2.cellular Concepts

RF

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RADIO FREQUENCY 

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The term Radio Frequency (RF or rf) refers to the electromagnetic field that is generated when an alternating current is input to an antenna. This field, also called an RF field or radio wave This can be used for wireless broadcasting and communications and many other purposes over a significant portion of the electromagnetic radiation spectrum In vacuum, all electromagnetic waves travel at the speed of light: c = 3x108 m/sec. 2

TRANSMISSION PROBLEMS  Many

problems may occur during the transmission of a radio signal.  Some of the most common problems are described below.   

Path Loss Shadowing Fading

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PATH LOSS  Path

loss occurs when the received signal becomes weaker and weaker due to increasing distance between MS and BTS, even if there are no obstacles between the transmitting (Tx) and receiving (Rx) antenna.  The path loss problem seldom leads to a dropped call because before the problem becomes extreme, a new transmission path is established via another BTS. 4

Shadowing  Shadowing

occurs when there are physical obstacles including hills and buildings between the BTS and the MS.  The obstacles create a shadowing effect which can decrease the received signal strength.  When the MS moves, the signal strength fluctuates depending on the obstacles between the MS and BTS. 5

Shadowing contd.., A

signal influenced by fading varies in signal strength.  Drops in strength are called fading dips.

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RF Propagation  Basic   

propagation models

Reflection Diffraction Scattering

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Reflection  Reflection

occurs when a propagating electromagnetic wave is incident upon an object which has very large dimensions when compared to the wavelength of the propagating wave  Reflection occurs from the surface of the earth and from buildings and walls

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Diffraction  Diffraction

occurs when the radio path between the transmitter and receiver is obstructed by a surface that has sharp irregularities (edges)  Building edges, roof tops

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Scattering 

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Scattering occurs when the medium through which the wave travels consist of objects with dimensions that are small compared to the wavelength . Rough surfaces , Sign posts Essentially, the radio waves interact with the physical environment along each of these paths. There are typically (unless you are in free-space) many paths from the transmitter to the receiver. Each path is called a multipath. 10

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Fading  Fading

occurs when there is more than one transmission path to the MS or BTS, and therefore more than one signal is arriving at the receiver.  This may be due to buildings or mountains, either close to or far from the receiving device.

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Rayleigh Fading  This

occurs when a signal takes more than one path between the MS and BTS antennas.  In this case, the signal is not received on a line of sight path directly from the Tx antenna.  Rather, it is reflected off buildings, for example, and is received from several different indirect paths.

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Rayleigh Fading

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Mobile Radio Propagation Effects 

Signal strength 

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Must be strong enough between base station and mobile unit to maintain signal quality at the receiver Must not be so strong as to create too much interference with channels in another cell.

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Attenuation due to rain 

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Presence of raindrops can severely degrade the reliability and performance of communication links

Trees near subscriber sites can lead to multipath fading Loss of strength of signal due to absorption by trees, plants, parks (greenary) is called vegetation losses

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SOLUTIONS TO TRANSMISSION PROBLEMS  Some

solutions to the problems described in previous sections are   

Channel Coding Interleaving Diversity

 Although

many of these do not entirely solve all problems on the radio transmission path, they do play an important part in maintaining call quality for as long as possible. 17

Channel Coding  In

digital transmission, the quality of the transmitted signal is often expressed in terms of how many of the received bits are incorrect.  This is called Bit Error Rate (BER).  BER defines the percentage of the total number of received bits which are incorrectly detected. 18

Channel Coding contd..,  This

percentage should be as low as possible. It is not possible to reduce the percentage to zero because the transmission path is constantly changing.

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Channel Coding contd..,  This

means that there must be an allowance for a certain amount of errors and at the same time an ability to restore the information, or at least detect errors so the incorrect information bits are not interpreted as correct.  Channel coding is used to detect and correct errors in a received bit stream. 20

Multiple Antennas Diversity 

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One way to overcome fading problem is to design receivers with multiple, say 2, antennas. Both antennas can receive the desired radio wave.

Transmitter Receiver

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Multiple Antennas (Cont’d) 

If receive antennas are adequately separated, then paths followed by radio waves to the first antenna are different from paths followed by radio waves to the second antenna.

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Result: fading of received signal on first antenna is different from fading of received signal on second antenna. Chances are if one antenna experiences a deep fade, the other does not.

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Receiver can adaptively choose the “stronger” antenna to determine the received radio wave at any given time.

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Tilt 

Mechanical Tilt

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Electrical Tilt

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CELLS A

cell may be defined as an area of radio coverage from one BTS antenna system.  It is the smallest building block in a mobile network and is the reason why mobile networks are often referred to as cellular networks.  Typically, cells are represented graphically by hexagons. 24

The Cellular concept

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Cell Characteristics 

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The Basic Union In The System  defined as the area where radio coverage is given by one base station. A cell has one or several frequencies, depending on traffic load.  Frequencies are reused, but not used in neighboring cells due to interference.  Each served by its own antenna  Served by base station consisting of transmitter, receiver, and control unit Use multiple low-power transmitters Cell sizes from some 100 m in cities to, e.g., 35 km on the country side (GSM) 26

Different types of cells

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Pico cells covering building interiors, Micro cells covering selected outdoor areas, and Macro cells for more wider areas like roads.

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Capacity and Frequency Re-use  

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It is the number of frequencies in a cell that determines the cell’s capacity. However, a cellular network can overcome this constraint and maximize the number of subscribers that it can service by using frequency re-use. Each company with a license to operate a mobile network is allocated a limited number of frequencies. Depending on the traffic load and the availability of frequencies, a cell may have one or more frequencies allocated to it.

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 It

is important when allocating frequencies that interference is avoided.  Frequency re-use means that two radio channels within the same network can use exactly the same pair of frequencies, provided that there is a sufficient geographical distance.  The tighter frequency re-use plan, the greater the capacity potential of the network. 30

Frequency Re-use contd..,  Interference

can be caused by a variety of

factors.  A common factor is the use of similar frequencies close to each other. The higher interference, the lower call quality.  To provide coverage to all the subscribers, frequencies must be reused many times at different geographical locations in order to provide a network with sufficient capacity. 31

Frequency Re-use contd.., 

The same frequencies can not be re-used in neighboring cells as they would interfere with each other so special patterns of frequency usage are determined during the planning of the network.

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Example of Frequency Reuse

Cells using the same frequencies 33

Frequency Re-use contd..,  These

frequency re-use patterns ensure that any frequencies being re-used are located at a sufficient distance apart to ensure that there is little interference between them.  The term “frequency re-use distance” is used to describe the distance between two identical frequencies in a re-use pattern.  The lower frequency re-use distance, the more capacity will be available in the network. 34

Frequency Hopping 

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For each cell(sector)the hopping sequence is the same. Information of the sequence is broadcast on the control channels. Hopping sequence must be different on each BTS site. Frequency that sends the control channels must remain on the same frequency and power level.

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Frequency Hopping contd..,

F1 F2 F3 F4 Time

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Interference  

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Co-channel Interference (C/I) Cellular networks are more often limited by problems caused by interference rather than by signal strength problems. Co-channel interference is caused by the use of a frequency close to the exact same frequency. The former will interfere with the latter, leading to the terms interfering frequency (I) and carrier frequency (C). 37

Adjacent channel interference (C/A) 

Although adjacent frequencies are at different frequencies to the carrier frequency they can still cause interference and quality problems.

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Approaches to Cope with Increasing Capacity   

Frequency borrowing – frequencies are taken from adjacent cells by congested cells Cell splitting – cells in areas of high usage can be split into smaller cells Cell sectoring – cells are divided into a number of wedge-shaped sectors, each with their own set of channels

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Cell Splitting

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Cell splitting   

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Decrease transmission power in base and mobile Results in more and smaller cells Reuse frequencies within same cell groups

Cell sectoring  

Directional antennas subdivide cell into 3 or 6 sectors Might also increase cell capacity by factor of 3 or 6

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The effects of decreasing cell site size:  Several small cells instead of a single transmitter=> frequency reuse  better efficiency  Lower power consumption in MS as a result more talk time for portables

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Handoff or Handover 

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A crucial component of the cellular concept is the notion of handoffs. Mobile phone users are by definition mobile, i.e., they move around while using the phone. Thus, the network should be able to give them continuous access as they move. This is not a problem when users move within the same cell. When they move from one cell to another, a handoff is needed. BS-1

BS-243

Handoff contd.., 

The process of changing cells during a call is called handover in GSM terminology.

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A user is transmitting and receiving signals from a given base station, say B1.

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Assume the user moves from the coverage area of one base station into the coverage area of a second base station, B2.

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B1 notices that the signal from this user is degrading.

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B2 notices that the signal from this user is improving. 44

Cluster  

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Combination of cells with unique frequency Adjacent cells are allocated different frequency so that they can overlap with out causing interference Min no of cells required to form a cluster is 3 A cluster is a group of cells in which all available frequencies have been used once and only once.

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Cell Patterns  Standard

model using 7 frequencies:

3 2

K=7

4 1 7

5

1

6

3

2

K=3

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f3 f1 f2 f3

f2 f3 f1

f3 f1 f2 f3

f2 f3 f1

f3 f1 f2 f3

3 cell cluster

f2 f4 f3 f6

f5 f1 f2

f3 f6 f7 f5

f2 f4 f3

f7 f5 f1 f2

7 cell cluster

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