Digital microwave communication principles
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  • {"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"}

Transcript

  • 1. Digital Microwave Communication Principles www.huawei.com Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td.
  • 2. Foreword  This course is developed to meet the requirement of Huawei Optical Network RTN microwave products.  This course informs engineers of the basics on digital microwave communications, which will pave the way for learning the RTN series microwave products later. Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 2
  • 3. Learning Guide  Microwave communication is developed on the basis of the electromagnetic field theory. Therefore, before learning this course, you are supposed to have mastered the following knowledge:  Network communications technology basics  Electromagnetic field basic theory Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 3
  • 4. Objectives  After this course, you will be able to explain:  Concept and characteristics of digital microwave communications  Functions and principles of each component of digital microwave equipment  Common networking modes and application scenarios of digital microwave equipment  Propagation principles of digital microwave communication and various types of fading  Anti-fading technologies  Procedure and key points in designing microwave transmission link Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 4
  • 5. Contents 1. Digital Microwave Communication Overview 2. Digital Microwave Communication Equipment 3. Digital Microwave Networking and Application 4. Microwave Propagation and Anti-fading Technologies 5. Designing Microwave Transmission Links Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 5
  • 6. Transmission Methods in Current Communications Networks Coaxial cable communication Optical fiber communication Microwave communication Microwave TE Microwave TE MUX/DEMUX MUX/DEMUX Satellite communication Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 6
  • 7. Microwave Communication vs. Optical Fiber Communication Microwave Communication Powerful space cross ability, little land occupied, not limited by land privatization Small investment, short construction period, easy maintenance Strong protection ability against natural disaster and easy to be recover Optical Fiber Communication Optical fiber burying and land occupation required Large investment ,long construction period Outdoor optical fiber maintenance required and hard to recover from natural disaster Limited frequency resources (frequency license required) Not limited by frequency, license not required Transmission quality greatly affected by climate and landform Stable and reliable transmission quality and not affected by external factors Limited transmission capacity Large transmission capacity Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 7
  • 8. Definition of Microwave  Microwave  Microwave is a kind of electromagnetic wave. In a broad sense, the microwave frequency range is from 300 MHz to 300 GHz. But In microwave communication, the frequency range is generally 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 and TEM wave for short. Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 8
  • 9. Development of Microwave Communication 155M Transmission capacity bit/s/ch) SDH digital microwave communication system 34/140M PDH digital microwave communication system 2/4/6/8M 480 voice channels Small and medium capacity digital microwave communication system Analog microwave communication system Late 1990s to now 1980s 1970s 1950s Note: Small capacity: < 10M Medium capacity: 10M to 100M Large capacity: > 100M Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 9
  • 10. Concept of Digital Microwave Communication  Digital microwave communication is a way of transmitting digital information in atmosphere through microwave or radio frequency (RF).  Microwave communication refers to the communication that use microwave as carrier .  Digital microwave communication refers to the microwave communication that adopts the digital modulation.  The baseband signal is modulated to intermediate frequency (IF) first . Then the intermediate frequency is converted into the microwave frequency.  The baseband signal can also be modulated directly to microwave frequency, but only phase shift keying (PSK) modulation method is applicable.  The electromagnetic field theory is the basis on which the microwave communication theory is developed. Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 10
  • 11. Microwave Frequency Band Selection and RF Channel Configuration (1) Generally-used frequency bands in digital microwave transmission:   7G/8G/11G/13G/15G/18G/23G/26G/32G/38G (defined by ITU-R Recommendations) 1.5 GHz 2.5 GHz Regional network 3.3 GHz Long haul trunk network 11 GHz Regional network, local network, and boundary network 2/8/34 Mbit/s 34/140/155 Mbit/s 2/8/34/140/155 Mbit/s GHz 1 2 3 4 5 8 10 Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. 20 30 Page 11 40 50
  • 12. Microwave Frequency Band Selection and RF Channel Configuration (2)  In each frequency band, subband frequency ranges, transmitting/receiving spacing (T/R spacing), and channel spacing are defined. Frequency range Low frequency band f0 (center frequency) High frequency band T/R spacing Protection spacing Channel spacing f1 T/R spacing Adjacent channel T/R spacing f2 fn Channel spacing f 1’ Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. f 2’ fn’ Page 12
  • 13. Microwave Frequency Band Selection and RF Channel Configuration (3) Frequency range (7425M–7725M) f0 (7575M) T/R spacing: 154M 28M f1=7442 7G Frequency f5 f2=7470 F0 (MHz) f1’=7596 f2’ f 5’ T/ Spacing R Channel Spacing Primary and Non- (MHz) (MHz) primary Stations Range Fn=f0-161+28n, 7425–7725 7575 154 28 Fn’=f0- 7+28n, (n: 1–5) 7575 161 7 7275 196 28 7597 196 28 7250–7550 7400 161 3.5 … … … … 7110–7750 Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. … Page 13
  • 14. Digital Microwave Communication Modulation (1)   Digital baseband signal is the unmodulated digital signal. The baseband signal cannot be directly transmitted over microwave radio channels and must be converted into carrier signal for microwave transmission. Channel bandwidth Baseband signal rate Digital baseband signal Modulation IF signal Service signal transmitted Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 14
  • 15. Digital Microwave Communication Modulation (2)   The following formula indicates a digital baseband signal being converted into a digital frequency band signal. A* COS(W t+φ ) c* Amplitude     Frequency Phase PSK and QAM are most frequently used in digital microwave. ASK: Amplitude Shift Keying. Use the digital baseband signal to change the carrier amplitude (A). Wc and φ remain unchanged. FSK: Frequency Shift Keying. Use the digital baseband signal to change the carrier frequency (Wc). A and φ remain unchanged. PSK: Phase Shift Keying. Use the digital baseband signal to change the carrier phase (φ). Wc and A remain unchanged. QAM: Quadrature Amplitude Modulation. ). Use the digital baseband signal to change the carrier phase (φ) and amplitude (A). Wc remains unchanged. Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 15
  • 16. Microwave Frame Structure (1)  RFCOH 171.072 Mbit/s 15.552 Mbit/s RFCOH STM-1 155.52 Mbit/s SOH Payload MLCM DMY XPIC ATPC WS RSC INI ID FA 11.84 Mbit/s 64 kbit/s 16 kbit/s 64 kbit/s 2.24 Mbit/s 864 kbit/s 144 kbit/s 32 kbit/s 288 kbit/s RFCOH: Radio Frame Complementary Overhead RSC: Radio Service Channel MLCM: Multi-Level Coding Modulation INI: N:1 switching command DMY: Dummy ID: Identifier XPIC: Cross-polarization Interference Cancellation FA: Frame Alignment ATPC: Automatic Transmit Power Control WS: Wayside Service Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 16
  • 17. Microwave Frame Structure (2)  RFCOH is multiplexed into the STM-1 data and a block multiframe is formed. Each multiframe has six rows and each row has 3564 bits. One multiframe is composed of two basic frames. Each basic frame has 1776 bits. The remaining 12 bits are used for frame alignment. FS 6 bits Multiframe 3564 bits FS Basic frame 1 6 bits Basic frame 2 6 bits 1776 bits ( 148 words ) 1776 bits (148 words) I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I b I I C2 I I I I I a I I b I I C2 I I C1 I I C1 I I C1 I I C1 I I C1 I I C1 I I C1 I I C1 12 bits (the 1st word) 12 bits (the 148th word) I: STM-1 information bit C1/C2: Two-level correction coding monitoring bits FS: Frame synchronization a/b: Other complementary overheads Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 17
  • 18. Questions  What is microwave?  What is digital microwave communication?  What are the frequently used digital microwave frequency bands?  What concepts are involved in microwave frequency setting?  What are the frequently used modulation schemes? Which are the most frequently used modulation schemes? Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 18
  • 19. Contents 1. Digital Microwave Communication Overview 2. Digital Microwave Communication Equipment 3. Digital Microwave Networking and Application 4. Microwave Propagation and Anti-fading Technologies 5. Designing Microwave Transmission Links Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 19
  • 20. Microwave Equipment Category Digital microwave System Analog microwave MUX/DEMUX Mode PDH SDH Capacity Small and medium capacity (2–16E1, 34M) Large capacity (STM-0, STM-1, 2xSTM1) (Discontinued) Trunk radio Structure Split-mount radio All outdoor radio Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 20
  • 21. Trunk Microwave Equipment • • High cost, large transmission capacity, more stable performance, applicable to long haul and trunk transmission MSTU: Main Signal Transmission Unit (transceiver, modem, SDH electrical interface, hitless switching) P M1 SCSU: Supervision, Control and Switching Unit M2 … … RF, IF, signal processing, and MUX/DEMUX units are all indoor. Only the antenna system is outdoor. BRU: Branch RF Unit BBIU: Baseband Interface Unit (option) (STM-1 optical interface, C4 PDH interface) SDH microwave equipment Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 21
  • 22. All Outdoor Microwave Equipment • All the units are outdoor. RF processing unit IF cable • Installation is easy. IF and baseband processing unit • The equipment room can be saved. Service and power cable All outdoor microwave equipment Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 22
  • 23. Split-Mount Microwave Equipment (1)  The RF unit is an outdoor unit (ODU). The IF, signal processing, and MUX/DEMUX units are integrated in Antenna the indoor unit (IDU). The ODU and IF cable IDU are connected through an IF cable.  The ODU can either be directly ODU (Outdoor Unit) mounted onto the antenna or connected to the antenna through a short soft waveguide.  IDU (Indoor Unit) Although the capacity is smaller than the trunk, due to the easy installation and maintenance, fast network construction, it’s the most widely used microwave equipment. Split-mount microwave equipment Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 23
  • 24. Split-Mount Microwave Equipment (2)  Unit Functions  Antenna: Focuses the RF signals transmitted by ODUs and increases the signal gain.  ODU: RF processing, conversion of IF/RF signals.  IF cable: Transmitting of IF signal, management signal and power supply of ODU.  IDU: Performs access, dispatch, multiplex/demultiplex, and modulation/demodulation for services. Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 24
  • 25. Split-Mount Microwave Equipment – Installation Direct Mount Separate Mount antenna (direct mount) antenna (separate mount) ODU Soft waveguide IF cable IF cable ODU 中频口 IDU IF port Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. IF port IDU Page 25
  • 26. Microwave Antenna (1) Parabolic antenna  Antennas are used to send and receive microwave signals. Parabolic antennas is common type of microwave antennas. Microwave antenna diameters includes: 0.3m, 0.6m, 1.2m, 1.8m,2.0m, 2.4m, 3.0m, 3.2metc. Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 26
  • 27. Microwave Antenna (2)  Different frequency channels in same frequency band can share one antenna. Channel Tx Rx Tx Rx Channel 1 1 1 1 n n n n Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 27
  • 28. Antenna Adjustment (1) Side lobe Half-power angle Side view Main lobe Tail lobe Side lobe Half-power angle Top view Main lobe Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Tail lobe Page 28
  • 29. Antenna Adjustment (2) During antenna adjustment, change the direction vertically or horizontally. Meanwhile, use a multimeter to test the RSSI at the receiving end. Usually, the voltage wave will be displayed as shown in the lower right corner. The peak point of the voltage wave indicates the main lobe position in the vertical or horizontal direction. Large-scope adjustment is unnecessary. Perform fine adjustment on the antenna to the peak voltage point.  When antennas are poorly aligned, 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.  The antennas at both ends that are well aligned face a little bit upward. Though 1–2 dB is lost, reflection interference will be avoided.  Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. AGC Voltage detection point VAGC Angle Side lobe position Main lobe position Page 29
  • 30. Antenna Adjustment (3)  During antenna adjustment, the two wrong adjustment cases are show here. One antenna is aligned to another antenna through the side lobe. As a result, the RSSI cannot meet the requirements. Wrong Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Wrong Page 30 Correct
  • 31. Split-Mount Microwave Equipment – Antenna (1)  Antenna gain  Definition: Ratio of the input power of an isotropic antenna Pio to the input power of a parabolic antenna Pi when the electric field at a point is the same for the isotropic antenna and the parabolic antenna.   2 Pio  πD  = Calculating formula of antenna gain: G =  ∗η Pi  λ  Half-power angle  Usually, the given antenna specifications contain the gain in the largest radiation (main lobe) direction, denoted by dBi. The half-power point, or the –3 dB point is the point which is deviated from the central line of the main lobe and where the power is decreased by half. The angle between the two half-power points is called the half-power angle.  0 0 Calculating formula of half-power angle: θ 0.5 = (65 ~ 70 ) λ D Half-power angle Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 31
  • 32. Split-Mount Microwave Equipment – Antenna (2)   Cross polarization discrimination  Suppression ratio of the antenna receiving heteropolarizing waves, usually, larger than 30 dB.  XdB = 10lgPo/Px  Po: Receiving power of normal polarized wave  Px: Receiving power of abnormal polarized wave   Antenna protection ratio   Attenuation degree of the receiving capability in a direction of an antenna compared with that in the main lobe direction. An antenna protection ratio of 180° is called front-to-back ratio. Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 32
  • 33. Split-Mount Microwave Equipment – ODU (1) ODU system architecture Uplink IF/RF conversion IF amplificat ion Frequency mixing Sideband filtering Local oscillation (Tx) ATPC Local oscillation (Rx) Supervi sion and control signal IF amplification Filtering Frequency mixing RF attenuation Power amplification Power detection RF loop Low-noise amplification Bandpass filtering Downlink RF/IF conversion Alarm and control Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 33
  • 34. Split-Mount Microwave Equipment – ODU (2)  Specifications of Transmitter  W orking frequency band Generally, trunk radios use 6, 7, and 8 GHz frequency bands. 11, 13 GHz and higher frequency bands are used in the access layer (e.g. BTS access).  Output power The power at the output port of a transmitter. Generally, the output power is 15 to 30 dBm. Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 34
  • 35. Split-Mount Microwave Equipment – ODU (3)  Local frequency stability If the working frequency of the transmitter is unstable, the demodulated effectived signal ratio will be decreased and the bit error ratio will be increased. The value range of the local frequency stability is 3 to 10 ppm.  Transmit Frequency Spectrum Frame The frequency spectrum of the transmitted signal must meet specified requirements, to avoid occupying too much bandwidth and thus causing too much interference to adjacent channels. The limitations to frequency spectrum is called transmit frequency spectrum frame. Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 35
  • 36. Split-Mount Microwave Equipment – ODU (4)  Specifications of Receiver  W orking frequency band Receivers work together with transmitters. The receiving frequency on the local station is the transmitting frequency of the same channel on the opposite station.  Local frequency stability The same as that of transmitters: 3 to 10 ppm  Noise figure The noise figure of digital microwave receivers is 2.5 dB to 5 dB. Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 36
  • 37. Split-Mount Microwave Equipment – ODU (5)  Passband To effectively suppress interference and achieve the best transmission quality, the passband and amplitude frequency characteristics should be properly chosen. The receiver passband characteristics depend on the IF filter.  Selectivity Ability of receivers of suppressing the various interferences outside the passband, especially the interference from adjacent channels, image interference and the interference between transmitted and received signals.  Automatic gain control (AGC) range Automatic control of receiver gain. With this function, input RF signals change within a certain range and the IF signal level remains unchanges. Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 37
  • 38. Split-Mount Microwave Equipment – ODU (6) Frequency range (7425M–7725M) T/R spacing: 154M Subband A 7442 Subband B f0(7575M) Subband C Subband A Subband B Subband C ODUs are of rich types and small volume. Usually, ODUs are produced by small manufacturers and integrated by big manufacturers. 7498 Non-primary station Primary station ODU specifications are related to radio frequencies. As one ODU cannot cover an entire frequency band, usually, a frequency band will be divided into several subbands and each subband corresponds to one ODU.  Different T/R spacing corresponds to different ODUs.  Primary and non-primary stations have different ODUs.  Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Types of ODUs = Number of frequency bands x Number of T/R spacing x Number of subbands x 2 (ODUs of some manufacturers are also classified by capacity. Page 38
  • 39. Split-Mount Microwave Equipment – IDU Service channel Tributary unit Microwave frame demultiplexing Modulat ion Demod ulation Tx IF Rx IF Line unit O&M interface Power interface Service channel Supervision and control DC/DC conversion Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 39 Cable interface Crossconne ction Microwave frame multiplexing IF unit From/to ODU
  • 40. Questions   What types are microwave equipment classified into? What units do the split-mount microwave equipment have? And what are their functions??  How to adjust antennas?  What are the key specifications of antennas?  What are the key specifications of ODU transmitters and receivers?  Can you describe the entire signal flow of microwave transmission? Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 40
  • 41. Summary  Classification of digital microwave equipment  Components of split-mount microwave equipment and their functions  Antenna installation and key specifications of antennas  Functional modules and key performance indexes of ODU  Functional modules of IDU  Signal flow of microwave transmission Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 41
  • 42. Contents 1. Digital Microwave Communication Overview 2. Digital Microwave Communication Equipment 3. Digital Microwave Networking and Application 4. Microwave Propagation and Anti-fading Technologies 5. Designing Microwave Transmission Links Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 42
  • 43. Common Networking Modes of Digital Microwave Ring network Chain network Add/Drop network Hub network Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 43
  • 44. Types of Digital Microwave Stations • Digital microwave stations are classified into Pivotal stations, add/drop relay stations, relay stations and terminal stations. Add/Drop relay station Relay station Terminal station Terminal station Pivotal station Terminal station Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 44
  • 45. Types of Relay Stations Passive • Back-to-back antenna • Plane reflector Relay station Active Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. • Regenerative repeater • IF repeater • RF repeater Page 45
  • 46. Active Relay Station   R adio Frequency relay station   An active, bi-directional radio repeater system without frequency shift. The RF relay station directly amplifies the signal over radio frequency.    Regenerator relay station   A high-frequency repeater of high performance. The regenerator relay station is used to extend the transmission distance of microwave communication systems, or to deflect the transmission direction of the signal to avoid obstructions and ensure the signal quality is not degraded. After complete regeneration and amplification, the received signal is forwarded. Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 46
  • 47. Passive Relay Station   Parabolic reflector passive relay station  The parabolic reflector passive relay station is composed of two parabolic antennas connected by a soft waveguide back to back.  The two-parabolic passive relay station often uses large-diameter antennas. Meters are necessary to adjust antennas, which is time consuming.  The near end is less than 5 km away. Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 47
  • 48. Plane Reflector Passive Relay Station Plane reflector passive relay station: A metal board which has smooth surface, proper effective area, proper angle and distance with the two communication points. It is also a passive relay microwave station.   Full-distance free space loss: d1(km) ϕ Ls = 142.1 + 20 log d1d 2 − 20 log a d 2(km) a = A cosϕ2 “a” is the effective area (m2) of the flat reflector. Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 48
  • 49. Passive Relay Station (Photos) Passive relay station (plane reflector) Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Passive relay station (parabolic reflectors) Page 49
  • 50. Application of Digital Microwave BTS backhaul transmission Complementary networks to optical networks (access the services from the last 1 km) Special transmission conditions (rivers, lakes, islands, etc.) Microwave application Emergency communications (conventions, activities, danger elimination, disaster relief, etc.) Redundancy backup of important links VIP customer access Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 50
  • 51. Questions  What are the networking modes frequently used for digital microwave?  What are the types of digital microwave stations?  What are the types of relay stations?  What is the major application of digital microwave? Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 51
  • 52. Contents 1. Digital Microwave Communication Overview 2. Digital Microwave Communication Equipment 3. Digital Microwave Networking and Application 4. Microwave Propagation and Anti-fading Technologies 5. Designing Microwave Transmission Links Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 52
  • 53. Contents 4. Microwave Propagation and Anti-fading Technologies  4.1 Factors Affecting Electric W ave Propagation  4.2 Various Fading in Microwave Propagation  4.3 Anti-fading Technologies for Digital Microwave Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 53
  • 54. Key Parameters in Microwave Propagation (1)   Fresnel Zone and Fresnel Zone Radius   Fresnel zone: The sum of the distance from P to T and the distance from P to R complies with the formula, TP+ PR-TR= n λ/2 (n= 1 , 2 , 3 , … ). The elliptical region encircled by the trail of P is called the Fre s ne l z o ne .   Fresnel zone radius: The vertical distance from P to the TR line in the Fresnel zone. The first Fresnel zone radius is represented by F1 (n=1). Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 54
  • 55. Key Parameters in Microwave Propagation (2)   Formula of the first Fresnel zone radius: F1 = 17.32 d1 (km) × d 2 (km) f (GHz ) × d (km)   The first Fresnel zone is the region where the microwave transmission energy is the most concentrated. The obstruction in the Fresnel zone should be as little as possible. With the increase of the Fresnel zone serial numbers, the field strength of the receiving point reduces as per arithmetic series. Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 55
  • 56. Key Parameters in Microwave Propagation (3)   A Clearance h3 h1 F M hc B hp h5 hs h4 h6 d1  d h2 d2   Along the microwave propagation trail, the obstruction from buildings, trees, and mountain peaks is sometimes inevitable. If the height of the obstacle enters the first Fresnel zone, additional loss might be caused. As a result, the received level is decreased and the transmission quality is affected. Clearance is used to avoid the case described previously.  The vertical distance from the obstacle to AB line segment is called the clearance of the obstacle on the trail. For convenience, the vertical distance hc from the obstacle to the ground surface is used to represent the clearance. In practice, the error is not big because the line segment AB is approximately parallel to the ground surface. If the first Fresnel zone radius of the obstacle is F1, then hc/ F1 is the relative clearance. Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 56
  • 57. Factors Affecting Electric W ave Propagation – Terrain The reflected wave from the ground surface is the major factor that affects the received level.  Straight line Reflection  Straight line Reflection Smooth ground or water surface can reflect the part of the signal energy transmitted by the antenna to the receiving antenna and cause interference to the main wave (direct wave). The vector sum of the reflected wave and main wave increases or decreases the composite wave. As a result, the transmission becomes unstable. Therefore, when doing microwave link design, avoid reflected waves as much as possible. If reflection is inevitable, make use of the terrain ups and downs to block the reflected waves. Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 57
  • 58. Factors Affecting Electric W ave Propagation – Terrain Different reflection conditions of different terrains have different effects on electric  wave propagation. Terrains are classified into the following four types:   Type A: mountains (or cities with dense buildings)    Type B: hills (gently wavy ground surface)    Type C: plain    Type D: large-area water surface    The reflection coefficient of mountains is the smallest, and thus the mountain terrain  is most suitable for microwave transmission. The hill terrain is less suitable. When designing circuits, try to avoid smooth plane such as water surface. Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 58
  • 59. Factors Affecting Electric W ave Propagation – Atmosphere   Troposphere indicates the low altitude atmosphere within 10 km from the  ground. Microwave antennas will not be higher than troposphere, so the electric wave propagation in aerosphere can be narrowed down to that in troposphere. Main effects of troposphere on electric wave propagation are listed below:  Absorption caused by gas resonance. This type of absorption can affect the microwave at 12 GHz or higher.  Absorption and scattering caused by rain, fog, and snow. This type of absorption can affect the microwave at 10 GHz or higher.  Refraction, absorption, reflection and scattering caused by inhomogeneity of atmosphere. Refraction is the most significant impact to the microwave propagation. Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 59
  • 60. Contents 4. Microwave Propagation and Anti-fading Technologies  4.1 Factors Affecting Electric Wave Propagation  4.2 Various Fading in Microwave Propagation  4.3 Anti-fading Technologies for Digital Microwave Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 60
  • 61. Fading in Microwave Propagation   Fading: Random variation of the received level. The variation is irregular and the reasons for this are various. Fading mechanism Fading time Frequency selective fading Page 61 Influence of fading on signal Flat fading Down fading Up fading Slow fading Fast fading Duct type fading K-type fading Scintillation fading Rain fading Absorption fading Free space propagation fading Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Received level
  • 62. Free Space Transmission Loss   Free space loss: A = 92.4 + 20 log d + 20 log f (d: km, f: GHz). If d or f is doubled, the loss will increase by 6 dB. d GTX GRX PRX = Receive power G = Antenna gain f Power level PTX = Transmit power A0 = Free space loss M = Fading margin G A0 PTX PRX G Receiving threshold M Distance Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 62
  • 63. Absorption Fading   Molecules of all substances are composed of charged particles. These particles have their own electromagnetic resonant frequencies. When the microwave frequencies of these substances are close to their resonance frequencies, resonance absorption occurs to the microwave.   Statistic shows that absorption to the microwave frequency lower than 12 GHz is smaller than 0.1 dB/km. Compared with free space loss, the absorption loss can be ignored. 10dB 1dB 0.1dB 0.01dB 60GHz 23GHz 12GHz 7.5GHz 1GHz Atmosphere absorption curve (dB/km) Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 63
  • 64. Rain Fading   For frequencies lower than 10 GHz, rain loss can be ignored. Only a few db may be added to a relay section.   For frequencies higher than 10 GHz, repeater spacing is mainly affected by rain loss. For example, for the 13 GHz frequency or higher, 100 mm/h rainfall causes a loss of 5 dB/km. Hence, for the 13 GHz and 15 GHz frequencies, the maximum relay distance is about 10 km. For the 20 GHz frequency and higher, the relay distance is limited in few kilometres due to rain loss.  High frequency bands can be used for user-level transmission. The higher the frequency band is, the more severe the rain fading. Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 64
  • 65. K-Type Fading (1)  Atmosphere refraction  As a result of atmosphere refraction, the microwave propagation trail is bent. It is considered that the electromagnetic wave is propagated along a straight line above the earth with an equivalent earth radius of  Re, Re = KR (R: actual earth radius.) The average measured K value is about 4/3. However, the K value of a specific section is related to the meteorological phenomena of the section. The K value may change within a comparatively large range. This can affect line-of-sight propagation. Re Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. R Page 65
  • 66. K-Type Fading (2)  Microwave propagation k > 1: Positive refraction k = 1: No refraction k < 1: Negative refraction Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 66
  • 67. K-Type Fading (3)  Equivalent earth radius  In temperate zones, the refraction when the K value is 4/3 is regarded as the standard refraction, where the atmosphere is the standard atmosphere and Re which is 4R/3 is the standard equivalent earth radius. k=∞ 4/3 1 2/3 Ground surface Actual earth radius (r) 2/3 1 4/3 k=∞ Ground surface Equivalent earth radius (r·k) Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 67
  • 68. Multipath Fading (1) Multipath fading: Due to multipath propagation of refracted waves, reflected waves, and scattered waves, multiple electric waves are received at the receiving end. The composition of these electric waves will result in severe interference fading.  Reasons for multipath fading: reflections due to non-uniform atmosphere, water surface and smooth ground surface.  Down fading: fading where the composite wave level is lower than the free space received level. Up fading: fading where the composite wave level is higher than the free space received level.    Non-uniform atmosphere   Water surface  Smooth ground surface. Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Ground surface Page 68
  • 69. Multipath Fading (2)  Multipath fading is a type of interference fading caused by multipath transmission. Multipath fading is caused by mutual interference between the direct wave and reflected wave (or diffracted wave on some conditions) with different phases.  Multipath fading grows more severe when the wave passes water surface or smooth ground surface. Therefore, when designing the route, try to avoid smooth water and ground surface. When these terrains are inevitable, use the high and low antenna technologies to bring the reflection point closer to one end so as to reduce the impact of the reflected wave, or use the high and low antennas and space diversity technologies or the antennas that are against reflected waves to overcome multipath fading. Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 69
  • 70. Multipath Fading – Frequency Selective Fading Received power (dBm) Flat Selective fading Normal Frequency (MHz) Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 70
  • 71. Multipath Fading – Flat Fading Up fading Received level in free space Threshold level (-30 dB) 1h Signal interruption Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 71
  • 72. Duct Type Fading Due to the effects of the meteorological conditions such as ground cooling in the night, burnt warm by the sun in the morning, smooth sea surface, and anticyclone, a non-uniform structure is formed in atmosphere. This phenomenon is called atmospheric duct. If microwave beams pass through the atmospheric duct while the receiving point is outside the duct layer, the field strength at the receiving point is from not only the direct wave and ground reflected wave, but also the reflected wave from the edge of the duct layer. As a result, severe interference fading occurs and causes interruption to the communications. Duct type fading Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 72
  • 73. Scintillation Fading When the dielectric constant of local atmosphere is different from the ambient due to the particle clusters formed under different pressure, temperature, and humidity conditions, scattering occurs to the electric wave. This is called scintillation fading. The amplitude and phase of different scattered waves vary with the atmosphere. As a result, the composite field strength at the receiving point changes randomly. Scintillation fading is a type of fast fading which lasts a short time. The level changes little and the main wave is barely affected. Scintillation fading will not cause communications interruption. Scintillation fading Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 73
  • 74.   Summary  The higher the frequency is and the longer the hop distance is, the more severe the fading is.  Fading is more severe at night than in the daylight, in summer than in winter. In the daylight, sunshine is good for air convection. In summer, weather changes frequently.  In sunny days without wind, atmosphere is non-uniform and atmosphere subdivision easily forms and hardly clears. Multipath transmission often occurs in such conditions.  Fading is more severe along water route than land route, because both the reflection coefficient of water surface and the atmosphere refraction coefficient above water surface are bigger.  Fading is more severe along plain route than mountain route, because atmosphere subdivision often occurs over plain and the ground reflection factor of the plain is bigger.  Rain and fog weather causes much influence on high-frequency microwave. Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 74
  • 75. Contents 4. Microwave Propagation and Anti-fading Technologies  4.1 Factors Affecting Electric Wave Propagation  4.2 Various Fading in Microwave Propagation  4.3 Anti-fading Technologies for Digital Microwave Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 75
  • 76. Anti-fading Technologies for Digital Microwave System (1) Category Effect Adaptive equalization System level countermeasure Automatic transmit power control (ATPC) Power reduction Forward error correction (FEC) Equipment level countermeasure Waveform distortion Power reduction Diversity receiving technology Power reduction and waveform distortion Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 76
  • 77. Anti-fading Technologies for Digital Microwave System (2)   Frequency domain equalization Multipath fading Signal frequency spectrum   Slope equalization Frequency spectrum after equalization The frequency domain equalization only equalizes the amplitude frequency response characteristics of the signal instead of the phase frequency spectrum characteristics.  The circuit is simple. Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 77
  • 78. Anti-fading Technologies for Digital Microwave System (3)   Time domain equalization Time domain equalization directly counteracts the intersymbol interference.  T C-n … T … C0 T Cn After Before -2Ts -Ts Ts Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. -2Ts -Ts Page 78 Ts
  • 79. Anti-fading Technologies for Digital Microwave System (4)  Automatic transmit power control (ATPC) Under normal propagation conditions, the output power of the transmitter is always at a lower level, for example, 10 to 15 dB lower than the normal level. When propagation fading occurs and the receiver detects that the propagation fading is lower than the minimum received level specified by ATPC, the RFCOH is used to let the transmitter to raise the transmit power.  Working principle of ATPC Modulator Transmitter ATPC Demodulator Receiver Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Receiver Demodulator ATPC Transmitter Page 79 Modulator
  • 80. Anti-fading Technologies for Digital Microwave System (5)  ATPC: The output power of the transmitter automatically traces and changes with the received level of the receiver within the control range of ATPC.  The time rate of severe propagation fading is usually small (<1%). After ATPC is configured, the transmitter works at a power 10 to 15 dB lower than the nominal power for over 99% of the time. In this way, adjacent channel interference and power consumption can be reduced.  Effects of ATPC:  Reduces the interference to adjacent systems and over-reach interference  Reduces up fading  Improves residual BER  Reduces DC power consumption Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 80
  • 81. Anti-fading Technologies for Digital Microwave System (6)  ATPC adjustment process (gradual change) High level -35 -45 Low level 21 -55 ATPC dynamic range -72 31 45 75 85 102 Link loss (dB) Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 81 Transmitter output level (dBm) Received level (dBm) -25
  • 82. Anti-fading Technologies for Digital Microwave System (7) Cross-polarization interference cancellation (XPIC)  680MHz 30MHz In microwave transmission, XPIC is  used to transmit two different signals over one frequency. The utilization ratio of the frequency spectrum is doubled. To avoid severe interference 340 MHz 80MHz 1 2 3 4 5 60MHz 6 7 8 1’ 5’ 4’ 5’ 6’ 7’ 8’ 680 MHz technology must be used. Electric field direction Shape of waveguide interface 4’ H (V) signals, the interference compensation 30MHz Vertical polarization 3’ V (H) between two different polarized Horizontal polarization 2’ 340MHz 80MHz 1 2 1X 2X 3 4 5 6 60MHz 7 8 6X 7X 1’ 2’ 3’ 6’ 7’ 8’ V (H) H (V) 3X 4X 5X 8X 1X’ 2X’ 3X' 4X’ 5X’ 6X’ 7X’ 8X’ Frequency configuration of U6 GHz frequency band (ITU-R F.384-5) Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 82
  • 83. Anti-fading Technologies for Digital Microwave System (8)  Diversity technologies For diversity, two or multiple transmission paths are used to transmit the same information and the receiver output signals are selected or composed, to reduce the effect of fading.  Diversity has the following types, space diversity, frequency diversity, polarization diversity, and angle diversity.  Space diversity and frequency diversity are more frequently used. Space diversity is economical and has a good effect. Frequency diversity is often applied to multi-channel systems as it requires a wide bandwidth. Usually, the system that has one standby channel is configured with frequency diversity.  H Space diversity (SD) Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. f1 f2 Frequency diversity (FD) Page 83
  • 84. Anti-fading Technologies for Digital Microwave System (9)   Frequency diversity  Signals at different frequencies have different fading characteristics. Accordingly, two or more microwave frequencies with certain frequency spacing to transmit and receive the same information which is then selected or composed, to reduce the influence of fading. This work mode is called frequency diversity.  Advantages: The effect is obvious. Only one antenna is required.  Disadvantages: The utilization ratio of frequency bands is low. f1 f2 Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 84
  • 85. Anti-fading Technologies for Digital Microwave System (10)  Space diversity Signals have different multipath effect over different paths and thus have different fading characteristics. Accordingly, two or more suites of antennas at different altitude levels to receive the signals at the same frequency which are composed or selected. This work mode is called space diversity. If there are n pairs of antennas, it is called n-fold diversity.   Advantages: The frequency resources are saved. Disadvantages: The equipment is complicated, as two or more suites of antennas are required.  Antenna distance: As per experience, the distance between the diversity antennas is 100 to 200 times the wavelength in frequently used frequency bands. f1  f1 Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 85
  • 86. Anti-fading Technologies for Digital Microwave System (11)  Dh calculation in space diversity + nl Tx l/2 Rx Dh h1 d  Approximately, Dh can be calculated according to this formula: Dh = (nl + l/2)d l: wavelength d: path distance h1: height of the antenna at the transmit end 2h1 Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 86
  • 87. Anti-fading Technologies for Digital Microwave System (12)   Apart from the anti-fading technologies introduced previously, here are two frequently used tips:  Method I: Make use of some terrain and ground objects to block reflected waves. Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 87
  • 88. Anti-fading Technologies for Digital Microwave System (13)   Method II: high and low antennas Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 88
  • 89. Protection Modes of Digital Microwave Equipment (1) Hybrid coupler With one hybrid coupler added between two ODUs and the antenna, the 1+1 HSB can be realized in the configuration of one antenna. Moreover, the FD technology can also be adopted.  The 1+1 HSB can also be realized in the configuration of two antennas. In this case, the FD and SD technologies can both be adopted, which improves the system availability.  Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 89
  • 90. Protection Modes of Digital Microwave Equipment (2)  N+1 (N≤3, 7, 11) Protection In the following figure, Mn stands for the active channel and P stands for the standby channel. The active channel and the standby channel have their independent modulation/demodulation unit and signal transmitting /receiving unit.  When the fault or fading occurs in the active channel, the signal is switched to the standby channel. The channel backup is an inter-frequency backup. This protection mode (FD) is mainly used in the all indoor microwave equipment.   Products of different vendors support different specifications. ch1 ch2 ch3 M1 M2 M1 M2 M3 M3 ch1 ch2 ch3 chP P P chP Switching control unit RFSOH Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Switching control unit Page 90
  • 91. Protection Modes of Digital Microwave Equipment (3) Configuration Protection Mode Remarks Application Terminal of the network 1+0 NP Non-protection 1+1 FD Channel protection Interfrequency 1+1 SD Equipment protection and channel protection Intrafrequency 1+1 FD+SD Equipment protection and channel protection Interfrequency N+1 FD Equipment protection and channel protection Interfrequency Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Select the proper mode depending on the geographical condition and requirements of the customer Large-capacity backbone network Page 91
  • 92. Questions  What factors can affect the microwave propagation?  What types of fading exists in the microwave propagation?  What are the two categories is the anti-fading technology?  What protection modes are available for the microwave? Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 92
  • 93. Summary  Importance parameters affecting microwave propagation  Various factors affecting microwave propagation  Various fading types in the microwave propagation (free space propagation fading, atmospheric absorption fading, rain or fog scattering fading, K type fading, multipath fading, duct type fading, and scintillation type fading)  Anti-fading technologies  Anti-fading measures adopted on the equipment: adaptive equalization, ATPC, and XPIC  Anti-fading measures adopted in the system: FD and SD  Protection modes of the microwave equipment Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 93
  • 94. Contents 1. Digital Microwave Communication Overview 2. Digital Microwave Communication Equipment 3. Digital Microwave Networking and Application 4. Microwave Propagation and Anti-fading Technologies 5. Designing Microwave Transmission Links Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 94
  • 95. Contents 5. Designing Microwave Transmission Links  5.1 Basis of Designing a Microwave Transmission Line  5.2 Procedures for Designing a Microwave Transmission Line Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 95
  • 96. Basis of Designing a Microwave Transmission Line  Requirement on the point-to-point line-of-sight communication  Objective of designing a microwave transmission line  Transmission clearance  Meanings of K value in the microwave transmission planning Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 96
  • 97. Requirement on a Microwave Transmission Line   Because the microwave is a short wave and has weak ability of diffraction, the normal communication can be realized in the line-of-sight transmission without obstacles. Line propagation Irradiated wave Antenna D Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 97
  • 98. Requirement on a Microwave Transmission Line  In the microwave transmission, the transmit power is very small, only the antenna in the accurate direction can realize the communication. For the communication of long distance, use the antenna of greater diameter or increase the transmit power. Direction demonstration of the microwave antenna Microwave antenna Half power angle of the microwave antenna Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. 3 dB Page 98
  • 99. Objective of Designing a Microwave Transmission Line  In common geographical conditions, it is recommended that there be no obstacles within the first Fresnel zone if K is equal to 4/3.  When the microwave transmission line passes the water surface or the desert area, it is recommended that there are no obstacles within the first Fresnel zone if K is equal to 1. The first Fresnel zone k = 4/3 Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 99
  • 100. Transmission Clearance (1)   The knife-edged obstacle blocks partial of the Fresnel zone. This also causes the diffraction of the microwave. Influenced by the two reasons, the level at the actual receive point must be lower than the free space level. The loss caused by the knife-edged obstacle is called additional loss. Diff rac tion Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 100
  • 101. Transmission Clearance (2)  When the peak of the obstacle is in the line connecting the transmit end and the receive end, that 8 is, the HC is equal to 0, the additional loss is equal to 6 4 2  When the peak of the obstacle is above the line connecting the transmit end and the receive end, the additional loss is increased greatly.  When the peak of the obstacle is below the line connecting the transmit end the receive end, the additional loss fluctuates around 0 dB. The transmission loss in the path and the signal receiving level approach the values in the free space transmission. Additional loss (dB) 6 dB. 0 -2 -4 -6 -8 -10 -12 -14 -16 -18 -20 -22 -24 -26 -28 -2.5-2.0-1.5-1.0-0.5 0 0.51.0 1.5 2.0 2.5 HC/F1 Loss caused by block of knife-edged obstacle Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 101
  • 102. Transmission Clearance (3)   Clearance calculation  Calculation formula for path clearance h1d 2 + h2 d1 hc = − hb − hs d The value of clearance is required greater than that of the first Fresnel Zone’s radius.  hb stands for the projecting hc h2 h1 height of the earth. hs d1 hb d d1d 2 hb = 0.0785 K  K stands for the atmosphere refraction factor. Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 102 d2
  • 103. Transmission Clearance (4)   To present the influence of various factors on microwave transmission, the field strength fading factor V is introduced. The field strength fading factor V is defined as the ratio of the combined field strength when the irradiated wave and the reflected wave arrive at the receive point to the field strength when the irradiated wave arrives at the receive point in the free space transmission.  h E 2 V = = 1 + ϕ − 2 ϕ cos π  ce  E0   F1      2     E : Combined field strength when the irradiated wave and reflected wave in E0 arrive at the receive point : Field strength when the irradiated wave arrives at the received point ϕ : the free space transmission factor Equivalent ground reflection Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 103
  • 104. Transmission Clearance (5)   ϕ The relation of the V and can be represented by the curve in the figure on the right.  In the case that Φ is equal to 1, with the influence of the earth considered, H C/F1 is equal to 0.577 when the signal receiving level is equal to the free space level the first time.  In the case that Φ is smaller than 1, H C/F1 is V ( dB ) 10 5 0 -5 φ=0.2 -10 φ=0.5 -15 approximately equal to 0.6 when the signal receiving level is equal to the free space level -25 the first time. φ=0.8 -20 -30 When the HC/F1 is equal to 0.577, the clearance is called the free space clearance, represented by H0 and expressed in the -35 -40 0.6 4 1.0 4 1.3 1 1.4 3 1.5 6 1.7 6 1.9 3 2.0 1 2.1 0 2.2 6 2.3 9 2.4 6 2.5 4 2.6 6 2.7 8 2.8 5 3.0 2  φ =1 following formula: H0 = 0.577F 1 = (λd1d2/d)1/2 Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. HC/F1=N Relation curve of V and Hc/F1 Page 104
  • 105. Meaning of K Value in Microwave Transmission Planning (1)  To make the clearance cost-effective and reasonable in the engineering, the height of the antenna should be adjusted according to the following requirements.  In the case that Φ is not greater than 0.5, that is, for the circuit that passes the area of small ground reflection factor like the mountainous area, city, and hilly area, to avoid over great diffraction, the height of the antenna should be adjusted according to the following requirements: When K = 2/3, HC ≥ 0.3F1 (for common obstacles) HC ≥ 0 (for knife-shaped obstacles)  The diffraction fading should not be greater than 8 dB in this case. Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 105
  • 106. Meaning of K Value in Microwave Transmission Planning (2)  In the case that Φ is greater than 0.7, that is, for the circuit that passes the area of great ground reflection factor like the plain area and water reticulation area, to avoid over great reflection fading, the height of the antenna should be adjusted according to the following requirements      When K = 2/3, HC ≥ 0.3F1 (for common obstacles)      HC ≥ 0 (for knife-edged obstacles)    When K = 4/3, HC ≈ F1    When K = ∞ , HC ≤ 1.35F1 (The deep fading occurs when HC = 21/2 F1.)  If these requirements cannot be met, change the height of the antenna or the route. Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 106
  • 107. Procedure for Designing a Microwave Transmission Line  Step 1 Determine the route according to the engineering map.  Step 2 Select the site of the microwave station.  Step 3 Draw the cross-sectional chart of the terrain.  Step 4 Calculate the parameters for site construction. Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 107
  • 108. Procedure for Designing a Microwave Transmission Line (1) Step 1    Determine the route according to engineering map. We should select the area that rolls as much as possible, such as the hilly area. We should avoid passing the water surface and the flat and wide area that is not suitable for the transmission of the electric wave. In this way, the strong reflection signal and the accordingly caused deep fading can be avoided. The line should avoid crossing through or penetrating into the mountainous area. The line should go along with the railway, road and other areas with the convenient transportation. Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 108
  • 109. Procedure for Designing a Microwave Transmission Line (2) Step 2  Select the site of the microwave station. The distance between two sites should not be too long. The distance between two relay stations should be equal, and each relay section should have the proper clearance.  Select the Z route to avoid the over-reach interference.  Avoid the interference from other radio services, such as the satellite communication system, radar site, TV station, and broadcast station. f1 f1 f1 f2 f2 f2 Over-reach interference Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. The signal from the first microwave station interferes with the signal of the same frequency from the third microwave station. Page 109
  • 110. Procedure for Designing a Microwave Transmission Line (3) Step 3 Draw the cross-sectional chart of the terrain.  Draw the cross-sectional chart of the terrain based on the data of each site.  Calculate the antenna height and transmission situation of each site. For the line that has strong reflection, adjust the mounting height of the antenna to block the reflected wave, or have the reflection point fall on the earth surface with small reflection factor.  Consider the path clearance. The clearance in the plain area should not be over great, and that in the mountainous area should not be over small. Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 110
  • 111. Procedure for Designing a Microwave Transmission Line (4) Step 4    Calculate the parameters for site construction. Calculate the terrain parameters when the route and the site are already determined. Calculate the azimuth and the elevation angles of the antenna, distance between sites, free space transmission loss and receive level, rain fading index, line interruption probability, and allocated values and margin of the line index. When the margin of the line index is eligible, plan the equipment and frequencies, make the approximate budget, and deliver the construction chart. Input There is special network planning software, and the commonly used is CTE Pathloss. Input Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 111
  • 112. Questions  What are the requirements for microwave communication?  What is the goal of microwave design?  What extra factors should be taken into consideration for microwave planning?  Can you tell the procedure for designing a microwave transmission line? Copyright © 2006 H uawei T echnologies Co., L All rights reserved. td. Page 112
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