14-05-2013, 02:28 PM
8 Cellular Networks -- From 1G to 5G
[attachment=53873]
Introduction
Cellular telephones and residential cordless telephones (wireless systems) were introduced in
the mid-1980s. These technologies are enjoying widespread public approval with a rapidly
increasing demand that has exceeded all early predictions. For example, AT&T predicted in
the mid-1980s that there would be about 1 million cellular telephone users by the year 2000
[Ziegler 1993]. However, the number of cellular telephone users exceeded 12 million way
back in 1994. The Cellular Telecommunications & Internet Association (CTIA) estimated
that there were over 90 million cell phone US subscribers in 2000 and about 140 million in
2002. The CTIA predicts a yearly increase of about 10%. At a global level, the number of
estimated subscribers exceeded 1 billion in 2003. This number is also expected to keep
growing for several years to come.
The cellular networks are evolving through several generations (see Figure 8-1). The older
generations (1G and 2G) provide lower data rates, while the new and future ones (3G and
beyond) offer higher data rates. Although the cellular networks are going through rapid
evolution, there are some common principles to all cellular networks. Section 8.2 discusses
these principles that are the foundations of cellular networks. Sections 8.3 through 8.8 review
the 1G through 4G systems and Section 8.11 introduces some cellular network engineering
issues.
Overview
Cellular networks are wireless WANs that establish a connection between mobile users.
Figure 8-2 shows a high level view of a cellular communication network. The cellular
network is comprised of many “cells” that typically cover 2 to 20 miles in area. The users
communicate within a cell through wireless communications. A Base Transceiver Station
(BTS), also known as a Base Station (BS), is accessed by the mobile units in each cell by
using wireless communications. One BTS is assigned to each cell. Regular cable
communication channels can be used to connect the BTSs to the Mobile Switching Center
(MSC), also known as Mobile Telecommunications Service Center (MTSC ). The MSC is
the heart of cellular networks – it determines the destination of the call received from a BTS
and routes it to a proper site, either by sending it to another BTS or to a regular telephone
network. Keep in mind that the communications are wireless within a cell only. The bulk of
cell-to-cell communication is carried through regular telephone lines (wireless local loops can
be used but are not essential). The MTSC uses two databases called Home Location Register
(HLR) and Visitor location Register (VLR) to locate the mobile users. We will discuss
location services and roaming support in a later section.
How Cellular Phones Work – A Quick Look
Each cell in a cellular network is assigned a band of frequencies. In general, 10 to 50
frequencies are assigned per cell. The allocated frequencies are divided into two types of
channels: control channels that are used to set up and maintain calls, and traffic channels
that are used to carry voice and data traffic. For the sake of simplicity, let us assume that each
channel is assigned a unique frequency and that one user can only use one channel. For
example, if the frequency band of a cell is subdivided into 30 traffic channels, then only 30
users can talk simultaneously. User 31 is blocked and gets a busy signal. This model changes
somewhat, as we will see, when we use CDMA systems.
Highlights of Cellular Networks
Let us capture the highlights of cellular networks by using our, by now, familiar framework
of data rates/distance covered, target applications, frequency allocation, location management,
and physical communications shown in Table 8-1.
Frequency Allocations
The cellular networks operate in licensed frequency bands roughly between 900 MHz and 2
GHz. There are several implications of this. First, the cellular operators compete with each
other for frequency allocations because the bandwidths, and consequently data rates, of
communication channels are restricted by government regulations. Because of this, cell
design for efficient frequency utilization is a major concern (see Section 8.2.4). An additional
implication is that universal adoption of cellular technology is contingent upon international
agreements between various licensing agencies.
Location Management
It is important to keep track of cellular users, since the location of a sender/receiver is
unknown prior to the start of communication and can change during the conversation. In
particular, rapid changes in location can occur because the cellular users can be traveling in
cars and trains. Due to this, handoffs between cells must be handled very efficiently.
The cellular systems have to keep track of the users as they move around rapidly through the
cells. As discussed in Chapter 5, a wide range of techniques for location management have
been introduced in cellular networks.
Cell Design and Frequency Utilization
Most cells are between 2 to 20 miles in radius depending on the population density and the
strength of the signal. Some cells are smaller (1.5 km appears to be a practical small limit)
while the others are larger (in fact a geosynchronous satellite can be thought of as a large cell
that can cover around 13,000 kilo meters). Each cell is served by a base transceiver station
(BTS), commonly known as a base station, consisting of transmitter, receiver, and a control
unit. Base station (BS) antennas are placed in high places such as churches and high rise
buildings. In several large cities, wireless operators pay around $500 per month for a base
station (yet another way for the landlords to make more money in large cities!). Cells are set
up such that antennas of all neighbors are equidistant from a base station. This is why
hexagonal patterns are used to represent cells.
[attachment=53873]
Introduction
Cellular telephones and residential cordless telephones (wireless systems) were introduced in
the mid-1980s. These technologies are enjoying widespread public approval with a rapidly
increasing demand that has exceeded all early predictions. For example, AT&T predicted in
the mid-1980s that there would be about 1 million cellular telephone users by the year 2000
[Ziegler 1993]. However, the number of cellular telephone users exceeded 12 million way
back in 1994. The Cellular Telecommunications & Internet Association (CTIA) estimated
that there were over 90 million cell phone US subscribers in 2000 and about 140 million in
2002. The CTIA predicts a yearly increase of about 10%. At a global level, the number of
estimated subscribers exceeded 1 billion in 2003. This number is also expected to keep
growing for several years to come.
The cellular networks are evolving through several generations (see Figure 8-1). The older
generations (1G and 2G) provide lower data rates, while the new and future ones (3G and
beyond) offer higher data rates. Although the cellular networks are going through rapid
evolution, there are some common principles to all cellular networks. Section 8.2 discusses
these principles that are the foundations of cellular networks. Sections 8.3 through 8.8 review
the 1G through 4G systems and Section 8.11 introduces some cellular network engineering
issues.
Overview
Cellular networks are wireless WANs that establish a connection between mobile users.
Figure 8-2 shows a high level view of a cellular communication network. The cellular
network is comprised of many “cells” that typically cover 2 to 20 miles in area. The users
communicate within a cell through wireless communications. A Base Transceiver Station
(BTS), also known as a Base Station (BS), is accessed by the mobile units in each cell by
using wireless communications. One BTS is assigned to each cell. Regular cable
communication channels can be used to connect the BTSs to the Mobile Switching Center
(MSC), also known as Mobile Telecommunications Service Center (MTSC ). The MSC is
the heart of cellular networks – it determines the destination of the call received from a BTS
and routes it to a proper site, either by sending it to another BTS or to a regular telephone
network. Keep in mind that the communications are wireless within a cell only. The bulk of
cell-to-cell communication is carried through regular telephone lines (wireless local loops can
be used but are not essential). The MTSC uses two databases called Home Location Register
(HLR) and Visitor location Register (VLR) to locate the mobile users. We will discuss
location services and roaming support in a later section.
How Cellular Phones Work – A Quick Look
Each cell in a cellular network is assigned a band of frequencies. In general, 10 to 50
frequencies are assigned per cell. The allocated frequencies are divided into two types of
channels: control channels that are used to set up and maintain calls, and traffic channels
that are used to carry voice and data traffic. For the sake of simplicity, let us assume that each
channel is assigned a unique frequency and that one user can only use one channel. For
example, if the frequency band of a cell is subdivided into 30 traffic channels, then only 30
users can talk simultaneously. User 31 is blocked and gets a busy signal. This model changes
somewhat, as we will see, when we use CDMA systems.
Highlights of Cellular Networks
Let us capture the highlights of cellular networks by using our, by now, familiar framework
of data rates/distance covered, target applications, frequency allocation, location management,
and physical communications shown in Table 8-1.
Frequency Allocations
The cellular networks operate in licensed frequency bands roughly between 900 MHz and 2
GHz. There are several implications of this. First, the cellular operators compete with each
other for frequency allocations because the bandwidths, and consequently data rates, of
communication channels are restricted by government regulations. Because of this, cell
design for efficient frequency utilization is a major concern (see Section 8.2.4). An additional
implication is that universal adoption of cellular technology is contingent upon international
agreements between various licensing agencies.
Location Management
It is important to keep track of cellular users, since the location of a sender/receiver is
unknown prior to the start of communication and can change during the conversation. In
particular, rapid changes in location can occur because the cellular users can be traveling in
cars and trains. Due to this, handoffs between cells must be handled very efficiently.
The cellular systems have to keep track of the users as they move around rapidly through the
cells. As discussed in Chapter 5, a wide range of techniques for location management have
been introduced in cellular networks.
Cell Design and Frequency Utilization
Most cells are between 2 to 20 miles in radius depending on the population density and the
strength of the signal. Some cells are smaller (1.5 km appears to be a practical small limit)
while the others are larger (in fact a geosynchronous satellite can be thought of as a large cell
that can cover around 13,000 kilo meters). Each cell is served by a base transceiver station
(BTS), commonly known as a base station, consisting of transmitter, receiver, and a control
unit. Base station (BS) antennas are placed in high places such as churches and high rise
buildings. In several large cities, wireless operators pay around $500 per month for a base
station (yet another way for the landlords to make more money in large cities!). Cells are set
up such that antennas of all neighbors are equidistant from a base station. This is why
hexagonal patterns are used to represent cells.