Data and Computer Communications - Chapter 10
The time variation of received signal power caused by changes in the transmission medium is referred to as ________, which is a major technical problem for cellular networks.
fading
Nonselective, or ________, is the type of fading in which all frequency components of the received signal fluctuate in the same proportions simultaneously.
flat fading
As the mobile units move they pass from cell to cell, which requires transferring of the call from one base transceiver to another. This process is called ________.
handoff
________ occurs at the edge of an impenetrable body that is large compared to the wavelength of the radio wave. When a radio wave encounters such an edge, waves propagate in different directions with the edge as the source. Thus, signals can be received even when there is no unobstructed line-of-sight from the transmitter.
Diffraction
________ are useful in city streets in congested areas, along highways, and inside large public buildings.
Microcells
Scattering effects are difficult to predict.
True
Key differences between first- and second-generation cellular systems.
-Digital traffic channels : 1G is purely analog and support voice channels using FM; 2G is digital and provide digital traffic channels -Encryption : 2G uses this, 1G doesn't -Error detection and correction : 2G uses this -Channel access : -in 1G each cell supports a number of channels, and at any given time a channel is allocated to only one user. -2G systems provide multiple channels per cell, but each channel is dynamically shared by a number of users using TDMA (time division multiple access) or CDMA (code division multiple access).
Second generation cellular networks were analog.
False - first generation. Second generation was digital
One of the most widely used models to develop guidelines for cell size was developed by ________.
Okumura/Hata
The term ________ refers to the provision of multiple fixed data rate logical channels to a given user, in which different data rates are provided on different logical channels. The traffic on each logical channel can be switched independently through the wireless and fixed networks to different destinations.
multirate
_______ carry a voice or data connection between users.
Traffic channels
Mobile radio communication introduces complexities not found in wire communication or in fixed wireless communication.
True
________ occurs if the size of an obstacle is on the order of the wavelength of the signal or less. An incoming signal is scattered into several weaker outgoing signals. At typical cellular microwave frequencies, there are numerous objects, such as lamp posts and traffic signs that can cause scattering. Thus, scattering effects are difficult to predict.
Scattering
________ is the type of fading that affects unequally the different spectral components of a radio signal. It is usually significantly only relative to the bandwidth of the overall communications channel.
Selective
List the 5 ways of increasing the capacity of a cellular system.
-Adding new channels -Frequency borrowing -Cell splitting -Cell sectoring -Microcells
Steps in a typical call between two mobile users within an area controlled by MTSO.
-Mobile unit initialization : when the mobil unit is turned on , it scans and selects the strongest setup control channel used for this system. -Mobile-originated call : a mobile unit originates a call by sending the number of the called unit on the preselected setup channel. -Paging : the MTSO then attempts to complete the connection to the called unit. The MTSO sends a paging message to certain BSs depending on the called mobile number. -Call accepted : the called mobile unit recognizes its number on the setup channel being monitored and responds to that BS, which sends the response to the MTSO. -Ongoing call : while the connection is maintained, the two mobile units exchange voice or data signals, going through their respective BSs and the MTSO. -Handoff : if a mobile unit moves out of range of one cell and into the range of another during a connection, the traffic channel has to change to one assigned to the BS in the new cell.
Since the early 1980s the most common first-generation system in North America has been the ________ developed by AT&T.
AMPS (advanced mobile phone service)
________ can be applied to transmissions that carry analog information or digital information and is used to combat intersymbol interference.
Adaptive equalization
A technique in widespread use in second generation cellular networks is ________.
CDMA (code division multiple access)
Second generation systems provide multiple channels per cell, but each channel is dynamically shared by a number of users using time division multiple access or ________.
CDMA (code division multiple access)
The dominant technology for third generation systems is ___________.
CDMA (code division multiple access)
________ are used to exchange information having to do with setting up and maintaining calls and with establishing a relationship between a mobile unit and the nearest base station.
Control channels
Even though cells are sufficiently distant from each other, they still need to be assigned different frequencies to avoid interference or crosstalk.
False - adjacent cells are assigned different frequencies to avoid interference or crosstalk. Cells sufficiently distant from each other can use the same frequency band.
Only one mobile user unit may be active and communicating with the base station at a time.
False - any number of mobile units may be active
4G mobile users can still connect with 3G network users but can no longer connect with PSTN subscribers.
False - can connect with PSTN subscribers also
The area to be covered in a cellular network is divided into cells in a pentagonal tile pattern that provide full coverage of the area and equidistant antennas.
False - hexagonal
All first and second generation cellular networks provide encryption.
False - only second generation. First generation systems send user traffic in the clear, providing no security.
In a mobile environment one of the two antennas is moving relative to the other, therefore the relative location of various obstacles moves also, so as not to hinder transmission.
False - the relative location of various obstacles changes over time, creating complex transmission effects.
______ is when the signal is spread out over a larger frequency bandwidth or carried on multiple frequency carriers.
Frequency diversity
The ________ connects calls between mobile units, makes connections between a fixed subscriber to the public network and a mobile subscriber to the cellular network, assigns the voice channel to each call, performs handoffs, and monitors the call for billing information.
MTSO (mobile telecommunications switching office)
The key technology for 4G cellular systems is ________.
OFDMA (orthogonal frequency division multiple access)
________ occurs when an electromagnetic signal encounters a surface that is large relative to the wavelength of the signal.
Reflection
Explain the paging function of a cellular system.
The mobile identification number (MIN) is broadcast over all the forward control channels throughout the cellular system. To complete a call to a mobile unit, the base stations in a number of cells will send out a page signal in an attempt to find the mobile unit and make the connection.
3G cellular systems support multiple simultaneous applications from a given user because of its multirate feature.
True
A wireless cellular system limits the opportunity to use the same frequency for different communications because the signals can interfere with one another even if geographically separated.
True
An important design goal for all 3G systems is to limit channel usage to 5 MHz.
True
Cells in areas of high usage can be split into smaller cells.
True
Cellular technology is the foundation of mobile wireless communications.
True
One of the minimum requirements of 4G cellular system is it must be based on an all-IP packet switched network.
True
One unwanted effect of multipath propagation is that multiple copies of a signal may arrive at different phases.
True
The efforts to compensate for the errors and distortions introduced by multipath fading fall into three general categories: forward error correction, diversity techniques, and ________.
adaptive equalization
Each cell in a cellular network is allocated a band of frequencies and is served by a ________ consisting of transmitter, receiver and control unit.
base station
Affecting not only mobile phones in automobiles, but even a mobile phone user walking down an urban street, ________ occurs when there are rapid variations in signal strength. As the mobile unit moves down a street in an urban environment, rapid variations in signal strength occur over distances of about one-half a wavelength.
fast fading
First generation cellular networks made use of ________ multiplexing.
frequency division
In cellular system operation, a ________ is a process used to identify the user and register its location.
hanshake
Each base station is connected to a ________, which serves multiple base stations.
mobile telecommunications switching office (MTSO)
Third generation wireless communication supports ___________.
multimedia and voice
N = the number of cells in a repetitious pattern, (each cell in the pattern uses a unique band of frequencies), is termed the ________.
reuse factor
If there is no clear line-of-sight to the transmitter, such as in an urban area at street level, then diffraction and ________ are the primary means of signal reception.
scattering
As the mobile user covers distances well in excess of a wavelength, the urban environment changes as the user passes buildings of different heights, vacant lots, and intersections. Over these longer distances, there is a change in the average received power level about which the rapid fluctuations occur. This is referred to as ________.
slow fading
A cellular network uses multiple lower power _________.
transmitters
First generation cellular networks ____________.
were analog