Wireless Communication

Transcript

COMPUCOM INSTITUTE OF TECHNOLOGY & MANAGEMENT, JAIPUR (DEPARTMENT OF ELECTRONICS & COMMUNICATION) Notes Wireless Communication (Subject Code: 7EC3) Prepared By: LOKESH KUMAR ARYA Class: B. Tech. IV Year, VII Semester Wireless Communication Line of sight microwave communication Syllabus UNIT 2: LINE OF SIGHT MICOWAVE COMMUNICATION- Link Engineering, Frequency planning, Free space loss, Fresnel zone clearance bending of radio beam, Effective earth radius, Building blocks of Transmitter & Receiver. Beyond the Syllabus Frequency Reuse Learning Objectives This unit gives the detailed knowledge about different parameters of data links in wireless communication. Prepared By: LOKESH KUMAR ARYA Page 2 Wireless Communication Line of sight microwave communication Unit 2 Line of sight microwave communication 2.1 Link Engineering: A link in a communication system can be defined as connecting two points separated at a distance, where one point may act as transmitter and another point act as a receiver. Implementation of data link is an integral part of communication engineering design and performance of data links significantly effect the overall communication system performance. Broadly data links are divided in four types: i. Wire line link ii. Radio wave link iii. Microwave link Radio link systems operate in the MHz to GHz range (microwaves). A microwave system consists of a number of ground base stations. Transmitting and receiving antennas must be in direct line of sight of each other. Radio link systems were introduced as an alternative to coaxial cable on long haul routes. They are also used for links to islands and difficult rural situations. Advantages of radio link systems include: · high bandwidth · low level of signal attenuation · can be used over rough terrain which would be unsuitable for cabled media Disadvantages of radio link systems include: · expensive over short distances · there can be no obstacles between the transmitting and receiving antennas · can suffer from interference due to climatic conditions and other microwave sources Digital Point-to-Point Microwave Link: The term digital communications covers a broad area of communications techniques, including digital transmission and digital radio. Digital transmission is the transmittal of digital pulses between two points in a communications system. Digital radio is the transmittal of digitally modulated analog carriers between two points in a communications system. Digital transmission systems require physical facility between the transmitter and receiver, such as a metallic wire pair, a coaxial cable or a fiber optic cable. In digital radio systems, the transmission medium is free space or the earth's atmosphere. Figure 2.1 shows simplified block diagrams of digital transmission point-to-point microwave system. In a digital transmission system, the original source information may be in digital Prepared By: LOKESH KUMAR ARYA Page 3 Wireless Communication Line of sight microwave communication or analog form. If it is in analog form, it must be converted to digital pulses prior to transmission and converted back to analog format the receive end. In a digital radio system, the modulating input signal and the demodulated output signal are digital pulses. The digital pulses could originate from a digital transmission system, from a digital source such as a mainframe computer or from the binary encoding of an analog signal. Transmission Lines A transmission line is a device that transfers energy (information) from one point to another with minimum amount of loss. Information can take the form of voice, video and data signals. In other words, the transmission line must be efficient. Efficiency is the real key to a transmission. Transmission media can be classified as either: Cabled · twisted pair · coaxial cable · fiber optic cable Non-cabled 2.2 · cellular radio systems · radio link systems · satellite system Frequency planning: Prepared By: LOKESH KUMAR ARYA Page 4 Wireless Communication Line of sight microwave communication Electromagnetic Spectrum for Telecommunications Electromagnetic Spectrum For wireless communication, antenna is needed. In transmission, antenna radiates electromagnetic energy in space and in reception, antenna picks up EM waves from surrounding medium. In general, there are three major ranges of frequencies which are used for wireless communication: i) Microwaves ii) Radio Waves iii) Infrared waves When electrons move, they create electromagnetic waves that can propagate through free space even in a vacuum. By attaching an antenna of the appropriate size to an electrical circuit, the electromagnetic waves can be broadcast efficiently and received by a receiver some distance away. All wireless communication is based on this principle. The electromagnetic spectrum is shown in the following figure. The radio, microwave, infrared and visible light portion of the spectrum can all be used for transmitting information by modulating the amplitude, frequency, or phase of the wave. Ultraviolet light, X- Ray and gamma rays would be even better, due to their higher frequencies but they are hard to produced and modulate, do not propagate well through buildings and are dangerous to living things. The bands listed below at the bottom of electromagnetic spectrum are the official ITU names and based on the wave lengths. So the LF band goes from 1km to 10 km (approximately 30 KHz to 300 KHz). The terms LF, MF Prepared By: LOKESH KUMAR ARYA Page 5 Wireless Communication Line of sight microwave communication and HF refer to low, medium and high frequency respectively. The amount of information that an electromagnetic wave can carry is related to its bandwidth. Frequency band for communication Table: Frequency bands: S. No. Frequency Band Frequency Range 1 Extremely Low Frequency (ELF) 0 to 3 KHz 2 Very Low Frequency (VLF) 3 KHz to 30 KHz 3 Low Frequency (LF) 30 KHz to 300 KHz 4 Medium Frequency (MF) 300 KHz to 3000 KHz 5 High Frequency (HF) 3 MHz to 30 MHz 6 Very High Frequency (VHF) 30 MHz to 300 MHz 7 Ultra-High Frequency (UHF) 300 MHz to 3000 MHz 8 Super high Frequencies (SHF) 3 GHz to 30.0 GHz (Microwave) 9 C-band 3600 MHz to 7025 MHz 10 X-band: 7.25 GHz to 8.4 GHz 11 Ku-band 10.7 GHz to 14.5 GHz 12 Ka-band 17.3 GHz to 31.0 GHz 13 Extremely High Frequencies (EHF) 30.0 GHz to 300 GHz (Millimeter Wave Signals) 2.3 14 Infrared Radiation 300 GHz to 430 THz 15 Visible Light 430 THz to 750 THz 16 Ultraviolet Radiation 1.62 PHz to 30 PHz 17 X-Rays 30 PHz to 30 EHz 18 Gamma Rays 30 EHz to 3000 EHz Free space loss: In telecommunication, free-space path loss (FSPL) is the loss in signal strength of an electromagnetic wave that would result from a line-of-sight path through free space (usually air), with no obstacles nearby to cause reflection or diffraction. It does not include factors such as the gain of the antennas used at the transmitter and receiver, nor any loss associated with hardware imperfections. A discussion of these losses may be found in the article on link budget. Prepared By: LOKESH KUMAR ARYA Page 6 Wireless Communication Line of sight microwave communication Free-space path loss formula Free-space space path loss is proportional to the square of the distance between the transmitter and receiver, and also proportional to the square of the frequency of the radio signal. For any type of wireless communication the signal disperses with distance. Therefore, an antenna with a fixed area will receive less signal power the farther it is from the transmitting antenna. For satellite communication this is the primary mode of signa signall loss. Even if no other sources of attenuation or impairment are assumed, a transmitted signal attenuates over distance because the signal is being spread over a larger and larger area. This form of attenuation is known as free space loss loss, which can be express press in terms of the ratio of the radiated power to the power received by the antenna or, in decibels, by taking 10 times the log of that ratio. For the ideal isotropic antenna, free space loss is The equation for FSPL is Pt (4pd ) (4pfd ) = = Pr l2 c2 2 2 where: · is the signal wavelength (in metres), · is the signal frequency (in hertz), · is the distance from the transmitter (in metres), · is the speed of light in a vacuum, 2.99792458 × 108 metres per second. This equation is only accurate in the far field where spherical spreading can be assumed; it does not hold close to the transmitter. Prepared By: LOKESH KUMAR ARYA Page 7 Wireless Communication Line of sight microwave communication Free-space path loss in decibels A convenient way to express FSPL is in terms of dB: LdB = 10 log Pt æ 4pd ö = 20 logç ÷ Pr è l ø For other antennas, we must take into account the gain of the antenna, which yields the following free space loss equation: Pt (4p ) (d ) (ld ) = (cd ) = = 2 Pr Gr Gt l Ar At f 2 Ar At 2 2 2 · Gt = gain of transmitting antenna · Gr = gain of receiving antenna · At = effective area of transmitting antenna · Ar = effective area of receiving antenna 2 The third fraction is derived from the second fraction using the relationship between antenna gain and eff effective area defined in Equation. We can recast the loss equation as LdB=20 log(λ)+ 20 log(d) log(d)-10 log(AtAr) = =-20 log(f)+ 20 log(d)-10 log(AtAr)+169.54 dB Thus, for the same antenna dimensions and separation, the longer the carrier wavelength (lower the carrier frequency), the higher is the free space path loss. It is interesting to compare Equations. Equation indicates that as the frequency increases, the free space loss also increas increases, es, which would suggest that at higher frequencies, losses become more burdensome. However, Equation shows that we can easily compensate for this increased loss with antenna gains. In fact, there is a net gain at higher frequencies, other factors remaining constant. Equation shows that at a fixed distance an increase in frequency results in an increased loss measured by 20log(f). However, if we take into account antenna gain, and fix antenna area, then the change in loss is measured by -20log(f) 20log(f) that is, there th is actually a decrease in loss at higher frequencies. Prepared By: LOKESH KUMAR ARYA Page 8 Wireless Communication 2.4 Line of sight microwave communication Fresnel Zone: If unobstructed, radio waves will travel in a straight line from the transmitter to the receiver. But if there are obstacles near the path, the radio waves reflecting off those objects may arrive out of phase with the signals that travel directly and reduce the power of the received signal. On the other hand, the reflection can enhance the power of the received signal if the reflection and the direct signals arrive in phase. Sometimes this results in the counterintuitive finding that reducing the height of an antenna increases the S+N/N ratio. Fresnel provided a means to calculate where the zones are where obstacles will cause mostly in phase and mostly out of phase reflections between the transmitter and the receiver. Obstacles in the first Fresnel will create signals that will be 0 to 90 degrees out of phase, in the second zone they will be 90 to 270 degrees out of phase, in third zone, they will be 270 to 450 degrees out of phase and so on. Odd numbered zones are constructive and even numbered zones are destructive. The concept of Fresnel zone clearance may be used to analyze interference by obstacles near the path of a radio beam. The first zone must be kept largely free from obstructions to avoid interfering with the radio reception. However, some obstruction of the Fresnel zones can often be tolerated, as a rule of thumb the maximum obstruction allowable is 40%, but the recommended obstruction is 20% or less. For establishing Fresnel zones, first determine the RF Line of Sight (RF LoS), which in simple terms is a straight line between the transmitting and receiving antennas. Now the zone surrounding the RF LoS is said to be the Fresnel zone. The general equation for calculating the Fresnel zone radius at any point P in the middle of the link is the following: where, Fn = The nth Fresnel Zone radius in metres d1 = The distance of P from one end in metres d2 = The distance of P from the other end in metres λ = The wavelength of the transmitted signal in metres Prepared By: LOKESH KUMAR ARYA Page 9 Wireless Communication Line of sight microwave communication Frequency Reuse Frequency reuse is a technique of reusing frequencies and channels within a communications system to improve capacity and spectral efficiency. Frequency reuse is one of the fundamental concepts on which comm commercial wireless systems are based that involves the partitioning of an RF radiating area (cell) into segments of a cell. One segment of the cell uses a frequency that is far enough away from the frequency in the bordering segment that it does not provide interference nterference problems. Frequency re re-use use in mobile cellular systems means that each cell has a frequency that is far enough away from the frequency in the bordering cell that it does not provide interference problems. The same frequency is used at least two cells apart from each other. This practice enables cellular providers to have many times more customers for a given site license. The key characteristic of a cellular network is the ability to re re-use use frequencies to increase both coverage and capacity. As described above, adjacent cells must use different frequencies, however there is no problem with two cells sufficiently far apart operating on the same frequency. The elements that determine frequency reuse are the reuse distance and the reuse factor. The reuse distance, D is calculated as where R is the cell radius and N is the number of cells per cluster. Cells may vary in radius in the ranges (1 km to 30 km). The boundaries of the cells can also overlap between adjacent cells and large cells can be divided into smaller cells. The frequency reuse factor is the rate at which which the same frequency can be used in the network. It is 1/K (or K according to some books) where K is the number of cells which cannot use the same frequencies for transmission. Prepared By: LOKESH KUMAR ARYA Page 10 Wireless Communication Line of sight microwave communication Common values for the frequency reuse factor are 1/3, 1/4, 1/7, 1/9 and 1/12 (or 3, 4, 7, 9 and 12 depending on notation). In case of N sector antennas on the same base station site, each with different direction, the base station site can serve N different sectors. N is typically 3. A reuse pattern of N/K denotes a further division in frequency among N sector antennas per site. Some current and historical reuse patterns are 3/7 (North American AMPS), 6/4 (Motorola NAMPS), and 3/4 (GSM). If the total available bandwidth is B, each cell can only use a number of frequency channels corresponding to a bandwidth of B/K, and each sector can use a bandwidth of B/NK. Code division multiple access-based systems use a wider frequency band to achieve the same rate of transmission as FDMA, but this is compensated for by the ability to use a frequency reuse factor of 1, for example using a reuse pattern of 1/1. In other words, adjacent base station sites use the same frequencies, and the different base stations and users are separated by codes rather than frequencies. While N is shown as 1 in this example, that does not mean the CDMA cell has only one sector, but rather that the entire cell bandwidth is also available to each sector individually. Depending on the size of the city, a taxi system may not have any frequency-reuse in its own city, but certainly in other nearby cities, the same frequency can be used. In a big city, on the other hand, frequency-reuse could certainly be in use. Recently also orthogonal frequency-division multiple access based systems such as LTE are being deployed with a frequency reuse of 1. Since such systems do not spread the signal across the frequency band, inter-cell radio resource management is important to coordinate resource allocation between different cell sites and to limit the inter-cell interference. There are various means of Inter-cell Interference Coordination (ICIC) already defined in the standard. Coordinated scheduling, multi-site MIMO or multi-site beam forming is other examples for inter-cell radio resource management that might be standardized in the future. Prepared By: LOKESH KUMAR ARYA Page 11