equipment that realizes the wireless functions; and a TA works as an
interface between the TE and the MT.
r Base Station (BS). The BS terminates the radio path on the network
side and provides connection to the network. It is composed of two
elements:
Base Transceiver Station (BTS). The BTS consists of a radio equip-
ment (transmitter and receiver–transceiver) and provides the radio
coverage for a given cell or sector.
Base Station Controller (BSC). The BSC incorporates a control capability
to manage one or more BTSs, executing the interfacing functions
between BTSs and the network. The BSC may be co-located with
a BTS or else independently located.
r Mobile Switching Center (MSC). The MSC provides an automatic
switching between users within the same network or other public
switched networks, coordinating calls and routing procedures. In gen-
eral, an MSC controls several BSCs, but it may also serve in different
capacities. The MSC provides the SSP function in a wireless IN.
r Visitor Location Register (VLR). The VLR is a database containing tem-
porary records associated with subscribers under the status of a vis-
itor. A subscriber is considered a visitor if such a subscriber is being
served by another system within the same home service area or by an-
other system away from the respective home service area (in a roam-
ing condition). The information within the VLR is retrieved from the
HLR. An VLR is usually co-located with an MSC.
r Home Location Register (HLR). The HLR is the primary database for
the home subscriber. It maintains information records on subscriber
current location, subscriber identifications (electronic serial number,
international mobile station identification, etc.), user profile (services
and features), and so forth. An HLR may be co-located with an MSC
or it may be located independently of the MSC. It may even be dis-
tributed over various locations and it may serve several MSCs. An
HLR usually operates on a centralized basis and serves many MSCs.
r Gateway (GTW). The GTW serves as an interface between the wireless
network and the external network.
r Service Control Point (SCP). The SCP provides a centralized element
to control service delivery to subscribers. It is responsible for higher-
level services that are usually carried out by the MSC in wireless
networks not using IN facilities.
r Service Transfer Point (STP). The STP is a packet switch device that
handles the distribution of control signals between different elements
in the network.
© 2002 by CRC Press LLC
r Intelligent Peripheral (IP). The IP processes the information of the sub-
scribers (credit card information, personal identification number,
voice-activated information, etc.) in support of IN services within
a wireless network.
r External Network. The external network constitutes the ISDN (Inte-
grated Services Digital Networks), CSPDN (Circuit-Switched Pub-
lic Data Network), PSPDN (Packed-Switched Public Data Network),
and, of course, PSTN (Public-Switched Telephone Network).
Note that a wireless network can be grossly split into a radio access network
(RAN) and a core network (CN). The RAN implements functions related to
the radio access to the network, whereas the CN implements functions related
to routing and switching. The RAN comprises the BSC, BTS, MT, and control
functionalities of the MS. The CN comprises the MSC, HLR, VRL, GTW, and
other devices implementing the switching and routing functions. This book is
primarily concerned with the radio aspects—the radio interface—of a wireless
network.
1.4 Protocol Architecture
A radio interface implements the wireless electromagnetic interconnec-
tion between a mobile station and a base station.[1] A general radio pro-
tocol contains the three lowest layers of the OSI/ISO Reference Model, as
follows:
r Physical Layer. The physical layer is responsible for providing a radio
link over the radio interface. Such a radio link is characterized by its
throughput and data quality. It is defined for the BTS and for the MT.
r Data Link Layer. The data link layer comprises two sublayers, as
follows:
Medium Access Control (MAC) sublayer. The MAC sublayer is respon-
sible for controlling the physical layer. It performs link quality
control and mapping of data flow onto this radio link. It is defined
for the BTS and for the MT. It may or may not exist in the BSC and
in the control functionalities of an MS.
Link Access Control (LAC) sublayer. The LAC sublayer is responsible
for performing functions essential to the logical link connection
such as setup, maintenance, and release of a link. It is defined for
BSC, BTS, MT, and control functionalities of the MS.
© 2002 by CRC Press LLC
r Network Layer. The network layer contains functions dealing with call
control, mobility management, and radio resource management. It is
mostly independent of radio transmission technology. Such a layer
can be transparent for user data in certain user services. It is defined
for BSC, BTS, MT, and control functionalities of the MS.
1.5 Channel Structure
A channel provides means of conveying information between two network
elements. Within the radio interface, three types of channels are specified:
radio frequency (RF) channel, physical channel, and logical channel.[1] These
channels are defined in the forward direction (downlink)—from BS to MS—or
in the reverse direction (uplink)—from MS to BS.
1.5.1 RF Channel
An RF channel is defined in terms of a carrier frequency centered within
a specified bandwidth, representing a portion of the RF spectrum. The RF
channel constitutes the means of carrying information over the radio interface.
It can be shared in the frequency domain, time domain, code domain, or space
domain.
1.5.2 Physical Channel
A physical channel corresponds to a portion of one or more RF channels
used to convey any given information. Such a portion is defined in terms of
frequency, time, code, space, or a combination of these. A physical channel
may be partitioned into a frame structure, with the specific timing defined
in accordance with the control and management functions to be performed.
Fixed or variable frame structures may be used.
1.5.3 Logical Channel
A logical channel is defined by the type of information it conveys. The logi-
cal channels are mapped onto one or more physical channels. Logical chan-
nels are usually grouped into control channels and traffic channels. Further
specifications concerning these channels vary according to the wireless net-
work. Logic channels may be combined by means of a multiplexing process,
using a frame structure. The following division and definitions are based on
Reference 1, and such a division, as depicted in Figure 1.3, reflects the basic
structure used in most wireless networks.
© 2002 by CRC Press LLC
FIGURE 1.3
Logical channels.
Traffic Channels
Traffic channels convey user information streams including data and voice.
Two types of traffic channels are specified:
r Dedicated Traffic Channel (DTCH). The DTCH conveys user informa-
tion. It may be defined in one or both directions.
r Random Traffic Channel (RTCH). The RTCH conveys packet-type data
user information. It is usually defined in one direction.
Control Channels
Control channels convey signaling information related to call management,
mobility management, and radio resource management. Two groups of
control channels are defined—dedicated control channels and common con-
trol channels:
© 2002 by CRC Press LLC
r Dedicated Control Channels (DCCH). A DCCH is a point-to-point chan-
nel defined in both directions. Two DCCHs are specified:
Associated Control Channel (ACCH). An ACCH is always allocated with
a traffic channel or with an SDCCH.
Stand-Alone Dedicated Control Channel (SDCCH). An SDCCH is allo-
cated independently of the allocation of a traffic channel.
r Common Control Channels (CCCH). A CCCH is a point-to-multipoint
or multipoint-to-point channel used to convey signaling information
(connectionless messages) for access management purposes. Four
types of CCCHs are specified:
Broadcast Control Channel (BCCH). The BCCH is a downlink channel
used to broadcast system information. It is a point-to-multipoint
channel listened to by all MSs, from which information is obtained
before any access attempt is made.
Forward Access Channel (FACH). The FACH is a downlink channel con-
veying a number of system management messages, including en-
quiries to the MS and radio-related and mobility-related resource
assignment. It may also convey packet-type user data.
Paging Channel (PCH). The PCH is a downlink channel used for paging
MSs. A page is defined as the process of seeking an MS in the event
that an incoming call is addressed to that MS.
Random Access Channel (RACH). The RACH is an uplink channel used
to convey messages related to call establishment requests and re-
sponses to network-originated inquiries.
1.6 Narrowband and Wideband Systems
Wireless systems can be classified according to whether they have a narrow-
band or wideband architecture. Narrowband systems support low-bit-rate
transmission, whereas wideband systems support high-bit-rate transmission.
A system is defined as narrowband or wideband depending on the band-
width of the transmission physical channels with which it operates. The sys-
tem channel bandwidth is assessed with respect to the coherence bandwidth.
The coherence bandwidth is defined as the frequency band within which all
frequency components are equally affected by fading due to multipath propa-
gation phenomena. Systems operating with channels substantially narrower
than the coherence bandwidth are known as narrowband systems. Wide-
band systems operate with channels substantially wider than the coherence
© 2002 by CRC Press LLC
bandwidth. In narrowband systems, all the components of the signal are
equally influenced by multipath propagation. Accordingly, although with
different amplitudes, the received narrowband signal is essentially the same
as the transmitted narrowband signal. In wideband systems, the various
frequency components of the signal may be differently affected by fading.
Narrowband systems, therefore, are affected by nonselective fading, whereas
wideband systems are affected by selective fading.
The coherence bandwidth, Bc , depends on the environment. It is approxi-
mately given by
Bc = (2π T)−1
in hertz, where T, in seconds, is the delay spread, as defined next. In a fading
environment, a propagated signal arrives at the receiver through multiple
paths. The time span between the arrival of the first and the last multipath
signals that can be sensed by the receiver is known as delay spread. The delay
spread varies from tenths of microseconds, in rural areas, to tens of microsec-
onds, in urban areas. As an example, consider an urban area where the delay
spread is T = 5µs. In such an environment, the coherence bandwidth is calcu-
lated as Bc = 32 kHz. Therefore, a system is considered to be narrowband if it
operates with channels narrower than 32 kHz. It is considered to be wideband
if it operates with channels several times wider than 32 kHz.
Another important definition within this context concerns coherence time.
The coherence time, Tc , is defined as the time interval during which the fad-
ing characteristics of the channel remain approximately unchanged (slow
change). This is approximately given as
Tc = (2 f m )−1
where f m is the maximum Doppler shift. The Doppler shift, in hertz, is given
as v/λ, where v, in m/s, is the speed of the mobile terminal and λ, in m, is the
wavelength of the signal.
1.7 Multiple Access
Wireless networks are multiuser systems in which information is conveyed
by means of radio waves. In a multiuser environment, access coordination can
be accomplished via several mechanisms: by insulating the various signals
sharing the same access medium, by allowing the signals to contend for the
access, or by combining these two approaches. The choice for the appropriate
© 2002 by CRC Press LLC
scheme must take into account a number of factors, such as type of traffic un-
der consideration, available technology, cost, complexity. Signal insulation is
easily attainable by means of a scheduling procedure in which signals are al-
lowed to access the medium according to a predefined plan. Signal contention
occurs exactly because no signal insulation mechanism is used. Access co-
ordination may be carried out in different domains: the frequency domain,
time domain, code domain, and space domain. Signal insulation in each do-
main is attained by splitting the resource available into nonoverlapping slots
(frequency slot, time slot, code slot, and space slot) and assigning each signal
a slot. Four main multiple access technologies are used by the wireless net-
works: frequency division multiple access (FDMA), time division multiple
access (TDMA), code division multiple access (CDMA), and space division
multiple access (SDMA).
1.7.1 Frequency Division Multiple Access
FDMA is certainly the most conventional method of multiple access and was
the first technique to be employed in modern wireless applications. In FDMA,
the available bandwidth is split into a number of equal subbands, each of
which constitutes a physical channel. The channel bandwidth is a function of
the services to be provided and of the available technology and is identified
by its center frequency, known as a carrier. In single channel per carrier FDMA
technology, the channels, once assigned, are used on a non-time-sharing ba-
sis. Thus, a channel allocated to a given user remains allocated until the end
of the task for which that specific assignment was made.
1.7.2 Time Division Multiple Access
TDMA is another widely known multiple-access technique and succeeded
FDMA in modern wireless applications. In TDMA, the entire bandwidth is
made available to all signals but on a time-sharing basis. In such a case,
the communication is carried out on a buffer-and-burst scheme so that the
source information is first stored and then transmitted. Prior to transmission,
the information remains stored during a period of time referred to as a frame.
Transmission then occurs within a time interval known as a (time) slot. The
time slot constitutes the physical channel.
1.7.3 Code Division Multiple Access
CDMA is a nonconventional multiple-access technique that immediately
found wide application in modern wireless systems. In CDMA, the entire
bandwidth is made available simultaneously to all signals. In theory, very
little dynamic coordination is required, as opposed to FDMA and TDMA in
© 2002 by CRC Press LLC
which frequency and time management have a direct impact on performance.
To accomplish CDMA systems, spread-spectrum techniques are used. (Ap-
pendix C introduces the concept of spread spectrum.)
In CDMA, signals are discriminated by means of code sequences or sig-
nature sequences, which correspond to the physical channels. Each pair of
transmitter–receivers is allotted one code sequence with which a communica-
tion is established. At the reception side, detection is carried out by means of a
correlation operation. Ideally, the best performance is attained with zero cross-
correlation codes, i.e., with orthogonal codes. In theory, for a synchronous
system and for equal rate users, the number of users within a given band-
width is dictated by the number of possible orthogonal code sequences. In
general, CDMA systems operate synchronously in the forward direction and
asynchronously in the reverse direction. The point-to-multipoint character-
istic of the downlink facilitates the synchronous approach, because one ref-
erence channel, broadcast by the base station, can be used by all mobile sta-
tions within its service area for synchronization purposes. On the other hand,
the implementation of a similar feature on the reverse link is not as simple
because of its multipoint-to-point transmission characteristic. In theory, the
use of orthogonal codes eliminates the multiple-access interference. There-
fore, in an ideal situation, the forward link would not present multiple-access
interference. The reverse link, in turn, is characterized by multiple-access in-
terference. In practice, however, interference still occurs in synchronous sys-
tems, because of the multipath propagation and because of the other-cell sig-
nals. The multipath phenomenon produces delayed and attenuated replicas of
the signals, with these signals then losing the synchronism and, therefore, the
orthogonality. The other-cell signals, in turn, are not time-aligned with the
desired signal. Therefore, they are not orthogonal with the desired signal and
may cause interference.
Channels in the forward link are identified by orthogonal sequences, i.e.,
channelization in the forward link is achieved by the use of orthogonal codes.
Base stations are identified by pseudonoise (PN) sequences. Therefore, in the
forward link, each channel uses a specific orthogonal code and employs a
PN sequence modulation, with a PN code sequence specific to each base sta-
tion. Hence, multiple access in the forward link is accomplished by the use
of spreading orthogonal sequences. The purpose of the PN sequence in the
forward link is to identify the base station and to reduce the interference. In
general, the use of orthogonal codes in the reverse link finds no direct appli-
cation, because the reverse link is intrinsically asynchronous. Channelization
in the reverse link is achieved with the use of long PN sequences combined
with some private identification, such as the electronic serial number of the
mobile station. Some systems, on the other hand, implement some sort of syn-
chronous transmission on the reverse link, as shall be detailed in the chapters
© 2002 by CRC Press LLC
that follow. In such a case, orthogonal codes may also be used with channel-
ization purposes in the reverse link.
Several PN sequences are used in the various systems, and they will be
detailed for the several technologies described in the following chapters. Two
main orthogonal sequences are used in all CDMA systems: Walsh codes and
orthogonal variable spreading functions (OVSF) (see Appendix C).
1.7.4 Space Division Multiple Access
SDMA is a nonconventional multiple-access technique that finds application
in modern wireless systems mainly in combination with other multiple-access
techniques. The spatial dimension has been extensively explored by wireless
communications systems in the form of frequency reuse. The deployment
of advanced techniques to take further advantage of the spatial dimension
is embedded in the SDMA philosophy. In SDMA, the entire bandwidth is
made available simultaneously to all signals. Signals are discriminated spa-
tially, and the communication trajectory constitutes the physical channels.
The implementation of an SDMA architecture is based strongly on antennas
technology coupled with advanced digital signal processing. As opposed to
the conventional applications in which the locations are constantly illumi-
nated by rigid-beam antennas, in SDMA the antennas should provide for
the ability to illuminate the locations in a dynamic fashion. The antenna
beams must be electronically and adaptively directed to the user so that,
in an idealized situation, the location alone is enough to discriminate the
user.
FDMA and TDMA systems are usually considered to be narrowband,
whereas CDMA systems are usually designed to be wideband. SDMA sys-
tems are deployed together with the other multiple-access technologies.
1.8 Summary
Wireless networks are multiuser systems in which information is conveyed
by radio waves. Modern wireless networks have evolved through different
generations: 1G systems, based on analog technology, aimed at providing
voice telephony services; 2G systems, based on digital technology, aimed at
providing a better spectral efficiency, a more robust communication, voice
privacy, and authentication capabilities; 2.5G systems, based on 2G systems,
aimed at providing the 2G systems with a better data rate capability; and 3G
systems that aim at providing for multimedia services in their entirety.
© 2002 by CRC Press LLC
References
1. Framework for the radio interface(s) and radio sub-system functionality for
International Mobile Telecommunications-2000 (IMT-2000), Recommendation
ITU-R M.1035.
2. The international intelligent network (IN), The International Engineering Con-
sortium, available at http://www.iec.org.
© 2002 by CRC Press LLC
2
Cellular Principles
2.1 Introduction
The electromagnetic spectrum is a limited but renewable resource that, if
adequately managed, can be reused to expand wireless network capacity.
Frequency reuse, in fact, constitutes the basic idea behind the cellular concept.
In a cellular system, the service area is divided into cells and portions of the
available spectrum are conveniently allocated to each cell. The main pur-
pose of defining cells in a wireless network is to delimit areas within which
channels or base stations are used at least preferentially. A cell, therefore, is
defined as the geographic area where a mobile station is preferentially served
by its base station. A mobile station moving out of its serving cell and into a
neighboring cell must be provided with sufficient resources from these cells
so that the already established communication will not be discontinued. Such
a process is known as handoff or handover. A group of cells among which the
whole spectrum is shared and within which no frequency reuse exists con-
stitutes a cluster. The number of cells per cluster defines the reuse pattern,
and this is a function of the cellular geometry. In an ideal situation, for om-
nidirectional transmission with antennas mounted high above the rooftops,
mobile stations at the same distance from the base station receive the same
mean signal power in all directions. In such a case, the cell shape can be de-
fined as a circle. Its radius is determined so as to have a circular area within
which base station and mobile stations receive a signal power exceeding a
given threshold. Circles, on the other hand, cannot fill a plane without leaving
gaps (holes) or exhibiting overlapped areas. The use of a circular geometry
may impose difficulties in the design of a cellular network. Regular poly-
gons, such as equilateral triangles, squares, and regular hexagons do not exhibit
these constraints. The choice for one or another cellular format depends on
the application. In practice, the coverage area differs substantially from the
© 2002 by CRC Press LLC
idealized geometric figures and “amoeboid” cellular shapes are more likely
to occur.
This chapter defines the issues related to the cellular concepts. The main
definitions that follow are based on an ITU Recommendation for IMT-2000.[1]
The concepts developed in Reference 1 generalize those that have been used
for conventional cellular networks.
2.2 Cellular Hierarchy
To maximize spectral efficiency as well as to minimize the number of han-
dovers, it is beneficial for the cells to be designed with different sizes and
formats. The design of different cells depends on several parameters, such as
mobility characteristics, output power, and types of services utilized. Cellular
layers are then defined with each layer containing cells of the same type in a
given service area. The layering of cells does not imply that all mobile stations
must be able to connect to all base stations serving the geographic area where
the mobile station is positioned. For example, the mobile station may not have
sufficient output power to access a given layer or may not be entitled to the
service provided by the cells of a given layer. The cellular hierarchy makes
use of four categories of cells: mega cells, macro cells, micro cells, and pico
cells.[1]
Mega cells provide coverage to large areas and are characterized by cells
presenting radii in the range 100 to 500 km. They are particularly useful for
remote areas with low traffic density or for areas without access to terrestrial
telecommunications networks. Mega cells are provided by low-orbit satellites
and the cell radius is a function of the satellite altitude, power, and antenna
aperture. Note that in a mega cell the distances between mobile stations and
the base station are very large. Because of their sizes, these cells must be both
flexible and robust to accommodate a wide range of user scenarios. They
must be able to support low-mobility as well as very high-mobility users.
Note that for nongeostationary orbits, the cells move because the satellites
move with respect to the Earth. Therefore, handovers may be necessary even
for stationary mobile stations.
Macro cells provide coverage to large areas and are characterized by cells
presenting radii of up to 35 km. Larger cell radii may be provided with the use
of directional antennas. The macro cells are outdoor cells that are illuminated
by high-power sites with the antennas mounted above the rooftops—on tow-
ers or on the tops of buildings. They serve low to medium traffic density and
support mobile speeds of up to 500 km/h.
Micro cells provide coverage to small areas and are characterized by cells
presenting radii of up to 1 km. They are outdoor cells that are illuminated
© 2002 by CRC Press LLC
by low-power antennas with the antennas mounted below the rooftops—on
lampposts or on building walls. They support medium to high traffic density
and mobile speeds of up to 100 km/h.
Pico cells provide coverage to small areas and are characterized by cells
presenting radii of up to 50 m. They are indoor cells supporting medium to
high traffic density and mobile speeds of up to 10 km/h.
In the real world, mega cells, macro cells, micro cells, and pico cells
coexist in the same environment. Digital technology has made it possible for
wireless systems to take full advantage of such a coexistence so that coverage
is improved, capacity is increased, load is balanced, and users are provided
with different services according to the mobility characteristics. More gen-
erally, pico, micro, macro, and mega cells are displaced in a hierarchy, the
so-called hierarchical cellular structure (HCS). In HCS wireless systems, very
low to very high mobility and in-building to satellite coverage provide for
the multimedia–anywhere–anytime wireless services. In HCS, several layers
of cells may coexist with the smallest cells occupying the lowest layer in the
hierarchy. The mobility and the class of service of the user determine the layer
within which the required service is to be provided. In a multilayered cellular
environment, the selection of which cell to serve a given call should be based
on criteria such as speed of the mobile station relative to the base station, cell
availability, and required transmission power to and from the mobile station.
2.3 System Management
The phases of a communication between mobile station and base station
encompass the establishment, maintenance, and release of the connection.
The management of these phases is carried out by several functions. These
functions include link quality measurement, cell selection, channel selection/
assignment, handover, and mobility support.[1]
2.3.1 Link Quality Measurement
During any given connection, forward and reverse links are continually moni-
tored to assess the radio link quality. The assessment is based on parameters
such as the received signal quality and the bit error rates.
2.3.2 Cell Selection
In advanced wireless networks, cell selection is a feature that can be provided.
Cell selection may be based on several criteria, including mobility and class
of service to be provided. It starts with the choice of the operator, a phase that
occurs as the mobile station is powered up. The selection of the operator may
© 2002 by CRC Press LLC
be based on user preferences, available networks, mobile station capabilities,
network capabilities, mobile station mobility, and service requirements. Once
a system has been selected by the mobile station, a base station is then searched
and its broadcast control channel monitored. Cell reselection may also occur,
and the following circumstances may trigger the reselection: unsuitability
of current cell due to interference or output power requirements, radio link
failure, network request, traffic load considerations, and user request.
2.3.3 Channel Selection/Assignment
Channel assignment algorithms are used to ascertain conveniently the avail-
able channels and to assign one or more of these channels to a call. The algo-
rithms vary in accordance with specific allocation policies, but they usually
take into account the following: system load, traffic patterns, service types,
service priorities, and interference situations. Channel assignment algorithms
are added-on features that differ for different system providers.
2.3.4 Handover
Handover is defined as “the change of Physical Channel(s) involved in a call
whilst maintaining the call.”[1] Handover constitutes a diversity technique
used to prevent mobile calls from being released when the mobile stations
experience a degraded radio condition. Many factors affect the received sig-
nal quality, one of which is the distance between mobile and base stations. A
signal degradation may occur, particularly when the mobile stations cross the
cell boundaries. Handovers may take place in several conditions: within the
cell (intracell handover), between cells in the same cell layer (intercell han-
dover), between cells of different layers (interlayer handover), or between
cells of different networks (internetwork handover).
In FDMA and TDMA wireless networks, handovers are “hard” (hard hand-
over). In hard handover, the communication with the old base station through
a given channel is discontinued, and a new communication with a new base
station, and necessarily through another channel, is established. Internetwork
handovers and handovers between systems of different technologies are
always hard handover.
In CDMA wireless networks, handovers are “soft,” and three kinds of soft-
type handovers are identified: soft handover, softer handover, and soft-softer
handover. In soft handover, the mobile station maintains communication si-
multaneously with the old base station and with one or more new base sta-
tions, provided that all base stations have CDMA channels with an identical
frequency assignment. Note that the soft handover constitutes a means of di-
versity for both the forward and reverse paths, and is supposed to occur in the
vicinities of the boundary of the cell, where the signal is presumably weaker. In
softer handover, the mobile station maintains communication simultaneously
© 2002 by CRC Press LLC
with two or more sectors of the same base station and certainly within the
same CDMA channel of that base station. Similar to the soft handover, the
softer handover also constitutes a means of diversity and is supposed to oc-
cur in the vicinities of the boundary of the coverage area of the sectors. In
soft-softer handover, the mobile station maintains communication simulta-
neously with two or more sectors of the same base station and with one or
more base stations (or their sectors).
Note that, whereas in the hard-type handover the number of resources uti-
lized remains unchanged, in the soft-type handover this is not true. In the
hard-type handover, the new resource is utilized only after the old resource
has been released. In the soft-type handover, a communication with more than
one resource is maintained throughout the duration of the process. However,
in the uplink direction only one channel is used by the mobile station. In
such a case, the involved base station receives this very channel and the selec-
tion of the best communication is performed by the mobile switching center.
In the downlink direction, on the other hand, each base station supporting
the handover transmits to the same mobile station, and the combination of
the received signals is carried out by the mobile station. Therefore, the in-
volved base stations must provide for additional channels for soft handover
purposes, with these additional channels used in the forward direction only.
From the transmission point of view, the handover process comprises two
phases: the evaluation phase and the execution phase. In the evaluation phase,
the rationale for performing a handover is continually assessed. Handovers
may be initiated for such reasons as operation and maintenance, radio chan-
nel capacity optimization, poor radio transmission conditions, signal level
variability, significant amount of interference, etc. The following criteria may
be used to initiate a handover for radio transmission reasons: signal strength
measurements, signal-to-interference ratio, bit error rates, distance between
mobile station and base station, mobile station speed, mobile station mobility
trends, and others. The handover is actually performed in the execution phase.
A handover may be initiated by the base station or by the mobile station.
Three handover strategies are possible: mobile-controlled handover
(MCHO); mobile-assisted handover (MAHO); and base-controlled handover
(BCHO). In MCHO, the mobile station controls the handover evaluation phase
as well as the handover execution phase. In MAHO, the base station controls
the handover process with the support of the mobile station (e.g., measure-
ments carried out by the mobile station). In BCHO, the base station controls
the handover evaluation phase as well as the handover execution phase.
2.3.5 Mobility Support
User mobility is supported by the following processes: logon–logoff and lo-
cation updating. In logon–logoff, messages are transmitted from the mobile
station to the network to notify the network of the terminal status. Such a
© 2002 by CRC Press LLC
procedure may be initiated by the mobile station as well as by the network. In
location updating, messages are exchanged between the mobile station and
the network to identify the area within which the mobile station is located. A
location area is defined as the geographic area, containing a group of cells, in
which the mobile station is to be sought by the network. The location update
is performed whenever a mobile station moves into a new location area. Note
that the smaller the location area, the higher the chance of locating the mobile
station within the network. On the other hand, the smaller the location area,
the higher the frequency with which the exchange of updating messages must
be carried out.
2.4 System Performance
Several aspects that affect the performance of the system must be addressed:
interference control, diversity strategies, variable data rate control, capacity
improvement techniques, and battery-saving techniques.
2.4.1 Interference Control
Synchronization is certainly one of the issues that must be examined for in-
terference control purposes. Some technologies require that base stations be-
longing to the same system must be synchronized. This is particularly true for
TDMA systems and some CDMA systems. In the same way, time synchroniza-
tion between base stations in different but geographically co-located systems
and time synchronization between user terminals and base stations are ele-
ments that have a great impact on interference. Power control is another im-
portant issue in interference control. Power control must be exercised because
of the near–far phenomenon, a feature inherent to mobile communications.
Because of the mobility feature, the powers of desired and interfering sig-
nals may vary according to the location of the mobile stations. Power control
must be performed so that intra- and intersystem interference is minimized.
The near–far phenomenon is more relevant in the multipoint-to-point trans-
mission (mobile stations to base station) than in the point-to-multipoint one
(base station to mobile stations). In the first case (multipoint-to-point), because
the mobile stations may be at different distances from the base station, the
various signals arriving at the base will have different strengths. In the second
case (point-to-multipoint), the various signals transmitted by the base reach a
given mobile station with approximately the same power loss, thus main-
taining power proportionality. In practice, however, both reverse link and
forward link require power control: the reverse link for the reasons already
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outlined, and the forward link to compensate for poor reception conditions
encountered by the mobile station.
2.4.2 Diversity Strategies
Diversity strategies are used to combat fading. The diversity methods take
advantage of the fact that fades occurring on independent channels (known
as branches) constitute independent events. Therefore, if certain information
is redundantly available on two or more branches, simultaneous fades are less
likely to occur. By appropriately combining the various branches, the quality
of the received signal is improved. Diversity may be achieved in several ways,
such as in space (spaced antennas), frequency, and time.
2.4.3 Variable Data Rate Control
Variable data rates may be accomplished by several means: direct support of
variable data rates over the air interface, variation of the number of bearer
channels so that multiple bearer channels are combined to deliver the desired
user rate, or packet access. Different wireless networks use different variable
data rate technologies.
2.4.4 Capacity Improvement Techniques
Network capacity may be improved by means of such techniques as slow fre-
quency hopping; dynamic power control; dynamic channel allocation; dis-
continuous transmission for voice, including voice activity detection, and
nonvoice services; and others. The applicability of one or another technique
is dependent on the multiple-access technology chosen.
2.4.5 Battery-Saving Techniques
Digital technologies facilitate the use of battery-saving techniques. These tech-
niques include output power control, discontinuous reception, and disconti-
nuous transmission.
2.5 Cellular Reuse Pattern
For quite a while, since the inception of modern wireless networks, the cellular
grid has been dominated by macro cells. The macrocellular network makes
use of a hexagonal cell array with the reuse pattern established with the
supposition that reuse distances are isotropic and that a cluster comprises a
© 2002 by CRC Press LLC
group of contiguous cells. In theory, high-power sites, combined with base
station antennas positioned well above the rooftops, provide for propaga-
tion symmetry, in which case, for system planning purposes, the hexagonal
coverage grid has proved appropriate. Further, macro cells are adequate for
low-capacity systems.
The expansion and the evolution of wireless networks can only be supported
by an ample microcellular structure, not only to satisfy the high traffic demand
in dense urban regions but also to provide for services requiring low mobi-
lity. The microcellular network concept differs from that of the macrocellular
concept widely employed in wireless systems. In microcellular systems, with
low-power sites and antennas mounted at street level (below the rooftops),
the supposed propagation symmetry of the macrocellular network no longer
applies and the hexagonal cell pattern does not make sense. The “micro-
scopic” structure of the environment (e.g., street orientation, width of the
streets, layout of the buildings, among others) constitutes a decisive element
influencing system performance. With the antennas mounted at street level,
the buildings lining each side of the street work as “waveguides,” in the radial
direction, and as obstructors, in the perpendicular direction. Therefore, the
propagation direction of the radio waves is greatly influenced by the envi-
ronment. Assuming the base stations to be positioned at the intersection of
the streets, a cell in such an environment is more likely to have a diamond
shape with the radial streets as the diagonals. In fact, a number of field meas-
urements and investigations[2– 8] show that an urban micro cell service area
can be reasonably well approximated by a square diamond.
The ubiquitous coverage of a service area based on a microcellular network
requires a much greater number of base stations as compared with the num-
ber of base stations required by macrocellular systems. Therefore, among the
important factors to be accounted for in microcellular system planning (cost
of the site, size of the service area, etc.), the per-subscriber cost is determi-
nant. This cost is intimately related to how efficiently the radio resources are
reutilized in a given service area. Reuse efficiency depends on the interfering
environment of the network and on how the involved technology can cope
with the interfering sources.
The study of interference in macrocellular systems is greatly eased by the
intrinsic symmetry of the problem. In the microcellular case, the inherent
asymmetry due to the microscopic structures introduces an additional com-
plication. In such a case, the interference is dependent not only on the distance
between transmitter and receiver but also, and mainly, on the line-of-sight
(LOS) condition of the radio path. Assume, for example, that base stations are
located at street intersections. Mobiles on streets running radially from the
base station may experience an interference pattern changing along the street
as they depart from the vicinity of their serving base station and approach new
street intersections. Near the serving base station, the desired signal is strong
© 2002 by CRC Press LLC
and the relevant interfering signals are obstructed by buildings (non-LOS,
or NLOS). Away from its serving base station and near new street intersec-
tions, mobile stations may have an LOS condition not only to their serving
base station but also to the interfering stations. The interfering situation will
then follow a completely distinct pattern on the perpendicular streets. Again,
the asymmetry of the problem is stressed by the traffic distribution, which
is more likely to comply with an uneven configuration with the main streets
accommodating more mobile users than the secondary streets.
2.6 Macrocellular Reuse Pattern
A macrocellular structure makes use of a hexagonal cellular grid. For the
hexagonal array, it is convenient to choose the set of coordinates as shown in
Figure 2.1. In Figure 2.1, the positive portions of the u and v axes form a 60◦
√
angle and the unit distance is 3R, where R is the cell radius. The distance d
between the centers of two cells, whose coordinates are, respectively, (u1 , v1 )
v
(u1, v1)
u
D
(u2, v2)
R
3R
FIGURE 2.1
Hexagonal cellular geometry.
© 2002 by CRC Press LLC
and (u2 , v2 ), is
d 2 = 3R2 [(u1 − u2 )2 + (v2 − v1 )2 − 2 cos (60◦ ) (u1 − u2 ) (v2 − v1 )]
√
Defining i = u2 − u1 , j = v2 − v1 , and 3R = 1, then
d2 = i2 + i j + j 2 (2.1)
where i and j range over the integers.
2.6.1 Reuse Factor (Number of Cells per Cluster)
Let D be the reuse distance, that is, the distance between two co-cells. There-
fore, for any given co-cells
D2 = i 2 + i j + j 2 (2.2)
Considering that reuse distances are isotropic and that a cluster is a group
of contiguous cells, the format of the clusters must be hexagonal. For two
adjacent clusters, D represents the distance between the centers of any two
co-cells within these clusters. Therefore, D is also the distance between the
centers of these two adjacent hexagonal clusters, D/2 their apothems, and
(D/2)/ cos 30◦ their radii. Let A be the area of the hexagonal cluster and a the
area of the hexagonal cell. Then,
√ 2
√
3 3 D/2 3D2
A= × =
2 cos 30◦ 2
and
√ √
3 3 3
a= ×R =
2
2 2
The number N of cells per cluster is N = A/a = D2 . Therefore,
N = i2 + i j + j2 (2.3)
Because i and j range over the integers, the clusters will accommodate only
a certain number of cells, such as 1, 3, 4, 7, 9, 12, 13, 16, 19, etc.
The layout of the cells within the cluster is attained having as principal
targets symmetry and compactness. Figure 2.2 shows some hexagonal repeat
patterns. The tessellation over the entire plane is then achieved by replicating
the cluster in an isotropic manner. In other words, if the chosen reuse pattern
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