Digital microwave communication principles

Editor's Notes

• #2: {"16":"In a digital microwave system, to transmit the digital information of orderwire, wayside services, bits employed by ATPC, and channel switching, additional bits that are called RFCOH will be added into the main data stream coming from the SDH MUX equipment. Suppliers plan the frame structure according to transmission rate, modulation schemes, error correction methods, and types of required additional information. Therefore, different suppliers may have different microwave frame structures. This figure shows the frame structure that employs multilevel coded modulation (MLCM). \n","44":"Terminal station: It refers to the microwave station that transmits services only in one direction. \nRelay station: It refers to the microwave station that transmits services in two directions and is required added to solve the problem existing in the microwave line of sight communication. The relay station is classified into two types, active relay station and passive relay station. \nAdd/Drop relay station: It refers to the microwave station that transmits services in two directions and adds/drops transmitted services. \nPivotal station: It refers to the microwave station that transmits services in three or more than three directions and transfers the services in transmission channels in different directions. It is also called the HUB station.\n","11":"(1) For long haul PDH microwave links (the distance between stations is generally longer than 15 km), 8 GHz frequency band is recommended. If the distance between stations is not longer than 25 km, 11 GHz frequency band can also be used. The specific frequency band shall be determined based on the local weather conditions and microwave transmission cross-section.\n(2) For short haul PDH microwave links (generally used in the access layer and the distance between stations is shorter than 10 km), 11/13/14/15/18 GHz frequency band is recommended. \n(3) For long haul SDH microwave links (the distance between stations is generally longer than 15 km), 5/6/7/8 GHz frequency band is recommended. If the distance between stations is not longer than 20 km, 11 GHz frequency band can also be used. The specific frequency band shall be determined based on the local weather conditions and microwave transmission cross-section.\n","83":"But as frequent recourses are becoming scarce currently and frequency diversity functions better only when the frequency spacing is large enough, space diversity is more often used. \n","61":"There are several kinds of fading according to the causes. \n1) Flat fading: The signal has the same level fading depth in the transmission bandwidth and the power is reduced.\n2) Frequency-selective fading: Waveform distortion caused by the frequency selectivity of fading.\nDirect wave is used in microwave propagation. The field strength at the receive point is the superposition of direct wave and ground-reflected wave.\n3) The propagation medium is the low-level aerosphere, ground and ground object along the path.\nWhen time conditions (such as season, day and night) and climate conditions (such as rain, fog, and snow) change, the temperature, temperature rate and stress of the atmosphere, position of ground reflection, and reflection coefficient change. These changes can cause the field strength at the receive point to change.\nSuch phenomenon is called radio propagation fading. \nObviously, fading is a random phenomenon.\nThe degree of fading is indicated by the fading factor VdB. The reasons for fading are mainly atmosphere and ground effect. \n","39":"The IDU implements the functions including service access, service grooming, multiplexing/demultiplexing, and modulation/demodulation. Thus the IDU is the main part of a set of microwave equipment. If we consider the IF board as the line board of optical network equipment, then the IDU is very much similar to Huawei case-shaped optical transmission equipment. An IDU contains service boards (SDE, SD1, SLE, SL1, PH1, PO1), cross-connect, power and clock board (PXC), system control and communication board (SCC). This figure shows the internal functional module structure of an IDU. \n","17":"The SDH frame is a block structure, composed of bytes, and has a fixed sequence. The microwave frame is different from the SDH frame. The microwave frame is composed of bits and the arrangement is irregular depending on application. \n","12":"Here are a few concepts about microwave frequency band setting. \nAfter selecting the microwave frequency band, configure the RF channels, that is, divide the frequency band into several smaller sub-bands to provide the spectrum required by the transmitter. We call these sub-bands channels. These channels are usually indicated by their respective center frequencies and sequence number. The channel bandwidth depends on the spectrum of the signal transmitted, or depends on the capacity and the modulation scheme employed. Therefore, when configuring RF channels, follow the principles listed below:\n(1) Make full use of the limited radio frequency band.\n(2) For one radio station, there must be enough spacing between the transmit frequency and receive frequency, to avoid serious interference on the receiver brought by the transmitter.\n(3) In a multi-channel system, there must be enough frequency spacing between two adjacent channels, to avoid mutual interference. \n(4) There must be enough protection spacing at the edges of the allocated frequency band, to avoid interference with the adjacent frequency bands.\n(5) Most RF channels are configured with equal spacing. \nAccording to the description of microwave relay system RF channel configuration in ITU-R F.746-3, equal spacing is the basic scheme employed first for RF channel configuration. The frequently used channel spacing is 2.5 MHz and 3.5 MHz, which belong to North American system and European system respectively. For 3.5 MHz channel spacing scheme, it is expected that the channel spacing will be further divided into 1.75 MHz, to support the small capacity transmission requirement of 1xE1 or 2xE1. \nThe following are the common parameters that are related to RF channel configuration:\nXS: the RF spacing between the center frequencies of the adjacent RF channels of the same polarization direction in the same transmission direction.\nYS: the RF spacing between the center frequencies of closest go channel and return channel.\nZS: the RF spacing between the center frequency of the outermost RF channel and the frequency at the edge of the frequency band. If the frequency spacing at the lower end is different from that at the upper end of the frequency band, then Z1S is used to indicate the frequency spacing at the lower end, and Z2S is used to indicate the frequency spacing at the upper end. \nDS: the spacing between the transmit and receive duplex frequencies. Within a specified channel, the spacing between a pair of fn and fn' is constant. \n","106":"This standard requirement shall be satisfied at the same time. If not, and if the transmission distance is within 20 km, ensure the conditions that the K value is 2/3 and then ensure that the K value is infinite. If the standard requirement still cannot be satisfied, SD need be used. \n","62":"Free space is an infinite space filled up with even and ideal propagation medium, in which electromagnetic wave is not affected by the factors such as blocking, reflection, diffraction, scattering, and absorption. \nThe concept of level fading contains a threshold level, a receive level, and a margin usually reserved. The margin may be not much for small-capacity systems. But for the current large-capacity digital microwave system, larger margin is required. The receive level in free space can be calculated by this formula:\nPr(dBm)=Pt+Gt+Gr-Lf-Lt-Lr-Lb\nPt: transmit power\nGt/Gr: antenna gain\nLf: free space transmission loss\nLt/Lr: loss of transmit/receive feed line \nLb: loss of branch system \n","51":"What are the networking modes frequently used for digital microwave?\nThe frequently-used networking modes include ring network, point-to-point chain network, hub network and add/drop network.\nWhat are the types of digital microwave stations?\nDigital microwave stations are classified into pivotal stations, add/drop relay stations, terminal stations, and relay stations.\nPivotal station: A station located in the backbone link to communicate with other stations in various directions.\nAdd/drop station: A station located in the middle of the link to add/drop tributaries and communicate with the two stations in two directions of the backbone link.\nTerminal station: A station located at either end of the link or at the endpoint of a tributary link.\nRelay station: A station located in the middle of the link without adding/dropping voice channels.\nWhat are the types of relay stations?\nRelay stations fall into passive relay stations and active relay stations. There are two types of passive relay stations: Back-to-back antenna and plane reflector. Active relay stations include regenerator stations, IF repeaters and RF repeaters.\nWhat are the major applications of digital microwave?\nDigital microwave is mainly used for complementary networks to optical networks (the last mile access), BTS backhaul transmission, redundancy backup of important links, VIP customer access, emergency communications (large conferences, disaster relief, etc.) and special transmission conditions (rivers, lakes, islands, etc.).\n","40":"What types are microwave equipment classified into?\nMicrowave equipment may be classified in different ways.\nBy system, it may fall into digital microwave equipment and analog microwave equipment. At present, the latter is already washed out and seldom used.\nBy capacity, it may fall into microwave equipment of small and medium capacity and microwave equipment of large capacity. Small and medium capacity refers to 2 – 16 E1s or 34M, and large capacity refers to STM-0, STM-1 and 2 x STM-1.\nBy structure, it may fall into trunk microwave equipment, split-mount microwave equipment and all outdoor microwave equipment.\nWhat units do the split-mount microwave equipment have? And what are their functions?\nThe split-mount microwave equipment is composed of four parts: Antenna, ODU, IF cable and IDU.\nAntenna: Focuses the RF signals transmitted by ODUs and increases the signal gain, thus enlarging the transmission distance.\nODU: Implements RF processing to realize IF/RF conversion of signals.\nIF cable: Transmits IF signals and IDU/ODU communication signals and also supplies power to ODUs.\nIDU: Performs access, grooming, multiplexing/demultiplexing and modulation/demodulation of services.\nHow to adjust antennas?\nThe objective of antenna adjustment is to align the main lobe of the local antenna to the main lobe of the opposite antenna.\nFirst fix the opposite antenna and then adjust the local antenna in the elevation or leveling direction. During elevation or leveling adjustment, use a multimeter to test RSSI at the receiving end and find at least three maximum values with the middle value being the biggest. The peak point of the voltage wave indicates the main lobe position in the elevation or leveling direction. Large-scope adjustment is unnecessary. Perform fine adjustment on the antenna to the peak voltage point.\nThe elevation and leveling adjustment methods are the same.\nWhen antennas are poorly aligned, only a small voltage may be detected in one direction. In this case, perform coarse adjustment on the antennas at both ends, so that the antennas are roughly aligned.\nThe antennas at both ends that are well aligned will face a little bit upward. Though 1–2 dB is lost, reflection interference will be avoided.\nWhat are the key specifications of antennas?\nAntenna gain, half-power angle, cross polarization decoupling, immunity, etc.\nWhat are the key specifications of ODU transmitters and receivers?\nKey specifications of transmitters: Operating frequency band, output power, local oscillator frequency stability, transmit frequency spectrum frame, etc.\nKey specifications of receivers: Operating frequency band, output power, local oscillator frequency stability, noise figure, passband, selectivity, AGC range, etc.\nCan you describe the entire signal flow of microwave transmission?\nWe may take the process of microwave transmission from the transmit end to the receive end to describe the signal flow of microwave transmission:\nIn the transmit end, the service access unit completes the access of the digital baseband signal, then the signal forms the microwave frame at the multiplexing unit, the microwave frame signal is modulated at the modulation unit into the IF signal, and the IF signal is sent to the ODU. After the ODU implements frequency mixing of the IF signal with the local transmit oscillator, the IF signal enters the sideband filter to become the RF signal. The converted RF signal is then amplified via the power amplifier and finally sent out via the antenna.\nIn the receive end, the antenna transmits the RF signal upon receipt of it to the ODU. The ODU first implements filtering to filter out some interference signals and then implements low-noise preamplification to improve the level of the received weak RF signal. The amplified signal undergoes frequency mixing with the local receive oscillator, and is then filtered to become the IF signal. The IF signal is then amplified and sent to the IDU. The IDU first demodulates the IF signal to get the digital baseband signal. Till now, the signal is still a complete microwave frame structure. The digital baseband signal is then sent to the multiplexing unit, where overheads and service signals are separated. The overheads are sent to the control unit and the service signals are sent to the cross-connect unit for service dispatching.\n","18":"What is microwave?\nMicrowave is a kind of electromagnetic wave. In a broad sense, the frequency range of microwave is 300 MHz to 300 GHz. In microwave communication, the frequency range generally is from 3 GHz to 30 GHz. According to the characteristics of microwave propagation, microwave can be considered as plane wave. The plane wave has no electric field and magnetic field longitudinal components along the propagation direction. The electric field and magnetic field components are vertical to the propagation direction. Therefore, it is called “transverse electromagnetic wave” or TEM for short.\nWhat is digital microwave communication? Digital microwave communication is a way of transmitting digital information in atmosphere on microwave or radio frequency (RF). It adopts the digital modulation scheme. The baseband signal is processed in the Intermediate Frequency (IF) unit. Then the signal is converted into the microwave frequency band through frequency conversion.\nWhat frequency bands are commonly used in digital microwave communication?\nAccording to ITU-R Recommendations, the common frequency bands include 7G/8G/11G/13G/15G/18G/23G/26G/32G/38G. Higher or lower bands may also be employed along with the development of technologies but the application is rare. Different bands are applied to different fields.\nWhat concepts are involved in microwave frequency setting?\nThe concepts include central frequency, transmit/receive spacing, channel spacing and protection spacing.\nWhat are the frequently used modulation schemes? Which are the most frequently-used?\nThe frequently-used modulation modes are ASK, FSK, PSK and QAM. The most frequently-used are PSK and QAM.\n","112":"What are the requirements for microwave communication?\nBecause the microwave is a short wave and has weak ability of diffraction, the normal communication can be realized only in the line-of-sight transmission without obstacles.\nIn microwave transmission, the transmit power is very small, so only the antenna in the accurate direction can realize the communication. The only way to implement long-haul communication is to use the antenna of a greater diameter or increase the transmit power of the antenna.\nWhat is the goal of microwave design?\nIn common geographical conditions, it is recommended that there be no obstacles within the first Fresnel zone if K is equal to 4/3.\nWhen the microwave transmission line passes the water surface or the desert area, it is recommended that there be no obstacles within the first Fresnel zone if K is equal to 1.\nWhat extra factors should be taken into consideration for microwave planning?\nMany factors should be considered in microwave planning. First, select the appropriate frequency band and channel configuration scheme according to the surrounding electromagnetic environment. Then select the appropriate links and sites. Generally, we should select the links with a small ground reflection factor. The selected sites should facilitate site construction and maintenance and ensure the line-of-sight communication between sites. Moreover, determine the appropriate clearance according to the K value and ground reflection factor, and then determine the mounting height and diameter of the antennas. Finally, calculate if the circuit indices, e.g. received level and link interruption rate, satisfy the requirements according to the local climate conditions. Add protection if necessary when the indices do not satisfy the requirements.\nCan you tell us the procedure for designing a microwave transmission line?\nFour steps:\nStep 1: Determine the circuit route according to the engineering map.\nStep 2: Select the site of the microwave station.\nStep 3: Draw the cross-sectional chart of the terrain.\nStep 4: Calculate the parameters for site construction.\n","85":"Space diversity can effectively solve the K-type fading caused by the interference of ground-reflective wave and direct wave, and the interference fading caused troposphere reflection. \n","8":"The wave with the radio frequency between 300 MHz and 300 GHz (or the wavelength between 1 meter and 1 millimeter) is called centimeter wave in microwave. \nTEM（Transverse Electric and Magnetic）\n","69":"The fading caused by the changes of K value. When the K value changes, it indicates that multipath fading is caused by ground reflection. \n","25":"The installation of the split-mount radio contains two parts, indoor installation and outdoor installation. Indoor installation is similar to case-shaped equipment installation. So we focus on outdoor installation. Outdoor installation includes installing the antenna and ODU. There are two methods. One is direct installation and the other is separate installation. \n","64":"Water droplets in rain or fog can cause scattering or absorption attenuation for electromagnetic wave.\n7G and 8G microwave can transmit for over 100 km. \n","31":"We can see that when the antenna diameter is determinate, the higher the operating frequency is, the smaller the half-power angle is. When the operating frequency is determinate, the bigger the antenna diameter is, the smaller the half-power angle is. And the smaller the half-power angle is, the more the energy is concentrated and the better the directional quality is. \nG=20log 7.33×D×F\n","9":"Before 1980, analog microwave had been playing a predominant role in communication. Since 1990, digital microwave technologies have been developing rapidly. Apart from the progress of technologies, the characteristic of digital signal, that is, keeping a good signal-to-noise ratio, is the key factor that ensures the long haul transmission capability. \nIt has been more than 50 years since microwave technologies developed. As a radio transmission scheme where the microwave frequency band signal adopts ground line-of-sight (LOS) propagation, microwave technologies have experienced the transition from analog microwave to digital microwave. The analog microwave and coaxial cable carrier transmission system are the two major methods used in the early stage for long haul transmission.\nThe earliest TV program transmission among cities adopts the microwave transmission channels. The small and medium capacity digital microwave equipment (8.34 Mbit/s) developed in 1970s has turned a new leaf for the digitalization of microwave. In late 1980s, the successful development of SDH digital microwave leads to the emerging of the Nx155 Mbit/s large capacity digital microwave system. Speaking of analog microwave, it was no longer used to construct networks in the end of 1980s, and now is used only in mountain stations owned by State Administration of Radio, Film and Television of China. \n","92":"What factors can affect microwave propagation?\nAnswer: The factors include terrain, atmosphere and climate.\nWhat types of fading exist in microwave propagation?\nAnswer: Fading may fall into many types by different classification methods.\nBy the mechanism of fading, fading may fall into duct type fading, k-type fading, scintillation fading, rain fading, absorption fading and free space propagation fading.\nBy fading time, fading may fall into fast fading and slow fading.\nBy received level, fading may fall into up fading and down fading.\nBy the influence of fading on signals, fading may fall into frequency selective fading and flat fading.\nWhat are the two categories of anti-fading technologies?\nAnswer: Equipment-level countermeasures and system-level countermeasures.\nThe equipment-level countermeasures include adaptive equalization, automatic transmit power control (ATPC) and forward error correction (FEC).\nThe system-level countermeasures include the diversity receiving technology.\nWhat protection modes are available for microwave?\nAnswer: 1+1 FD, 1+1 SD, 1+1 FD+SD, N+1 FD, etc.\n","48":"The relay efficiency of the reflector is higher than that of back-to-back antennas. \n","98":"This is a diagram of the transmit power of an antenna.\nWhy the microwave antenna adopts a parabolic surface instead of a round one? The principle is the same as the torch.\nAdjust the receive level of the antenna to the maximum. The meters include a multimeter and a NEC voltage regulator. The margin between the receive level of the main lobe and the side lobe can be over 10 dB.\nWhen adjusting the antenna, 0.5 watt indicates 3 dB. What is the half-power angle used for? The antenna shall be adjusted into the range of the half-power angle. \nThe iron tower can shake sometimes. Check whether the shaking affect the half-power angle or not. Generally, acquire the shaking direction and range of the iron tower when proposing requirements. \nAdding Note 4 \n","76":"In digital microwave transmission, the receive power decrease or waveform distortion is caused by various fading forms such as atmosphere, ground, and climate. This may further cause the circuit performance to downgrade. Therefore, proper anti-fading measures shall be taken to improve the performance of the transmission circuit system. \nIn addition, to apply microwave communication to the areas with so difficult propagation conditions (such as long distance, sea surface, and swamp) that other transmission technologies cannot satisfy the requirements, anti-fading measures shall also be taken.\nIn a digital microwave transmission system, the degradation factors can be divided into time-variant and time-invariant factors. Level fading, frequency selective fading, and rain fading belong to time-variant degradation factors. And the incomplete system belongs to time-invariant degradation factors. From the degradation phenomenon perspective, these factors can cause waveform distortion, or the increase of interference noise and heat noise. \nFor waveform distortion, the automatic equalization technique and various diversity combining techniques that enable the frequency characteristic to become flat are very effective. \nTo reduce the interference noise, the effective techniques are:\nInterference compensation technique used for cross polarization waves\nDiversity combining technique used to increase the receive level and decrease the interference noise\nAntenna technique used to improve the antenna directivity and avoid receiving interference electromagnetic wave. \nFor heat noise, these non-linear compensation techniques and diversity combining techniques that are used to increase transmit power or to prevent from the decrease of receive power can be adopted. \n","65":"What is the earth radius? In microwave, the earth radius used is 6370 km. The circumference of earth is over 40,000 km.\nFor the purpose of calculation, the concept of equivalent earth radius is used. Electromagnetic wave is considered as a straight line. The actual earth radius "a" is equivalent to "ae". The basic principle is that the clearance between the radial and the ground remains the same. \n","10":"The original communication does not contain the concept of network. Instead, it is point-to-point communication. There were no switches. It was manual switch at the beginning, then stored program control (SPC) switch, and time division and space division technologies were adopted later. The current complex networks are all derived from the primal simple networks. \nThe microwave transmission media is located in the troposphere which is the lowest layer. Above troposphere, there is the stratosphere, the use of which is now under research. \n"}