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Simultaneous broadcasting on the same frequency encountered unprecedented challenges

Views:6     Author:Site Editor     Publish Time: 2020-06-17      Origin:Site


Inherent shortcomings of co-channel simulcast


  1. Co-channel simulcast cannot completely eliminate co-channel interference


Because the same-frequency simulcast system adopts the same-frequency technology, in order to obtain clear voice quality in the overlapping coverage area, the system must achieve "three identical guarantees", that is to ensure that in the overlapping coverage area, the terminal receives wireless signals from multiple base stations. Meet the same frequency, same phase, same amplitude and minimum field strength guarantee requirements. The communication quality guarantee of the existing co-channel simulcast system mainly relies on the co-channel interference tolerance of the narrow-band FM system itself (typical value is 8dB). Although it can improve the downlink of adjacent simulcast base stations to a certain extent through technical means The communication quality when the signal strength difference is below the co-frequency interference tolerance (that is, less than 8dB), but from the perspective of technical principles, in the technical system of narrow-band frequency modulation, the co-frequency interference area of the analog co-frequency simulcast system is There must be. Similar to analog co-channel simulcast, co-channel interference zones also exist in digital co-simulcast. Under the background of narrower bandwidth (carrier channel from 25kHz to 12.5kHz) and higher data transmission rate (from 1.2kbps to 9.6kbps), the proportion of frequency change transition time when transmitting different digital symbols will be more Larger, the interference caused is also stronger than when analog voice is transmitted. It is more difficult to develop a higher-rate digital co-channel simulcast technology within the narrowband FM system. If you also want digital co-simulcast to achieve the same level of coverage as analog co-simulcast, it is obviously unrealistic. In areas with severe co-channel interference, the analog co-channel simulcast voice can be recognized by human ears despite the large interference, while digital co-channel simulcast has no sound output due to high bit error rate, which is also certain To a certain extent, it affects the coverage level of digital co-simulcast.


   Therefore, although some manufacturers claim that the use of co-frequency, in-phase and receiver selection techniques can completely eliminate co-frequency interference, from the perspective of technical principles and practical applications, co-frequency interference is inevitable and objective. The use of co-frequency correction and transmission synchronization techniques only reduces the impact of co-frequency interference to a certain extent. In areas where the downlink signal strength of adjacent simulcast base stations is comparable, the actual call effect is still poor. In the application scenarios of wide-area coverage and large-scale networking, it will be disastrous to use the same frequency simulcast, which will cause the normal coverage of the overlapping coverage area.


   Most current co-channel simulcast applications cover band or chain areas or simply cover a certain city, so the adverse effects of co-channel interference areas are not particularly prominent. However, if large-scale development in the future, to achieve continuous and seamless coverage across the province, the adverse effects of co-channel interference areas will be significantly amplified.


   2. Small cluster simulcast communication network capacity


   Simultaneous simulcast All simulcast channels work at the same frequency and must be occupied at the same time. It is impossible to increase user capacity through multiplexing like clusters. The working principle of the simulcast system is that all the base stations use the same frequency for each call (whether group call or single call) in the simulcast area. No matter how many base stations there are in the area, each base station must allocate channels, wasting A lot of channel resources. In addition, the simulcast area is actually equivalent to a base station. Since the simulcast coverage area of multiple base stations is generally much larger than that of a common base station, the number of users is usually relatively large, and the corresponding traffic volume is also relatively high, which is likely to cause base station channel congestion and affect Dispatch command. Although the channel congestion can be alleviated by increasing the number of base station carrier frequencies in the simulcast area, due to the limitation of the transmission capacity of the control channel, in order to reduce the collision on the control channel and ensure the rapid connection capability of the base station, the carrier frequency of a base station should not exceed 8 Pcs. Therefore, by increasing the carrier frequency of the base station to meet the use of a large number of users, the effect is obvious only within the 8 carrier frequency. Although the channel congestion of more than 8 carrier frequency is alleviated, the collision probability of the control channel will increase rapidly, and the calling experience will be obvious Get worse.


  3. Larger cluster co-simulcast network investment is large


Digital simulcast will cause significant error and frame loss when the symbol deviation between base stations exceeds 1/4 (52us, distance between stations is 15km), so simulcast must have denser base stations than clusters, usually for better results If the distance between base stations is less than 5km, a large amount of base station investment will increase, and the cost of operation and maintenance will also increase.


  Because the synchronization phase of the simulcast base station is required to transmit, a higher quality link is required. For example, E1 must be used. The ordinary IP link is not effective, and the impact is the increase in link cost.


  To make the simulcast base station reach the same user capacity as the cluster, it is necessary to increase the base station carrier frequency exponentially, resulting in increased investment.


Challenges brought by the development of new technologies


   1. Challenges brought by the development of digital trunking technology


   Co-simultaneous broadcasting is a technology that appeared 20 years ago to solve the problem of weak handover capability of analog trunking communication systems and the inability of interconnection of systems from different manufacturers. In the development of wireless communication technology, digital trunking systems already have relatively mature technical standards and relatively superior technical architecture. The PDT digital trunking system adopts mature handover technology. It has achieved continuous lines across the base station during the call, and during the call, the base station can immediately allocate the call channel when moving to a base station that does not have members of the group.


   "User mobility management" is an important part of the cluster system. The quality of "user mobility management" directly affects the efficiency of the cluster system. When a user talks, the trunking system allocates the channel resources of the base station according to the registration of the base station where the user is located, that is, on-demand allocation, which can maximize the channel utilization. The simulcast system does not have the "user mobility management" function. When a user talks, each base station allocates channel resources regardless of the presence of users, and the channel resource utilization rate is very low.


  The voice call service of the cluster system can be distinguished by different levels, and the priority call rights of important users can be guaranteed to ensure smooth command and dispatch at critical moments. Although the digital simulcast system can also carry out group calls, the number of groups should not be too large. The group lacks permission settings, and the call channel will often be occupied. If the digital simulcast system does not group and adopts the one-call method, most users will be disturbed by unrelated calls during the call.


Short data and GPS positioning have become essential functions for police officers in their daily duties. Since the simulcast system does not have a "user mobility management" function, data can only be sent in all base stations throughout the network, and the channel resource utilization efficiency is very low. If there are many users using short data services, it will seriously affect the basic voice call service.


   2. Challenges brought by the development of IP interconnection technology


The IP interconnection system mainly refers to the exchange of voice, data and control packets between digital repeaters scattered in different locations through Ethernet based on the TCP/IP protocol in the multi-base station IP interconnection network mode, that is, each repeater is connected via the Internet Together, a larger range of conventional digital communication networks can be formed to enable private network communication throughout the entire area.


Under the same transmission power, the IP interconnected digital site signals can cover 3-5 kilometers in densely built areas, 12-16 kilometers in flat areas, and even the prairie beyond 30 kilometers can be covered by IP networks. Multiple digital sites expand communication coverage without co-channel interference. The simulcast system uses the same frequency technology. In order to ensure the voice quality of the overlapping coverage area and strict co-frequency, same-phase, and same amplitude, the digital simulcast system is strictly guaranteed that the phase error is less than 1/8 symbol (distance difference 7.8kM). Therefore, in the case where both IP interconnection and co-channel simulcast use the same transmission power, the coverage area of co-channel simulcast is 2/3 of the coverage area of the IP interconnection system.


   The simulcasting simulcasting system itself has high hardware and software costs and needs site survey and equipment room construction. In the case of ensuring links and power supply, the waterproof performance and construction conditions should also be fully considered. The IP interconnection system can be directly connected to the IP network through the network cable. Generally, the IP interconnection base station has a high protection level, and the system can be completed without the need to build a separate computer room. Therefore, the hardware and software costs and construction costs of the device itself are relatively low compared to co-channel simulcasting, and the cost performance of network construction is high.


  Compared with the same frequency simulcast, the network using IP interconnection can achieve large-area coverage and large-scale networking, and the call quality in overlapping coverage areas will be well guaranteed. The characteristics of its system are wide coverage, good compatibility and high cost performance.


  3. Challenges brought by wireless ad hoc network technology


   Simultaneous frequency co-casting generally adopts a clock synchronization server or GPS timing to ensure the clock synchronization of the entire system. If the clock synchronization server is subject to high time delay and stability requirements on the link, a wired link is generally used. When constructing the same-frequency simulcast network, the parameters of each system need to be configured in advance, which cannot be deployed quickly and does not have strong disaster resistance.


The wireless ad hoc network has the advantage of using single frequency point communication to save frequency resources in the simulcast system. The wireless ad hoc network devices use the air wireless link to complete voice and data transmission without any wired links, which not only effectively reduces the system Cost; and broke through the geographical restrictions caused by wired communication resources. The self-organizing network base station supports self-organizing cascading among multiple nodes, which can be freely set up according to the needs of the site. The chain, tree, star, and mesh networking can be deployed flexibly. The same-frequency simulcast generally adopts wired networking, which requires manual configuration and cannot be automatically networked at all. In comparison, wireless ad hoc networks have unparalleled advantages in disaster prevention and destruction. The wireless link is used to complete voice and data transmission. When a base station in the wireless ad hoc network fails or fails, the device node will automatically search for connectable devices for networking, and other surrounding nodes will automatically supplement to ensure the reliability of the entire network system.


   The wireless ad hoc network also uses single frequency point communication to save frequency resources, but because co-frequency simulcast generally uses wired interconnection, the wireless ad hoc network has natural advantages in flexibility, security, and implementability.


The way out of the same frequency and broadcast


The co-channel simulcast system develops with the development of analog wireless communication. In the rapid development of wireless communication digitization, the co-channel simulcast system has a small user capacity and weak user mobility management capabilities. The original advantages are no longer obvious. Simulcasting encounters challenges. Using simulcasting or cluster simulcasting systems in cities or densely populated areas is not a wise choice. It is obviously unrealistic to expect digital simulcasting to achieve the same level of scale as analog simulcasting. Since most co-frequency simulcast applications cover band or chain areas, or simply cover a certain city, the areas with severe co-frequency interference are adjusted by fine-tuning the transmission power and timing synchronization deviation of each base station, so The adverse effects of the same frequency interference area are not particularly prominent, and can basically meet the needs of applications. For areas where frequency resources are tight or users do not cover a large number of bands or chains, co-simulcasting is still a good choice. From the perspective of emergency dispatch command application, the simplest equipment is often the most reliable, and the market share and penetration rate of low-cost products are higher, and the products with smaller size and longer use time on a single charge can be better. To meet the needs of emergency dispatch and command, if the same frequency simulcast product can achieve small size, long single charge time, simple and fast deployment, the same frequency simulcast will still have a good prospect as an emergency backup network.


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