward error correction coding to overcome channel errors. In the case where the channel error probability fluctuates sharply (such as the existing mobile channel), in order to obtain a certain transmission quality, the forward error correction coding must increase the redundancy check bits according to the currently estimated worst case, which will cause Waste of bandwidth resources. As far as the limited wireless channel is concerned, the bandwidth resource is obviously unable to meet the requirements. For this reason, consider combining ARQ technology with forward error control, called HARQ technology. HARQ is divided into two categories: In class I HARQ, the forward coding of the sending end must have certain error correction capabilities. When the receiving end finds an error, it first uses forward error correction coding to correct the error. If the error is corrected, send a feedback message (ACK) of the current packet reception success to the sender, otherwise send a reception failure message (NACK). If the sender receives the ACK, it will continue to send the next data packet, otherwise, it will resend the erroneous data packet. It can be seen that Class I ARQ requires strong forward error correction coding. In applications where the error rate is low, it will result in a waste of bandwidth resources, but in an environment with a high error rate, it can obtain better throughput than other types of ARQ mechanisms. effectiveness. Type II ARQ only requires forward error correction coding to have error detection capability. According to the theory about channel coding error correction capability, this can play a role in saving bandwidth. When the receiving end finds an error, it sends a retransmission request; the sending end only transmits the error-correcting check code corresponding to the erroneous data. When the receiving end receives, such as

If the error still cannot be corrected, continue to send the retransmission request, the sender can choose to retransmit the entire error data and check code, or can choose to send a check code with stronger error correction capabilities, depending on the control strategy can be adjusted . In view of the high error rate of the wireless channel, HARQ-I is usually used for wireless transmission with a feedback channel. Figure 2 shows a wireless video transmission system using HARQ-I. The dotted box in the figure represents the flow of error control during transmission. According to the design principle of HARQ-I, after the receiver finds an error, it first performs forward error correction (the first layer of error barrier in the figure). If it cannot be corrected and the current system meets the delay limit, it sends an ACK request to let the sender re Pass the error data (second layer error barrier). Such retransmission can be repeated until the receiving end receives correct data or the retransmission delay exceeds the system delay limit. If the correct data packet cannot be obtained after the retransmission, the error concealment technique will be used at the receiving end to perform error recovery (layer 3 error barrier). It can be seen that the basic idea of â€‹â€‹this mechanism is to use ARQ technology to recover errors after making mistakes, so here it is named "best effort" ARQ mechanism (BestEffortARQ, BEA, RQ).

Since the video signal has a strong spatio-temporal correlation, and the encoding end cannot completely remove this correlation, the decoding end can use these residual correlations to recover a certain quality of video. The quality of restoration is also closely related to the texture and movement of the restored part. Generally speaking, for the part with a smoother texture and a single movement, the restoration effect is better than other cases. In this case, if you use BEARQ to retransmit this part of the video, it will obviously cause a waste of bandwidth.

In order to overcome this waste of bandwidth, in practical applications, due to the close relationship between the channel error rate and the number of retransmissions, and each retransmission consumes a certain bandwidth, so the bandwidth and channel required to successfully transmit a data packet The error rate is related. Taking this factor into account, the core idea of â€‹â€‹using the concept of bandwidth-distortion cost function is that under certain conditions of packet loss rate, channel bandwidth and transmission delay, the reception quality of the terminal video and the bandwidth used in transmission are not only The rate-distortion performance of the source is also related to the channel error rate (packet loss rate) and terminal error recovery technology. Use it as a criterion for measuring whether the video packet should be retransmitted. On this basis, the optimized end-to-end transmission mechanism is adopted. In this mechanism, the error recovery mode of each part is pre-determined by the encoder side according to the current channel status and the error concealment algorithm adopted by the decoder side. To decide whether to adopt ARQ or error recovery. This effectively avoids the waste of bandwidth resources due to unnecessary retransmissions and improves the efficiency of system bandwidth usage.

In this paper, a Bandwidth-DistorTIon (BD) model for lossy channel video transmission is proposed. This model is a dual model of the RD model after considering channel errors, and has a form similar to RD. A video transmission system, its performance is mainly measured from two aspects: throughput efficiency and receiver reconstruction distortion.

Three important stages that wireless video surveillance will go through

According to the operation practice of network video surveillance business in recent years, the development of wireless network video surveillance technology and the development of different user groups' demand for wireless video surveillance, the author believes that the future development of domestic wireless network video surveillance business will go through the following three stages :

The first stage: the typical application stage of the industry mainly based on the wireless video surveillance application of major industry customers

At present, surveillance systems for high-end industry users, such as domestic safety projects, traffic road monitoring, and electronic surveillance video surveillance for inspection and quarantine, are mostly large-scale metropolitan or even national industry video surveillance systems. Most high-end industry users are now in the early stages of building large-scale video surveillance projects, which have very high requirements for surveillance systems, including not only the wired side images can be seen in real time, the video is stored well, the PTZ control and other commands respond quickly, etc. At the same time, it also increases the application requirements for wireless video capture (such as traffic patrol, safe city mobile patrol, urban management mobile patrol and law enforcement, etc.) and mobile video viewing and control.

Since the current profitable operator video surveillance platforms are basically in the first phase of industry users, government, finance and telecommunications are still the main application areas of video surveillance. The deep integration of wireless network video surveillance and specific industries will become network video surveillance The inevitable trend of market development.

The second stage: the niche application stage mainly based on the innovative application of commercial monitoring and part of the family promotion

Small and medium-sized business users are not only another potential large-scale user group of traditional wired network video surveillance services, but also another important field of wireless network video surveillance applications. This type of user refers to small shops, medium-sized chain business organizations, medical institutions, and educational institutions that require distributed monitoring and access. Due to the characteristics of users such as mobility, remote mobile access, and high requirements for work efficiency, the penetration rate of wireless network video surveillance services in such applications will have a higher starting point and greater room for improvement.

In addition to the basic monitoring business requirements such as mobile video viewing, business users' business requirements for wireless video surveillance often require a combination of surveillance systems and enterprise business systems. For example, medical services may use video surveillance platforms to carry out mobile telemedicine services (such as Emergency diagnosis and ambulance on ambulance), the bank will use the mobile video surveillance of the cash transport vehicle for management and control, and the school will use the monitoring platform for radio teaching, wireless electronic proctoring and other applications.

For business users, a wireless video surveillance platform that integrates well with their own business is a good way to expand their business. Therefore, the guiding strategy for operators to commercial customers should be to build a platform and find a wide range of integrators to jointly develop customer-oriented business application. If telecom operators actively guide the development of killer-level monitoring applications suitable for commercial users, the size of the user base will soon be expanded.

The third stage: with a wide range of personal and family applications, commercial and

Industry applications are in full bloom, and the general application stage of the integration of wireless video surveillance and other video applications such as IPTV, video conferencing and voice services.

With the improvement of people's living standards and the requirements for security monitoring, the market demand for home security alarms and wireless video surveillance services has gradually increased. In the future, the digital home network can provide communication information, home entertainment, security monitoring, and life services. In terms of application, it can fully meet people's informatized home life needs.

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