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EEE440 Modern Communication Systems

EEE440 Modern Communication Systems. Wireless and Mobile Communications. Introduction. What is wireless communication? What is mobile communication? Different types of mobile wireless systems Cellular Wireless LAN Wireless MAN Wireless PAN Mobile Ad-hoc network (MANET).

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EEE440 Modern Communication Systems

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  1. EEE440 Modern Communication Systems Wireless and Mobile Communications

  2. Introduction • What is wireless communication? • What is mobile communication? • Different types of mobile wireless systems • Cellular • Wireless LAN • Wireless MAN • Wireless PAN • Mobile Ad-hoc network (MANET)

  3. Characteristics of Mobile Wireless radio propagation 6 common effects • Propagation loss • Fading • Large Scale (Shadow fading) • Small scale (multipath fading) • Doppler shift • Frequency-selective fading • Time-selective fading

  4. Characteristics of Mobile Wireless radio propagation • In free space propagation, the power incident on a receiving antenna is given by the free space received power equation • In wireless environment where obstacles exist, the average power decrease with distance at a rate greater than d2 (usually 3 or 4) • This is commonly known as the propagation loss

  5. Characteristics of Mobile Wireless radio propagation • The actual power received over a relatively long distance will vary randomly about the average power • A good approximation reveals that the power measured in decibel follows a gaussian or normal distribution centred about its average value with some standard deviation ranging typically from 6 to 10dB. • The power probability distribution is commonly called a log-normal distribution • This is commonly referred to as shadow fading

  6. Characteristics of Mobile Wireless radio propagation • The actual power received over a much smaller distance also vary considerably due to the destructive/constructive interference of multiple signals that follow multiple paths • This is commonly referred to as multipath fading

  7. Characteristics of Mobile Wireless radio propagation • The three effects can be modelled by the following equation Multipath fading Propagation loss Shadow fading

  8. Characteristics of Mobile Wireless radio propagation • Terminal mobility with respective to the incoming wave introduces a frequency shift called Doppler shift • Signal fades due to the movement of the terminal

  9. Characteristics of Mobile Wireless radio propagation • The effect of multipath fading depends of the signal bandwidth • For a relatively large bandwidth, different frequency components of the signal being handled differently over the propagation path leading to signal distortion called frequency selective fading • This is manifested in inter-symbol interference (ISI) due to successive digital symbols overlap into adjacent symbol intervals • For narrower signal bandwidth, non-selective of flat fading occur

  10. Characteristics of Mobile Wireless radio propagation • Time selective fading occurs when the channel changes its characteristics during the transmission of the signal • The change in the channel characteristics is proportional to the receiver mobility

  11. Propagation loss • The average power measured at the receiver at a distance d from the transmitter is given by • g(d) represents the path loss with the general expression

  12. Propagation loss • There are many models for the path loss • A common two-ray model is most often used

  13. Propagation loss: Two-ray model • The two-ray model is the simplest representation that models the effect on the average received power of multiple rays due to reflection, diffraction and scattering • It treats the case of a single reflected ray • Provides reasonably accurate results in macrocellular environment with relatively high BS antenna and/or L.O.S conditions • Assumes that the signal arrives directly through a L.O.S path and indirectly through perfect relfection from a flat ground surface

  14. Propagation loss: Two-ray model

  15. Propagation loss: Two-ray model • The reflected signal shows up with a delay relative to the direct path signal and as a consequence, may add constructively (in phase) or destructively (out of phase) • Propagation starts out with an R2 falloff rate and then transitions to a R4 falloff rate at greater ranges. • The "point" where this transition occurs is often called the Fresnel breakpoint. • The nulls are representative of points where direct and reflected signals cancel while the humps show points where signals add. • In practice, ground reflections are usually somewhat diffuse (rough mirror instead of polished) and so the sharp nulls get filled in.  • In macrocellular communications systems, operating distances are usually large enough so that signal strength can be thought of as falling off at an R4 rate.

  16. Propagation loss: Other models • Various measurement based empirical laws have been developed to estimate median path loss for moderate to large macrocells based on frequency, building environment, and antenna heights; most notably the Okumura & Hata models • Generally describes median path loss as Median Path Loss (dB) = A + B log10( R ) where: A is median path loss in dB at 1km B is the rate at which median signal strength falls off (10 times ) R is range in km

  17. Propagation loss: Other models • Free space A&B values at 1900 MHz are A=98 dB and B=20 consistent with an R2 signal strength fall-off rate • In dense residential areas with a 100' basestation antenna height, model indicates A&B values of A=132 dB and B=38 for an R3.8 signal strength fall-off rate.

  18. Propagation loss: Microcells • For microcells, some investigators use Where n1 , n2 are two separate integers db is a measured breakpoint

  19. Propagation loss: Okumura model • One of the most common model used for signal prediction in large urban macrocells • Applicable over distance of 1-100km and frequency ranges of 150-1500 MHz • Measurement made in Tokyo using base station heights between 30-100m • The empirical propagation loss formula at distance d parameterised by the carrier frequency fc is given by

  20. Propagation loss: Hata model

  21. Propagation loss: Hata model

  22. Shadow fading • Long term shadow fading due to variations in radio signal power due to encounters with terrain obstructions such as hills or manmade structures such as buildings • The measured signal power differ substantially at different locations even though at the same radial distance from a transmitter • Represents the medium scale fluctuations of the radio signal strength over distances from tens to hundreds of meters

  23. Shadow Fading • Many empirical studies demonstrate that the received mean power fluctuates about the average power with a log-normal distribution • Can be well-approximated by a gaussian random variable with standard deviation, δ

  24. Shadow fading Consider the signal power equation in dB. The shadow-fading random variable x, measured in dB is taken to be a zero-mean gaussian random variable with variance δ2

  25. Shadow fading • Ignoring the multipath effect, α • The term pdB is the local-mean power modelled as a gaussian random variable with average value • The pdf for pdB is

  26. Shadow fading • Typical value of δ range from 6 to 10dB

  27. Multipath fading • The actual power received over a much smaller distance vary considerably due to the destructive/constructive interference of multiple signals that follow multiple paths to the receiver • The direct ray is actually made up of many rays due to scattering multiple times by obstructions along its path, all travelling about the same distance • Each of these rays appearing at the receiver will differ randomly in amplitude and phase due to the scattering

  28. Multipath fading

  29. Multipath fading • It is found that the multipath can be modelled by using the Rayleigh/Ricean statistics • With Rayleigh statistics, the pdf of the random variable α is given by

  30. Multipath fading

  31. Multipath fading • Rayleigh fading is viewed as a reasonable model for urban environments where there are many objects in the environment that scatter the radio signal before it arrives at the receiver • there is no dominant propagation along a LOS between the transmitter and receiver. • The central limit theorem holds that, if there is sufficiently much scatter, the channel impulse response will be well-modelled as a Gaussian process irrespective of the distribution of the individual components • such a process will have zero mean and phase evenly distributted between 0 and 2π radians. • The envelope of the channel response will therefore be Rayleigh distributed

  32. Multipath fading

  33. Multipath fading • If the environment is such that, in addition to the scattering, there is a strongly dominant signal seen at the receiver, usually caused by a LOS, then the mean of the random process will no longer be zero, varying instead around the power-level of the dominant path. • Such a situation may be better modelled as Rician fading.

  34. Multipath fading

  35. Doppler shift • How rapidly the channel fades will be affected by how fast the receiver and/or transmitter are moving • Motion causes Doppler shift in the received signal components • the change in frequency of a wave for a receiver moving relative to the transmitter

  36. Doppler shift • Say a mobile phone moving at velocity v km/hr in the x direction and the radio wave impinging on the receiver at an angle βk • The motion introduces a Doppler frequency shift, fk = vcos βk/λ • If the ray is directed opposite to the mobile’s motion (β=0), then fk=v/λ • The frequency of the signal has increased by the Doppler spread, fk

  37. Frequency selective fading • The effect of multipath fading on the reception of signals depends on the signal bandwidth • For relatively large bandwidth, different parts of the transmitted signal spectrum are attenuated differently, • This is manifested in the inter-symbol interference (ISI) • For narrower bandwidth signals, non-selective of flat fading occur • The delay spread, is the variation in the propagation delays of multiple scattered rays • Digital symbol intervals, Ts smaller than 5 or 6 times the delay spread,ds give rise to frequency selective fading (Ts <2πds) • Typical values of delay spread are 0.2µs (rural area), 0.5µs (suburban area), 3-8µs (urban area), <2 µs (urban microcell) and 50-300ns (indoor picocell)

  38. Frequency selective fading • Slow or fast fading depends on the coherence time, Tc • Coherence time is the measure of period over which the fading process is correllated • Tc is related to the delayspread, Tc=1/ds • The fading is said to be slow if the symbol duration, Ts is smaller than the coherence time. • Frequency selective fading occur when the signal bandwidth exceeds the coherence bandwidth, B=1/2πds

  39. Time selective fading • Occurs when the channel changes its characteristics during signal transmission • Directly proportional to receiver mobility • Receiver mobility causes the signal to change rapidly enough in comparison with the coherence time, Tc=0.18/fk • The Doppler effect leads to time selective fading • However, if the signal itself changes rapidly enough with respect to the reciprocal of the Doppler frequency spread, fk , distortion will not happen • There is a minimum bandwidth beyond which the time selective fading can be eliminated • Ts > Tc.9/(16πfk)

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