Wireless communication

2. • Unit I Wireless Channels:
• Large scale path loss – Path loss models: Free Space and Two-Ray models -
Link Budget design – Small scale fading- Parameters of mobile multipath
channels – Time dispersion parameters- Coherence bandwidth – Doppler
spread & Coherence time, Fading due to Multipath time delay spread – flat
fading – frequency selective fading – Fading due to Doppler spread – fast
fading – slow fading.

3. WHAT IS WIRELESS COMMUNICATION
• Wireless communications is the transmission of voice and data without
cable or wires
• Examples of Wireless Devices
• Cordless phones are wireless devices, as are TV remote controls, radios, and
GPS systems. Other wireless devices include phones, tablets, Bluetooth mice
and keyboards, wireless routers, and most devices that don't use wires to
transmit information.

4. Large scale path loss
• Path loss, which measures the loss of energy of a wave propagating between
the transmitter and the receiver, is the main parameter in the design of
wireless networks.
• Propagation models are focused on predicting the average received signal
strength at a given distance from the transmitter, as well as the variability of
the signal strength in close spatial proximity to a particular location.

5. • Propagation models that predict the mean signal strength for an arbitrary
transmitter-receiver (TR) separation distance are useful in estimating the
radio coverage area of a transmitter and are called large-scale propagation
models.

6. • As the mobile moves away from the transmitter over much larger distances,
the local average received signal will gradually decrease, and it is this local
average signal level that is predicted by large-scale propagation models.
Typically, the local average received power is computed by averaging signal
measurements over a measurement track of 5ߣ to 40 ߣ.

7. Free-Space Propagation Model
• Free space propagation model is used to predict the received signal strength
when transmitter and receiver have clear, unobstructed Line Of Sight path
between them.
• The free space propagation model assumes a transmit antenna and a receive
antenna to be located in an otherwise empty environment. Neither absorbing
obstacles nor reflecting surfaces are considered. In particular, the influence
of the earth surface is assumed to be entirely absent.

8. • In free space radio signals propagate as light does i.e., they follow a straight
line. If such a straight line exists between a sender and a receiver it is called
line-of-sight (LOS).Even if no matter exists between the sender and the
receiver, the signal still experiences the free space loss. The received power Pr
is proportional to 1/d2 with d being the distance between sender and
receiver (inverse square ).

9. • The received power decays as a function of T-R separation distance raised to
some power. Path Loss: Signal attenuation as a positive quantity measured in
dB and defined as the difference (in dB) between the effective transmitted
power and received power.
• Free space power received by a receiver antenna separated from a radiating
transmitter antenna by a distance d, is given by Friis free space equation:

10. • Free Space Model:
• The free space model predicts that received power decays as a function of
the T-R separation
• The free space power received by a receiver antenna which is separated from
a radiating transmitter antenna by a distance d, is given by the Friis free space
equation:

13. Two Ray Ground Reflection Model
• Two ray model considers both the direct path and a ground reflected
propagated path between transmitter and receiver.

15. • A two-ray model, which consists of two overlapping waves at the receiver,
one direct path and one reflected wave from the ground.
• The total received E-field ETOT is the result of the direct line of sight
component ELOS and the ground reflected component Eg.
• Referring to Figure (above),ht is the height of the transmitter and hr is the
height of the receiver. If E0is the free space electric field (in V/m) at a
reference distance d0 from the transmitter then for d>d0,

16. • In wireless communication, the two-ray ground reflection model is a
simplified propagation model often used to analyze signal propagation in
outdoor environments. This model assumes that a transmitted signal
propagates directly from the transmitter to the receiver, as well as via a
ground-reflected path. Here's a breakdown of the key components and
assumptions of the two-ray model:

17. • Reflected Path: In addition to the direct path, the signal also travels from the transmitter to
the receiver after reflecting off the ground surface. This path represents the ground-
reflected component of the signal.
• Assumptions:
• Perfect reflection: The ground surface is assumed to perfectly reflect the signal, meaning no absorption
or scattering occurs.
• Flat Earth assumption: The model assumes a flat Earth, which is a simplification suitable for short-
range communication or when the curvature of the Earth is negligible over the propagation distance.
• No obstacles: The model assumes no obstacles (e.g., buildings, trees) between the transmitter and the
receiver that could block or scatter the signal.

18. • Propagation Loss: The received signal power is the combination of the
powers received along both the direct and reflected paths. Due to the longer
path length of the reflected path, it generally experiences greater attenuation
compared to the direct path.
• Received Signal Strength: The received signal strength at the receiver is the
sum of the powers of the direct and reflected paths, which can be expressed
mathematically as:

19. Pr​=Pt(d2GtGr​​+(2d+h)2GtGr)
Where:
•��Pr is the received power.
•��Pt is the transmitted power.
•��Gt and ��Gr are the gains of the transmitter and
receiver antennas, respectively.
•�d is the direct path distance.
•ℎh is the height of the antenna above the ground.

20. • Diversity Gain: One advantage of the two-ray model is that it provides
diversity gain by utilizing both the direct and reflected paths. This can
improve the reliability of the communication link, especially in scenarios
where one of the paths experiences fading or attenuation.
• Overall, while the two-ray ground reflection model is a simplified
representation of real-world propagation, it provides valuable insights into
the behavior of wireless signals in outdoor environments, particularly in
scenarios with clear line-of-sight paths and flat terrain.

21. Link Budget design
• A link budget is a systematic way of accounting for all of the gains and
losses in a telecommunication system. It is crucial in the design of wireless
communication systems to ensure reliable and efficient communication.
Here's how you can design a link budget:

22. • Define System Requirements: Understand the requirements of your communication system, including the
desired coverage area, data rate, modulation scheme, frequency band, and quality of service (QoS)
parameters.
• Identify Key Parameters:
• Transmitter Parameters: Transmitted power (��Pt), antenna gain (��Gt), and any losses in the
transmitter chain.
• Propagation Losses: Account for free-space path loss, terrain losses, and atmospheric losses based on the
operating frequency and environment.
• Receiver Parameters: Receiver sensitivity (�minSmin), receiver noise figure (��NF), and antenna gain
(��Gr).
• Other Losses: Cable losses, connector losses, and miscellaneous losses.

23. • Calculate Path Loss:
• Use appropriate path loss models such as the Friis transmission equation or
empirical models like the Okumura-Hata or COST231 models depending on
the operating environment (urban, suburban, rural, etc.).
• Consider factors like distance, frequency, antenna heights, and environmental
conditions.

24. • Transmitter (Tx) and Receiver (Rx): The communication system consists
of a transmitter and a receiver. The transmitter sends out electromagnetic
waves, and the receiver picks up these waves.
• Direct Path: The signal travels directly from the transmitter to the receiver
without reflection. This path represents the line-of-sight (LOS) component
of the signal.

25. • Calculate Transmit Power: Determine the required transmit power based
on the path loss and receiver sensitivity. Use the link margin to account for
fading, shadowing, and other uncertainties.

26. SMALL SCALE FADING
• In wireless communication, fading is a phenomenon in which the strength and
quality of a radio signal fluctuate over time and distance. Fading is caused by a
variety of factors, including multipath propagation, atmospheric conditions, and the
movement of objects in the transmission path.
• Fading can have a significant impact on the performance of wireless
communication systems, particularly those that operate in high-frequency bands.
•

28. • Small Scale Fading
• Small-scale fading is a common issue in wireless communication.
• It happens when a signal is transmitted from a transmitter to a receiver and it
experiences multiple signal paths due to reflection, diffraction, and scattering
from objects in the environment.
• These signal paths can cause interference and distortion to the signal,
resulting in fluctuations of the signal strength at the receiver.

29. • Small-scale fading is called “small-scale” because the variations occur over short
distances, such as a few centimeters to a few meters.
• Small-scale fading can happen very quickly, sometimes in microseconds or less.
• It is primarily caused by the multipath propagation of the signal.
• Overall, small-scale fading is a common issue in wireless communication that affects
the quality of the received signal. However, with proper mitigation techniques, it is
possible to maintain reliable communication even in the presence of small-scale
fading.

30. Multipath delay spread
• Multipath delay spread is a type of small-scale fading that occurs when a transmitted
signal takes multiple paths to reach the receiver.
• The different components of the signal can arrive at the receiver at different times,
causing interference and rapid variations in signal amplitude and phase.
• Multipath delay spread can cause Inter-Symbol Interference (ISI), where symbols in
the transmitted signal overlap and interfere with each other, leading to errors in the
received signal.
•

31. • The root means square (RMS) delay spread is a measure of the dispersion of
the signal and determines the frequency-selective characteristics of the
channel.
• A higher RMS delay spread indicates a more frequency-selective channel,
while a lower RMS delay spread indicates a flatter, more frequency-invariant
channel.
• Multipath delay spread can be mitigated by using techniques such as
equalization, diversity, and adaptive modulation.

32. • Equalization techniques are used to compensate for the time dispersion
caused by multipath delay spread.
• Diversity techniques are used to combine multiple signal paths to mitigate
the effects of fading.
• Adaptive modulation techniques are used to adjust the modulation scheme
and data rate based on the channel conditions, allowing the system to adapt
to changes in the channel and maintain a reliable communication link.

33. Doppler Spread
• Doppler spread is a type of small-scale fading that occurs when there is
relative motion between the transmitter and the receiver.
• The relative motion causes a shift in the frequency of the transmitted signal,
known as the Doppler shift.
• The Doppler shift causes different frequency components of the signal to
arrive at the receiver with different phases and amplitudes.

34. • This results in rapid variations in signal amplitude and phase, which can
cause fading and errors in the received signal.
• The Doppler spread is a measure of the rate of change of the Doppler shift
and determines the time-varying characteristics of the channel.
• A higher Doppler spread indicates a faster time variation in the channel,
while a lower Doppler spread indicates a slower time variation.

35. • Doppler spread can be mitigated by using techniques such as equalization,
diversity, and adaptive modulation.
• Equalization techniques are used to compensate for the time dispersion
caused by Doppler spread.
• Diversity techniques are used to combine multiple signal paths to mitigate
the effects of fading.

36. • Adaptive modulation techniques are used to adjust the modulation scheme
and data rate based on the channel conditions, allowing the system to adapt
to changes in the channel and maintain a reliable communication link.
• Doppler spread is an important consideration in the design of wireless
communication systems, particularly for mobile applications where there is
often significant relative motion between the transmitter and the receiver.

37. PARAMETERS OF MOBILE MULTIPATH
CHANNELS
• Mobile multipath channels are characterized by several parameters that
describe the behavior of signals as they propagate through the wireless
medium. Some of the key parameters include:
• Time dispersion parameters The time dispersive properties of wide band
multipath channels are most commonly quantified by their mean excess delay
and rms delay spread. The mean excess delay is the first moment of the
power delay profile and is defined as

40. • These delays are measured relative to the first detectable signal arriving at the
receiver at τo = 0. Typical values of rms delay spread are on the order of
microseconds in outdoor mobile radio channels and on the order of
nanoseconds in indoor radio channels.
• Note that the rms delay spread and mean excess delay are defined from a
single power delay profile which is the temporal or spatial average of
consecutive impulse response

41. • measurements collected and averaged over a local area. Typically, many
measurements are made at many local areas in order to determine a statistical
range of multipath channel parameters for a mobile communication system
over a large-scale area .
• The maximum excess delay (X dB) of the power delay profile is defined to
be the time delay during which multipath energy falls to X dB below the
maximum.

42. COHERENCE BANDWIDTH
• It is a measure of the range of frequencies over which the channel can be
considered flat(i.e. channel passes all spectral components with equal gain
and linear phase).
• It is a definition that depends on RMS Delay Spread. Two sinusoids with
frequency separation greater than Bc are affected quite differently by the
channel. If we define Coherence(consistency) Bandwidth (BC) as the range
of frequencies over which the frequency correlation is above 0.9, then

44. • Coherence time is the time duration over which the channel impulse
response is essentially invariant. If the symbol period of the baseband signal
(reciprocal of the baseband signal bandwidth) is greater the coherence time,
then the signal will distort(alter), since channel will change during the
transmission of the signal.

46. TIME DISPERSION PARAMETERS
• In wireless communication systems, time dispersion refers to the
phenomenon where a transmitted signal spreads out over time due to various
factors such as multipath propagation, reflections, and scattering in the
transmission medium.
• Time dispersion can cause inter-symbol interference (ISI), where symbols
from one bit period interfere with symbols from adjacent bit periods, leading
to errors in data transmission. To mitigate the effects of time dispersion,
various parameters are considered:

47. • Delay Spread: Delay spread is a measure of the time difference between the arrival
of the earliest and latest significant signal components at the receiver. It
characterizes the spread of signal energy over time due to multipath propagation.
Large delay spreads increase the likelihood of ISI and can degrade system
performance.
• Coherence Bandwidth: Coherence bandwidth is the range of frequencies over
which the channel response remains relatively constant. It is inversely related to the
delay spread. A wider coherence bandwidth implies a shorter delay spread and vice
versa. Designing communication systems to operate within the coherence
bandwidth helps mitigate the effects of time dispersion.

48. • Channel Correlation Time: Channel correlation time is the time duration over
which the channel response remains correlated. It is related to the coherence time
of the channel. Short channel correlation times imply fast fading channels, while
long correlation times imply slow fading channels. Understanding the channel
correlation time helps in adapting modulation and coding schemes to suit the
channel conditions.
• Doppler Spread: Doppler spread is caused by the relative motion between the
transmitter, receiver, and scatterers in the environment. It results in the spreading of
the signal spectrum due to the Doppler effect. Doppler spread is proportional to
the relative velocity between the transmitter and receiver and can cause frequency-
selective fading, which is closely related to time dispersion.

49. • Rake Receiver: A rake receiver is a technique used to combat the effects of
multipath propagation by combining multiple delayed replicas of the
received signal. Each replica corresponds to a different propagation path. By
combining these replicas, the rake receiver can mitigate the effects of time
dispersion and improve the reliability of communication

50. DOPPLER SPREAD & COHERENCE
TIME
• Doppler Spread:
• Doppler spread refers to the frequency spread caused by the relative motion
between the transmitter, receiver, and scatterers in the propagation environment. As
a mobile terminal moves, the relative velocity between the transmitter and receiver
causes a frequency shift in the received signal due to the Doppler effect. This shift
results in a spread of frequencies, known as the Doppler spread.
•

52. • Effect on Communication: Doppler spread causes frequency-selective
fading, where different frequency components of the signal experience
varying levels of attenuation and phase shifts. This can lead to distortion and
degradation of the received signal, particularly in high-mobility scenarios.

53. • Coherence Time:
• Coherence time is the duration over which the channel response remains
relatively constant. It is inversely proportional to the Doppler spread. In
other words, higher Doppler spreads result in shorter coherence times, and
vice versa.

55. • Effect on Communication: Coherence time is a critical parameter for designing
communication systems, especially for mobile applications. Understanding the
coherence time helps in selecting appropriate modulation schemes, coding
techniques, and channel estimation methods to adapt to the varying channel
conditions caused by mobility-induced Doppler spread.
• In summary, Doppler spread and coherence time are closely related parameters that
characterize the effects of mobility on wireless communication channels. Doppler
spread introduces frequency variations due to relative motion, while coherence time
quantifies the duration over which the channel remains predictable. These
parameters are essential for designing robust and reliable mobile communication
systems.

56. FADING DUE TO MULTIPATH TIME
DELAY SPREAD
• Multipath propagation, an inherent feature of a mobile communications
channel, results in a received signal that is dispersed in time. Each path has its
own delay and the time dispersion leads to a form of intersymbol
interference.
• Delay spread is a measure of the multipath profile of a mobile
communications channel. It is generally defined as the difference between
the time of arrival of the earliest component (e.g., the line-of-sight wave if
there exists) and the time of arrival of the latest multipath component.

57. • Delay spread is a random variable, and the standard deviation is a common
metric to measure it. This measure is widely known as the root-mean-
square delay spread στ.

58. FLAT FADING
• The wireless channel is said to be flat fading if it has constant gain and linear
phase response over a bandwidth which is greater than the bandwidth of the
transmitted signal.
• In this type of fading all the frequency components of the received signal
fluctuate in same proportions simultaneously. It is also known as non-
selective fading.
• The effect of flat fading is seen as decrease in SNR. These flat fading
channels are known as amplitude varying channels or narrowband channels.

59. • Flat fading is typically encountered in environments where there are minimal
multipath effects, such as in free space or over short distances where the
signal encounters few obstacles. In such scenarios, the received signal can be
characterized by a single complex gain, making the channel relatively easy to
equalize.
• Flat fading simplifies the design of communication systems because it allows
for the use of simple equalization techniques, such as zero-forcing
equalization or matched filtering, to mitigate the effects of channel distortion

60. FREQUENCY SELECTIVE FADING
• Frequency-selective fading, also known as frequency-selective or dispersive
fading, is a phenomenon in wireless communication systems where different
frequency components of a transmitted signal experience varying levels of
attenuation and phase shift as they propagate through the communication
channel.
• This variation in attenuation and phase across the frequency spectrum results
from multipath propagation, where multiple delayed versions of the
transmitted signal arrive at the receiver with different path lengths and phase
offsets.

61. • A consequence of frequency selective fading is intersymbol
interference where symbols received over the direct or the shortest reflecting
paths are interfered with by previous symbols arriving at the same time over
longer delay paths. Frequency selective fading is reduced by equalization,
where a digital filter in the receiver counteracts the effects of the channel.

62. FADING DUE TO DOPPLER SPREAD
• This phenomenon is known as the Doppler shift. Signals traveling along
different paths can have different Doppler shifts, corresponding to different
rates of change in phase. The difference in Doppler shifts between different
signal components contributing to a signal fading channel tap is known as
the Doppler spread.
• Channels with a large Doppler spread have signal components that are each
changing independently in phase over time. Since fading depends on whether
signal components add constructively or destructively, such channels have a
very short coherence time.

63. FAST FADING
• Fast fading, also known as fast channel fading, refers to rapid fluctuations in
the received signal strength or phase over short periods of time, typically on
the order of milliseconds or microseconds.
• This rapid variation occurs due to changes in the propagation environment,
such as movement of the transmitter, receiver, or objects in the surrounding
environment, causing the characteristics of the communication channel to
change quickly.

64. • Fast Fading results due to following:
➨High Doppler Spread
➨Coherence Time < Symbol Period
• ➨Channel impulse response changes rapidly within the symbol duration.
➨It occurs for very low data rates.

65. SLOW FADING
• Slow fading, also known as large-scale fading or shadowing, refers to the
gradual variations in the received signal strength over relatively long distances
or time scales, typically on the order of seconds to minutes or longer.
• Unlike fast fading, which involves rapid fluctuations in signal strength or
phase, slow fading manifests as smooth and gradual changes in signal
strength due to the spatial or temporal characteristics of the propagation
environment.

66. • Low Doppler Spread
➨Coherence Time >> Symbol Period
• ➨Impulse response changes much slower than the transmitted signal.