Development of Fixed Wireless Access Networks

Fixed Wireless Access networks are still very mainstream access technologies. Many people may not know about fixed wireless access networks. It does not matter. After reading this article, you will certainly have a lot of GAINS. I hope this article will teach you more things, will have a comprehensive understanding of the application of the Fixed Wireless Access Network.

I. advantageous applications of Fixed Wireless Access Networks
In recent years, the demand for Internet access by communication users has continued to grow rapidly, while the construction progress of fixed wired network PSTN with Asymmetric Digital user line ADSL and cable modem is not fast, as a result, Fixed Wireless Access to FWA and Fixed Wireless Access are considered more in the last kilometer of the urban communication Access network ). Of course, this will be a broadband wireless access network. The so-called multi-point multi-channel allocation Service (MMDS), Multipoint Multichannel Distribution Service) fixed wireless network is a typical example of FWA. It uses frequencies below 5 GHz, such as 3.5 GHz and GHz, to set up a fixed transceiver Base Station BTS and Base Tranceiver Station within a certain region ). In addition to the original telephone communication, many users are eager to connect to Aintemet so that they can communicate with other users through e-mails and frequently request a large amount of data information from appropriate websites, the above broadband FWA can be fully utilized to install appropriate antennas and email receiving machines on the roofs of their offices and home locations, after a base station BTS, which is close to them and several kilometers away, it connects to a fixed urban communication network to achieve the required communication purpose. This does not have to rely entirely on the traditional lead-clad copper cable access network, and does not have to wait for the construction of ADSL and modem line equipment to complete, and the wireless FWA can ensure the same satisfactory service performance, it has the advantages of Fast Construction, easy scalability, simple maintenance, and low cost, and is easy to cater to the needs of the majority of communication users. In areas that do not have a wired access network, or areas where it is difficult to create a wired access network, the fixed wireless access network can give full play to its advantages, and the construction and application are given priority. This is why such technology has been vigorously promoted internationally in recent years. However, the New Fixed Wireless Access Network must be FWA with a wide bandwidth to meet the needs of the user's development of data communication trends. As a matter of fact, the wireless cellular network of mobile communication has evolved from the current second generation to the coming third generation, and even to the far-reaching fourth generation. It also attaches great importance to the construction and development of broadband networks, meet the needs of users to increase their use of high-speed data communication and access to the Internet.

Generally, in order to enable optimal propagation of radio waves in the air, fixed wireless access to FWA lines tends to achieve "Line of Sight transmission" LOS, Line of Sight ). In this article, we mentioned that a user installs a receiving and sending antenna on the office and home roof to communicate with the base station BTS through LOS. In recent years, FWA has made significant improvements in technical design, striving to achieve "Non-line-of-sight transmission" NLOS and Non-LOS ), it is no longer required that the user's receiving and sending antennas must be installed at the high point on the roof, but to allow the antennas to be mounted on the outdoor wall or even on the indoor table. This is a major improvement and is very beneficial for the promotion and application of FWA. That is to say, in order to make the wireless FWA compete with the wired ADSL-modem, we must strive for NLOS-FWA. In this way, the client is easy to install and the performance is equivalent to that of ADSL-modem.

Ii. Difficult to popularize line of sight transmission
In order to be practical, FWA of the Fixed Wireless Access Network must be widely adapted to various situations on the ground. Regardless of the terrain, region, and population density, wireless communication must be smooth. In addition, compared with the wireless cellular network of mobile communication, this fixed wireless network shows its superiority in terms of data transmission speed, access reliability, and spectrum usage efficiency.

Generally, FWA is designed based on line-of-sight transmission LOS, that is, the radio wave transmission between each user and the Access Point BTS is based on line-of-sight transmission LOS. Each user is equipped with a precisely oriented, narrow, and fine-beam antenna with a high gain. Such wireless lines can be independent from neighboring routes. Generally, radio waves may have several different latencies of multi-path propagation, resulting in inter-code interference and fading. The user devices are now required to have a high targeting orientation on the roof antenna, so that they are similar to the Radio Transmission Conditions of the base station, so that they only need a small fading margin, in addition, modulation and demodulation do not require a complex equalizer. However, practical experience shows that up to 30% of FWA users are connected to their respective base stations in line with the LOS condition. In addition, if the partition and base station conditions change or adjust, the user's antenna needs to re-adjust its direction, which is quite troublesome. Therefore, FWA's comprehensive design will never allow us to rely solely on LOS transmission. Maybe FWA is divided into more small and partially covered zones, and a large number of base stations are set up to help increase the user's LOS transmission, but this makes the entire FWA network structure complex, it is not easy to use, and the cost is improved.

Iii. Non-line-of-sight transmission should be taken seriously
In order to promote the application of Fixed Wireless Access Networks and expand the frequency band width and transmission data rate of wireless access, the fixed wireless access must not be limited to LOS, instead, we should allow more users to use non-line-of-sight transmission between the base station and the actual situation. According to the latest International Journal reports: a typical NLOS-FWA requires a data transfer rate of 6 Mbit/s, the spectrum utilization efficiency should be 2 bit/s • Hz, the transmission coverage distance should reach 10 km for the roof cable, 5 km for the wall cable, and 3 km for the indoor antenna. Their transmission performance should be similar to the DSL of the wired network and the reliability should reach 0.999.

Regardless of the upstream or downstream lines, the Spectrum Utilization Efficiency refers to the BTS per base station and the bits per second per Hz: SE = rM/kB, in formula, M indicates the average transfer volume and bit number of each RF channel. r indicates the code speed of the forward Correction Code. Therefore, rM is the effective transfer volume when users access the media. This transfer volume is related to the performance of the actual wireless path, distance loss, transmission power, noise level, and other factors. B Indicates the access bandwidth, including the Protection Band. K indicates the space reuse factor, which is determined by the number of base stations. In this way, the coverage of each residential area should have its own SE. If each base station increases the number of partitions and uses different RF channels, the channel transmission volume may be increased to improve the spectrum efficiency. The uplink and downlink lines are considered more important because they are related to Internet applications. However, the uplink lines can also be calculated to meet performance requirements.

In NLOS-FWA, the performance of wireless channels is often expressed by the signal to noise and Interference Ratio SINR. This means that any receiver antenna must meet the predefined SINR requirements. This is of course closely related to the fading phenomenon of wireless channel transmission. Therefore, the diversity system is often used in wireless large-line devices to grasp and reduce the occurrence of latency difference. In general, necessary practical measures should be taken, or efforts should be made to minimize k or maximize rM to obtain the maximum spectrum utilization efficiency SE.

Iv. New Fixed Wireless Access Network Technology
As mentioned above, the next-generation fixed wireless access network will adopt NLOS for non-line-of-sight transmission, which requires high system performance. Correspondingly, newer technologies must be used. In short, he said, the physical layer PHY will use a pay-as-you-go antenna to implement diversity transfer and space multiplexing, and will also use modulation methods that are not affected by latency differences, so that the modulation and demodulation process can still be effectively carried out when the channel is damaged. In the media access control layer MAC), the line is re-sent and adaptive modulation to deal with high and changing bit rate. The overall goal is to improve the ability to transmit data correctly.

First, the client and the base station use the automatic request re-sending and segmentation facilities, that is, according to the ARQF automatic request re-sending fragmentation) protocol, in addition to the IP-based ARQ automatic request re-sending, also, grouping data into atomic data units (ADU, atomic or elementary data unit) has the advantage that, in the process of data transmission, if grouping errors occur, you only need to resend a small part of the group to eliminate the group error and ensure satisfactory TCP/IP performance. Secondly, the New Fixed Wireless Access Network will adopt Adaptive Modulation and encoding technology to enable the data transmission rate to adapt to the SINR and fading degree of the channel. This has been successfully used in mobile communication cellular networks. For general data rates, adaptive modulation can use two-phase and four-phase-shifting keying (BPSK and PSK ). For high data rates, the orthogonal Amplitude Modulation of 64QAM or 256QAM is used. The former allows user data to be transmitted to a remote base station without interference. The latter can improve the spectrum utilization.

The new antenna includes a pay-as-you-go antenna, smart signal processing, and coding technology. They are not only used for each base station, but also installed on the user end. Therefore, new fixed wireless access networks must be equipped with new antennas. In addition, this new multi-payment antenna uses space-based SD, Spatial Diversity), and each antenna is not affected by fading at the same time. They increase the array gain and improve the signal-to-noise ratio of the communication system, the spatial diversity of 2-3 antennas may be 10-15 dB better than that of a single large line. These conditions indicate that the new antenna uses spatial diversity, which may extend the coverage distance and facilitate the reuse of frequency, and may also reduce the cost of power amplifier and prolong the life of the Dry Battery in the hosts. A practical test has been conducted on such a new fixed wireless access system. From the results, it is learned that, if every antenna is separated by 0.5-1 wavelength, the space diversity gain may be obtained. For example, when the carrier frequency is GHz, the antenna is separated by 6 ~ 12cm has obvious advantages. If dual-polarization antennas are used, the antenna interval requirement is reduced.

At both ends of a wireless line, you can consider using a pay-as-you-go antenna to increase the data rate of the wireless transmission signal. This means that the appropriate use of the space Reuse Technology SM, Spatial Multiplexing) may bring benefits, because the Fixed Wireless Access system often needs to transmit a high data rate, the space reuse measures are very suitable. It is not difficult to set up a certain number of antennas at both ends of the receiving and receiving end, but it can be expected to achieve significant reuse effect. For example, the sending end and the receiving end are configured with a three-payment antenna and a three-payment antenna, it may be expected to achieve 3 times the spectrum utilization efficiency. For example, a high-data-rate signal is divided into three low-data-rate signals at the sender end according to the digital interval sequence. Each signal is modulated and sent by a 3 Fu antenna. At the recipient's receiving end, the 3 Fu antenna receives the signal, and the signal is demodulated, and finally merged into a high-speed signal. Such a diversity multiplexing scheme can be implemented in the predefined SINR. In many cases, the best effect is achieved by combining the spatial diversity SD and spatial multiplexing SM.

In addition to the above spatial Diversity SD, we can also consider the use of Frequency Diversity FD, Frequency Diversity) and Time Diversity TD, Time Diversity) technology. Multi-carrier Orthogonal Frequency Division Multi-Channel OFDM, Orthogonal OFDM) system is transmitted by multi-channel narrowband Orthogonal sub-carrier at the same time. In recent years, it has been applied in some practical scenarios and proved to have good results. The ARO mentioned earlier in this article is time diversity and has been officially used in some fixed wireless access networks. It indicates that it is beneficial to overcome the damage caused by the fading of radio waves. According to the actual performance test results, the diversity and adaptive modulation technologies are suitable for Fixed Wireless Access Networks with non-line-of-sight transmission. The antenna of the client device does not need to be precisely aligned with the base station, but can adopt a wide-bundle antenna. The multi-channel influence caused by this can be solved by AROF and diversity technology, and satisfactory results can be obtained.

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