Wireless NETWORKS NET 434 Topic # 3 Wireless Transmission and Channel Figure 4-6 Sine Wave WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998 Figure 4-8 WCB/McGraw-Hill Amplitude Change The McGraw-Hill Companies, Inc., 1998 Figure 4-9 Frequency Change WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998 Figure 4-10 WCB/McGraw-Hill Phase Change The McGraw-Hill Companies, Inc., 1998 Figure 4-11 Time and Frequency Domain WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998 Figure 4-12 WCB/McGraw-Hill Examples The McGraw-Hill Companies, Inc., 1998 Figure 4-13 WCB/McGraw-Hill Signal with DC Component The McGraw-Hill Companies, Inc., 1998 Figure 4-14 WCB/McGraw-Hill Complex Waveform The McGraw-Hill Companies, Inc., 1998 Figure 4-15 Bandwidth WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998 Figure 4-16 WCB/McGraw-Hill Digital Signal The McGraw-Hill Companies, Inc., 1998 Figure 4-17 Amplitude, Period, and Phase for a Digital Signal WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998 Figure 4-18 WCB/McGraw-Hill Bit Rate and Bit Interval The McGraw-Hill Companies, Inc., 1998 Figure 4-19 Harmonics of a Digital Signal WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998 Figure 4-20 Exact and Significant Spectrums WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998 Figure 4-22 Corruption Due to Insufficient Bandwidth WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998 Figure 4-23 WCB/McGraw-Hill Bandwidth and Data Rate The McGraw-Hill Companies, Inc., 1998 Relationship between Data Rate and Bandwidth › Nyquist theorem – In information theory, Nyquist theorem tells the maximum rate at which information can be transmitted over a communications channel of a specified bandwidth in the absence of noise. Theoretical maximum data rate over a channel of Bandwidth B is given by 𝐶 = 2𝐵𝑙𝑜𝑔2 (𝑀) (bits/sec) – Where 𝐵 is the bandwidth of the channel. M is the number of discrete signal or voltage levels. › Shannon’s Capacity – In information theory, shannon theorem tells the maximum rate at which information can be transmitted over a communications channel of a specified bandwidth in the presence of noise. 𝐶 = 𝐵𝑙𝑜𝑔2 (1 + 𝑆𝑁𝑅) (bits/sec) – Where 𝐵 is the bandwidth of the channel. M is the number of discrete signal or voltage levels. – Where 𝑆𝑁𝑅 is given by 𝑑𝐵, 𝑑𝐵𝑚 – The decibel (dB) measures the relative strengths of two signals or one signal at two different points. – The (dB) is negative if a signal is attenuated and positive if the signal is amplified. 𝑑𝐵 = 10𝑙𝑜𝑔10 (𝑃2 /𝑃1 ) – The dB tells if power is lost or gained. – Signal to noise (decibels) 1 dB = 10 log10 S/N. – Ex: S/N = 10 => 10 dB; S/N =100 => 20 dB,1000 is 30dB – Decibel is used to measure signal power in milli watt. In this case it is referred to as 𝑑𝐵𝑚 . 𝑑𝐵𝑚 = 10𝑙𝑜𝑔10 (𝑃𝑚 ) – Example: › -3dB+7dB-3dB=1dB. › There is an overall gain in the system The expression 𝐸𝑏 /𝑁𝑜 › 𝐸𝑏 𝑁𝑜 – The expression 𝐸𝑏 𝑁𝑜 is the equivalent of 𝑆𝑁𝑅 for digital communication. The parameter is the ratio of signal energy per bit to noise power density per Hertz. – 𝑅 is the bit rate. – 𝐸𝑏 , the energy per bit can be represented 𝑆𝑇𝑏 . Where 𝑆 is the signal power and 𝑇𝑏 is the time to send one bit. Watt=1J/s. 𝑅 = 1/𝑇𝑏 . 𝐸𝑏 𝑆/𝑅 = , 𝑁𝑜 𝑁𝑜 – The metric of performance in digital communication systems is a 𝐸 plot of the bit error probability (𝑃𝑏 ) versus 𝑏 . The graph is a 𝑁𝑜 waterfall curve. The expression 𝐸𝑏 /𝑁𝑜 – The ratio 𝐸𝑏 𝑁𝑜 is important because the bit error rate is a decreasing function of this ratio. – – System describes a permissible probability of error. – There is a given 𝐸𝑏 𝑁𝑜 for a given probability of error that has to be achieved. – As the bit rate 𝑅 increases, the signal power must increase or the bandwidth of the channel W must be increased to maintain the same 𝐸𝑏 . 𝑁𝑜 Transmission impairments › With any communications system, the received signal may differ from the signal that is transmitted due to various transmission impairments. – For analog signals, these impairments can degrade the signal quality. For digital signals, bit errors may be introduced, such that a binary 1 is transformed into a binary 0 or vice versa. Transmission impairments › Common types of transmission impairments: – Attenuation – Delay distortion – Noise › Attenuation – The strength of a signal falls off with distance over any transmission medium. – For guided media, this reduction in strength, or attenuation, is generally exponential and thus is typically expressed as a constant number of decibels per unit distance. For twisted-pair and coaxial cable, loss varies exponentially with distance (linear in decibels). Transmission impairments – For microwave (and radio frequencies), the loss is expressed as 4𝜋𝑑 2 𝐿 = 10𝑙𝑜𝑔10 ( ) λ – where d is the distance and λ is the wavelength, in the same units. Loss varies as the square of the distance. – Repeaters or amplifiers may be placed farther apart for microwave systems—10 to 100 km is typical. – Attenuation introduces three considerations for the transmission engineer. › First, a received signal must have sufficient strength so that the electronic circuitry in the receiver can detect the signal. › Second, the signal must maintain a level sufficiently higher than noise to be received without error. › Third, attenuation varies with frequency. – The first and second problems are dealt with by attention to signal strength and the use of amplifiers or repeaters. – For a point-to-point link, the signal strength of the transmitter must Transmission impairments › Distortion Delay spread – Delay distortion occurs because the velocity of propagation of a signal through a guided medium varies with frequency. – For a band-limited signal, the velocity tends to be highest near the center frequency and fall off toward the two edges of the band. – Thus various frequency components of a signal will arrive at the receiver at different times, resulting in phase shifts between the different frequencies. – This effect is referred to as delay distortion because the received signal is distorted due to varying delays experienced at its constituent frequencies. Transmission impairments – Delay distortion (Inter symbol interference) is particularly critical for digital data. – Consider that a sequence of bits is being transmitted. Because of delay distortion, some of the signal components of one bit position will spill over into other bit positions, causing inter symbol interference, which is a major limitation to maximum bit rate over a transmission channel. › Noise – For any data transmission event, the received signal will consist of the transmitted signal, modified by the various distortions imposed by the transmission system, plus additional unwanted signals that are inserted somewhere between transmission and reception. – Undesired signals are referred to as noise. – Noise is the major limiting factor in communications system performance. › Noise may be divided into four categories: Transmission impairments – Crosstalk – Impulse noise › Thermal noise – Thermal agitation of electrons. It is present in all electronic devices and transmission media and is a function of temperature. – Thermal noise is uniformly distributed across the bandwidths typically used in communications systems and hence is often referred to as white noise. – Thermal noise cannot be eliminated and therefore places an upper bound on communications system performance. – Because of the weakness of the signal received by satellite earth stations, thermal noise is particularly significant for satellite communication. – The amount of thermal noise to be found in a bandwidth of 1 Hz in any device or conductor is 𝑊𝑎𝑡𝑡 𝑁𝑜𝑖𝑠𝑒 𝑃𝑜𝑤𝑒𝑟 𝐷𝑒𝑛𝑠𝑖𝑡𝑦 𝑁𝑜 = 𝐾𝑇( ) 𝐻𝑧 Transmission impairments – The noise is assumed to be independent of frequency. Thus the thermal noise in watts present in a bandwidth of B Hertz can be expressed as: N=KTB. › Intermodulation noise – Intermodulation noise arises when signals at different frequencies share the same transmission medium. – The effect of intermodulation noise is to produce signals at a frequency that is the sum or difference of the two original frequencies or multiples of those frequencies. – For example, the mixing of signals at frequencies and might produce energy at the frequency This derived signal could interfere with an intended signal at the frequency. › Cross talk – It is an unwanted coupling between signal paths. It can occur by electrical coupling between nearby twisted pairs or, rarely, coax cable lines carrying multiple signals. Transmission impairments – Crosstalk can also occur when microwave antennas pick up unwanted signals; although highly directional antennas are used. – microwave energy does spread during propagation. – Typically, crosstalk is of the same order of magnitude as, or less than, thermal noise. › Impulse Noise – Impulse noise, non-continuous, consisting of irregular pulses or noise spikes of short duration and of relatively high amplitude. – Causes, including external electromagnetic disturbances, such as lightning, and faults and flaws in the communications system. – Impulse noise is a minor annoyance for analog data. Example, voice transmission may be corrupted by short clicks and crackles with no loss of intelligibility. – Impulse noise the primary source of error in digital data communication. Example, a sharp spike of energy of 0.01 s duration would not destroy any voice data but would wash out about 560 bits of digital data being transmitted at 56 kbps. Transmission Impairments Wireless Line of sight › Impairments specific to wireless line-of-sight transmission – Free Space Loss – Atmospheric Absorption – Multipath – Refraction › Free Space Loss – For any type of wireless communication the signal disperses with distance. Antenna with a fixed area farther away from the antenna will receive less power. – 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. – Transmission Impairments Wireless Line of sight – Free space loss is expressed as a ratio of the power transmitted from the antenna 𝑃𝑡 to the power received at the receiving antenna 𝑃𝑟 . For an isotropic antenna it is given by the following expression: – For an antenna with a gain Transmission impairments › Interference – Another source of impairment is interference. With the growing popularity of microwave, transmission areas overlap and interference is always a danger. Thus the assignment of frequency bands is strictly regulated. › Atmospheric Absorption – An additional loss between the transmitting and receiving antennas is atmospheric absorption. Water vapor and oxygen contribute most to attenuation. – Attenuation is increased with rainfall. The effects of rainfall become especially noticeable above 10 GHz. – At frequencies below 15 GHz, the attenuation is less. – Rain and fog (suspended water droplets) cause scattering of radio waves that results in attenuation. In this context, the term scattering refers to the production of waves of changed direction or frequency when radio waves encounter matter. – This can be a major cause of signal loss. Thus, in areas of Transmission impairments › Multipath – For wireless (Satellite/Point to point microwave), there is a relatively free choice of where antennas are to be located, they can be placed so that if there are no nearby interfering obstacles, there is a direct line-of-sight path from transmitter to receiver. – Mobile telephony, there are obstacles in abundance. – The signal reflected by obstacles resulting in multiple copies of the signal with varying delays. Extreme cases, there may be no direct – The composite signal can be either larger or smaller than the direct signal.(depending on diff in the path lengths of the direct and reflected waves) WIRELESS PROPAGATION › Wireless Propagation Modes – A signal radiated from an antenna travels along one of three routes: › Ground wave › Sky wave, › Line of sight (LOS). – Ground Wave Propagation › Ground wave propagation follows the contour of the earth. › This effect is found in frequencies up to about 2 MHz. › Electromagnetic waves in this frequency range are scattered by the atmosphere and they do not penetrate the upper atmosphere. › The best-known example of ground wave communication is AM radio. Ground Wave Propagation Sky Wave Propagation Line of Sight Propagation WIRELESS PROPAGATION – Sky Wave propagation › Sky wave propagation, a signal from an earth-based antenna is reflected from the ionized layer of the upper atmosphere (ionosphere) back down to earth due to refraction. › A sky wave signal can travel through a number of hops, bouncing back and forth between the ionosphere and the earth’s surface. › This effect is found in frequencies from 2-30 MHz › With this propagation mode, a signal can be picked up thousands of kilometers from the transmitter. › Amateur radio, BBC world service, Voice of America – Line of Sight propagation › Mode of propagation above above 30 MHz, › Neither ground wave nor sky wave propagation modes operate, › Communication must be by line of sight. › For satellite communication, a signal above 30 MHz is not reflected by the ionosphere and therefore a signal can be WIRELESS PROPAGATION – For ground-based communication, the transmitting and receiving antennas must be within an effective line of sight of each other. › Optical and Radio Line of Sight – With no intervening obstacles, the optical line of sight can be expressed as 𝑑 = 3.57 ℎ – where d is the distance between an antenna and the horizon in kilometers and h is the antenna height in meters. The effective, or radio, line of sight to the horizon is expressed by 𝑑 = 3.57 𝐾ℎ – where K is an adjustment factor generally taken as 𝐾 = 4/3 to account for the refraction. – The maximum distance between two antennas for LOS propagation is – 3.57 𝐾ℎ1 +3.57 𝐾ℎ2 – where ℎ1 and ℎ2 and are the heights of the two antennas. Optical and Radio Horizons Refraction › Velocity of electromagnetic wave is a function of density of material – ~3 x 108 m/s in vacuum, less in anything else › As wave moves from one medium to another, its speed changes – Causes bending of direction of wave at boundary – Towards more dense medium – Causes sudden change of direction at transition between media – May cause gradual bending if medium density is varying – Density of atmosphere decreases with height – Results in bending towards earth of radio waves