1.2.2.1 Fading Channels
The signal arriving at a receiver is a combination of many components arriving from various directions
as a result of multipath propagation. This depends on terrain conditions and local buildings and structures,
causing the received signal power to fluctuate randomly as a function of distance. Fluctuations on
the order of 20 dB are common within the distance of one wavelength (I
λ). This phenomenon is called
fading. One may think this signal as a product of two variables.
The first component, also referred to as the short-term fading component, changes faster than the
second one and has a Rayleigh distribution. The second component is a long-term or slow-varying
© 2002 by CRC Press LLC
quantity and has lognormal distribution [17, 25]. In other words, the local mean varies slowly with
lognormal distribution and the fast variation around the local mean has Rayleigh distribution.
A movement in a mobile receiver causes it to encounter fluctuations in the received power level. The
rate at which this happens is referred to as the fading rate in mobile communication literature [26] and
it depends on the frequency of transmission and the speed of the mobile. For example, a mobile on foot
operating at 900 MHz would cause a fading rate of about 4.5 Hz whereas a typical vehicle mobile would
produce the fading rate of about 70 Hz.
1.2.2.2 Doppler Spread
The movement in a mobile causes the received frequency to differ from the transmitted frequency because
of the Doppler shift resulting from its relative motion. As the received signals arrive along many paths,
the relative velocity of the mobile with respect to various components of the signal differs, causing the
different components to yield a different Doppler shift. This can be viewed as spreading of the transmitted
frequency and is referred to as the Doppler spread. The width of the Doppler spread in frequency domain
is closely related to the rate of fluctuations in the observed signal [22].
1.2.2.3 Delay Spread
Because of the multipath nature of propagation in the area where a mobile is being used, it receives
multiple and delayed copies of the same transmission, resulting in spreading of the signal in time. The
root-mean-square (rms) delay spread may range from a fraction of a microsecond in urban areas to on
the order of 100
μsec in a hilly area, and this restricts the maximum signal bandwidth between 40 and
250 kHz. This bandwidth is known as coherence bandwidth. The coherence bandwidth is inversely
proportional to the rms delay spread. This is the bandwidth over which the channel is flat; that is, it has
a constant gain and linear phase.
For a signal bandwidth above the coherence bandwidth the channel loses its constant gain and linear
phase characteristic and becomes frequency selective. Roughly speaking, a channel becomes frequency
selective when the rms delay spread is larger than the symbol duration and causes intersymbol interference
(ISI) in digital communications. Frequency-selective channels are also known as dispersive channels
whereas the nondispersive channels are referred to as flat-fading channels.
1.2.2.4 Link Budget and Path Loss
Link budget is a name given to the process of estimating the power at the receiver site for a microwave
link taking into account the attenuation caused by the distance between the transmitter and the receiver.
This reduction is referred to as the path loss. In free space the path loss is proportional to the second
power of the distance; that is, the distance power gradient is two. In other words, by doubling the distance
between the transmitter and the receiver, the received power at the receiver reduces to one fourth of the
original amount.
For a mobile communication environment utilizing fading channels the distance power gradient varies
and depends on the propagation conditions. Experimental results show that it ranges from a value lower
than two in indoor areas with large corridors to as high as six in metal buildings. For urban areas the
path loss between the base and the cell site is often taken to vary as the fourth power of the distance
between the two [22].
Normal calculation of link budget is done by calculating carrier to noise ratio (CNR), where noise
consists of background and thermal noise, and the system utility is limited by the amount of this noise.
However, in mobile communication systems the interference resulting from other mobile units is a
dominant noise compared with the background and man-made noise. For this reason these systems are
limited by the amount of total interference present instead of the background noise as in the other case.
In other words, the signal to interference ratio (SIR) is the limiting factor for a mobile communication
system instead of the signal to noise ratio (SNR) as is the case for other communication systems. The
calculation of link budget for such interference-limited systems involves calculating the carrier level,
above the interference-level contributed by all sources [27].
© 2002 by CRC Press LLC
1.2.3 Multiple Access Schemes
The available spectrum bandwidth is shared in a number of ways by various wireless radio links. The
way in which this is done is referred to as a multiple access scheme. There are basically four principle
schemes. These are frequency division multiple access (FDMA), time division multiple access (TDMA),
code division multiple access (CDMA), and space division multiple access (SDMA) [29-40].
1.2.3.1 Frequency Division Multiple Access Scheme
In an FDMA scheme the available spectrum is divided into a number of frequency channels of certain
bandwidth and individual calls use different frequency channels. All first-generation cellular systems use
this scheme.
1.2.3.2 Time Division Multiple Access Scheme
In a TDMA scheme several calls share a frequency channel [29]. The scheme is useful for digitized speech
or other digital data. Each call is allocated a number of time slots based on its data rate within a frame
for upstream as well as downstream. Apart from the user data, each time slot also carries other data for
synchronization, guard times, and control information.
The transmission from base station to mobile is done in time division multiplex (TDM) mode whereas
in the upstream direction each mobile transmits in its own time slot. The overlap between different slots
resulting from different propagation delay is prevented by using guard times and precise slot synchronization
schemes.
The TDMA scheme is used along with the FDMA scheme because there are several frequency channels
used in a cell. The traffic in two directions is separated either by using two separate frequency channels or
by alternating in time. The two schemes are referred to as frequency division duplex (FDD) and time division
duplex (TDD), respectively. The FDD scheme uses less bandwidth than TDD schemes use and does not
require as precise synchronization of data flowing in two directions as that in the TDD method. The latter,
however, is useful when flexible bandwidth allocation is required for upstream and downstream traffic [29].
1.2.3.3 Code Division Multiple Access Scheme
The CDMA scheme is a direct sequence (DS), spread-spectrum method. It uses linear modulation with
wideband pseudonoise (PN) sequences to generate signals. These sequences, also known as codes, spread
the spectrum of the modulating signal over a large bandwidth, simultaneously reducing the spectral
density of the signal. Thus, various CDMA signals occupy the same bandwidth and appear as noise to
each other. More details on DS spread-spectrum may be found in Reference [36].
In the CDMA scheme, each user is assigned an individual code at the time of call initiation. This code
is used both for spreading the signal at the time of transmission and despreading the signal at the time
of reception. Cellular systems using CDMA schemes use FDD, thus employing two frequency channels
for forward and reverse links.
On forward-link a mobile transmits to all users synchronously and this preserves the orthogonality
of various codes assigned to different users. The orthogonality, however, is not preserved between different
components arriving from different paths in multipath situations [34]. On reverse links each user
transmits independently from other users because of their individual locations. Thus, the transmission
on reverse link is asynchronous and the various signals are not necessarily orthogonal.
It should be noted that these PN sequences are designed to be orthogonal to each other. In other
words, the cross correlation between different code sequences is zero and thus the signal modulated with
one code appears to be orthogonal to a receiver using a different code if the orthogonality is preserved
during the transmission. This is the case on forward-link and in the absence of multipath the signal
received by a mobile is not affected by signals transmitted by the base station to other mobiles.
On reverse link the situation is different. Signals arriving from different mobiles are not orthogonalized
because of the asynchronous nature of transmission. This may cause a serious problem when the base
station is trying to receive a weak signal from a distant mobile in the presence of a strong signal from a
© 2002 by CRC Press LLC
nearly mobile. This situation where a strong DS signal from a nearby mobile swamps a weak DS signal
from a distant mobile and makes its detection difficult is known as the "near-far" problem. It is prevented
by controlling the power transmitted from various mobiles such that the received signals at the base
station are almost of equal strength. The power control is discussed in a later section.
The term
wideband CDMA (WCDMA) is used when the spread bandwidth is more than the coherence
bandwidth of the channel [37]. Thus, over the spread bandwidth of DS-CDMA, the channel is frequency
selective. On the other hand, the term
narrowband CDMA is used when the channel encounters flat
fading over the spread bandwidth. When a channel encounters frequency-selective fading, over the spread
bandwidth, a RAKE receiver may be employed to resolve the multipath component and combine them
coherently to combat fading.
A WCDMA signal may be generated using multicarrier (MC) narrowband CDMA signals, each using
different frequency channels. This composite MC-WCDMA scheme has a number of advantages over
the single-carrier WCDMA scheme. It not only is able to provide diversity enhancement over multipath
fading channels but also does not require a contiguous spectrum as is the case for the single-carrier
WCDMA scheme. This helps to avoid frequency channels occupied by narrowband CDMA, by not
transmitting MC-WCDMA signals over these channels. More details on these and other issues may be
found in Reference [37] and references therein.
1.2.3.4 Comparison of Different Multiple Access Schemes
Each scheme has its advantages and disadvantages such as complexities of equipment design, robustness
of system parameter variation, and so on. For example, a TDMA scheme not only requires complex time
synchronization of different user data but also presents a challenge to design portable RF units that
overcome the problem of a periodically pulsating power envelope caused by short duty cycles of each
user terminal. It should be noted that when a TDMA frame consists of
N users transmitting equal bit
rates, the duty cycles of each user is 1/N. TDMA also has a number of advantages [29].
1. A base station communicating with a number of users sharing a frequency channel only requires
one set of common radio equipment.
2. The data rate, to and from each user, can easily be varied by changing the number of time slots
allocated to the user as per the requirements.
3. It does not require as stringent power control as that of CDMA because its interuser interference
is controlled by time slot and frequency-channel allocations.
4. Its time slot structure is helpful in measuring the quality of alternative slots and frequency channels
that could be used for mobile-assisted handoffs. Handoff is discussed in a later section.
It is argued in Reference [34] that, though there does not appear to be a single scheme that is the best
for all situations, CDMA possesses characteristics that give it distinct advantages over others.
1. It is able to reject delayed multipath arrivals that fall outside the correlation interval of the PN
sequence in use and thus reduces the multipath fading.
2. It has the ability to reduce the multipath fading by coherently combing different multipath
components using a RAKE receiver.
3. In TDMA and FDMA systems a frequency channel used in a cell is not used in adjacent cells to
prevent co-channel interference. In a CDMA system it is possible to use the same frequency channel
in adjacent cells and thus increase the system capacity.
4. The speech signal is inherently bursty because of the natural gaps during conversation. In FDMD
and TDMA systems once a channel (frequency and/or time slot) is allocated to a user, that channel
cannot be used during nonactivity periods. However, in CDMA systems the background noise is
roughly the average of transmitted signals from all other users and thus a nonactive period in
speech reduces the background noise. Hence, extra users may be accommodated without the loss
of signal quality. This in turn increases the system capacity.
© 2002 by CRC Press LLC
1.2.3.5 Space Division Multiple Access
The SDMA scheme also referred to as space diversity uses an array of antennas to provide control of
space by providing virtual channels in angle domain [38]. This scheme exploits the directivity and beamshaping
capability of an array of antennas to reduce co-channel interference. Thus, it is possible that by
using this scheme simultaneous calls in a cell could be established at the same carrier frequency. This
helps to increase the capacity of a cellular system.
The scheme is based on the fact that a signal arriving from a distant source reaches different antennas
in an array at different times as a result of their spatial distribution, and this delay is utilized to differentiate
one or more users in one area from those in another area. The scheme allows an effective transmission
to take place between a base station and a mobile without disturbing the transmission to other mobiles.
Thus, it has the potential such that the shape of a cell may be changed dynamically to reflect the user
movement instead of currently used fixed size cells. This arrangement then is able to create an extra
dimension by providing dynamic control in space [39, 40]. A number of chapters in this book deal with
various aspects of antenna array processing.
Advantages of using small cells in cellular systems include increased network capacity, improved coverage and network efficiency, as well as better user experience. Disadvantages can include higher deployment costs due to increased infrastructure requirements, potential interference issues with macro cells, and challenges in managing a dense network of small cells.
A seahorse is multicellular, meaning it is made up of multiple cells that work together to form tissues, organs, and organ systems.
disadvantages - radiation can ionize and damage cells and is very expensive to use. advantages - can go in lots of detail, and results are usually very clear
the total count includes dead as well as living cells
Yes, humans are multicellular organisms composed of trillions of cells that work together to form tissues, organs, and organ systems.
Advantages: Photoelectric cells produce electricity without emissions, reducing environmental impact. They are also scalable and can be used in remote locations. Disadvantages: They can be expensive to install initially and are dependent on sunlight, making them less reliable in areas with inconsistent sunlight. Additionally, the manufacturing process for photoelectric cells can have environmental impacts.
what are the advantages and disadvantages primary cells?
diasadvantages... low efficiency
advantages: our body can produce more red blood cells
list of advantages and disadvanteges of division of labour
advantages: our body can produce more red blood cells
Cellular division is the cellular equivalent of reproduction. It is how cells reproduce.
arshia harms human cells
diasadvantages... low efficiency
diasadvantages... low efficiency
advantages: our body can produce more red blood cells
Cellular phones operate using radio waves. With cellular transmission, a local area (ex. a city) is divided in multiple cells. The signals from the cell are transmitted to a receiver and integrated into the regular phone systems. Hence the name of cellular comes from these cells.
A seahorse is multicellular, meaning it is made up of multiple cells that work together to form tissues, organs, and organ systems.