Advantages and disadvantages with using of small cells in cellular systems?

Answer 1

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

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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].

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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

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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.