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Wireless Mobile Communication
Question Bank with Solutions
2 MARKS QUESTIONS previous question paper questions
- What is Microcell? State it Drawbacks
Microcell network is served by a low power cellular base station and is connected to base station by fiber/microwave link, Drawbacks : Dropped Calls,Cell Dragging Cell Dragging : Slow moving mobiles (pedestrians) may have very slow decay in RSSI and may stay with one Base Station until it has moved deep within another cell. This results in Channel Interference.
2.What are the three basic propagation mechanisms which impact propagation in a mobile communication system?
Reflection ,Diffraction and Scattering are the basic propagation mechanism which impact propagation in mobile communication system. Reflection : occurs when propagating electromagnetic wave impinges upon on object which has very large dimension compared to the wavelength of propagating wave. It occurs from surface of earth and from buildings and walls. Diffraction : Radio path between transmitter and receiver is obstructed by a surface that has sharp irregularities (edges). Scattering : Occurs when medium through which wave travels consists of objects with dimensions that are small compared to the wavelength and where the number of obstrucles per unit volume is large
3.If a cellular operator is allocated 12.5 Mhz for each simplex band and if Bt is 12.5 Mhz Bguard is
10khz and Bc is 30khz find the number of channels available in a FDMA system
The number of channels available in FDMA system is N= Bt - 2Bguard Bc = 12.5 10^6 -2(1010^3 ) 3*10^3 =^416
4 Define Jamming Margin
The level of interference (jamming) that a system is able to accept and still maintain a specified bit error ratio even though the signal to noise ratio is decreasing. Or It is the maximum jamming power to signal power ratio that a spread spectrum receiver can tolerate while still maintaining the specified bit error rate
of average powers of interference j and data signal ps
- What is the cut off frequency of baseband ,Gaussian ,pulse shaping filter used in GSM System?
Baseband filter cut off frequency from few kilohertz to 20 Mhz & Gaussian filter 5 Mhz
6.Why Hexagon Geometry are always proffered ?Explain
Hexagon Compared to circle has largest area and therefore has large number of users.
A Cell must be designed to serve the weakest mobile within footprint
Hexagon cell is universally adopted and manageable in handling performance analysis & System modeling
Base Station transmitter placed either in centre of cell or in edge of cell,
- What is trunking in cellular Radio system?
Trunking allows a large number of users to share small number of channels in a cell by providing access to each user on demand from set of available channels.
In a trunked system each user will be allocated a channel on a per call basis and when terminated the pervious occupied channel is returned to pool of available channels.
- Find the far field distance for an antenna with maximum dimension of 1m and operating
frequency of 900 mhz
Df =2D^2 /lambda = lambda = C /V = 3*10^8 /900 * 10^6 =.
Df = 2*(1)^2 / .33 = 6m
ESSAY Questions
- Explain adjacent and co channel interference Interference and System Capacity
Interference is the major limiting factor in the performance of cellular radio systems. Sources of interference another mobile in the same cell a cell in progress in a neighboring cell other base stations operating in the same frequency band...
- the average received power at a distance d from the transmitting antenna is approx. by
OR
where P 0 is the power received at a close-in reference point in the far field region of the
antenna at a small distance d 0 from the transmitting antenna, and n is the path loss exponent
(is the reduction of power density of electromagnetic wave propogates through space)
Adjacent Channel Interference
- Results from imperfect receiver filters which allow nearby frequencies to leak into the pass band.
- Near-far effect (the adjacent channel interference is particularly serious.)
- An adjacent channel user is transmitting in very close range to s subscriber’s receiver, while the receiver attempts to receive a base station on the desired channel.
- It also occurs when a mobile close to a base station transmits on a channel close to one being used by a weak mobile.
- The base station may use by a weak mobile. The base station may have difficulty in discriminating the desired mobile user from the “bleed over” caused by the close adjacent channel mobile
- Adjacent channel interference can be minimized through careful filtering and channel assignments.
- Since each cell is given only a fraction of the available channels, a cell need not be Assigned channels which are all adjacent in frequency.
- By keeping the frequency separation between each channel in a given cell as large as possible, the adjacent channel interference may be reduced considerably.
- Channel allocation schemes also prevent a secondary source of adjacent channel interference by avoiding the use of adjacent channels in neighboring cell sites..
- If the subscriber is at the distance d1 and interferer is at d2 then Signal To Interference Ration is o (^) S/I = (d1/d2)n
- where n is path loss exponent
Power Control for Reducing Interferences
n r d
d P = P ( )− 0
0
( ) ( ) 10 log( ) 0
0 d
d Pr dBm = P dBm − n
- In practical cellular radio and personal communication systems, the power levels transmitted by every mobile unit are under constant control by the serving base stations.
- This is done to ensure that each mobile transmits the smallest power necessary on the reverse channel.
- Power control not only helps prolong battery life, also reduces the signal to interference ratio on the reverse channel.
- It is especially important for CDMA systems, because every user in every cell share the same radio channel. (to reduce the co-channel interference).
- Derive the expression for blocking probability (Erlang B formula ) of a trunked system which provides no queuing for blocked calls
Trunking and Grade of Service
- Cellular radio systems rely on trunking to accomodate a large number of users in a limited radio spectrum
- In a trunked radio system, each user is allocated a channel on a per call basis and upon termination of the call, the previously occupied channel is immediately returned to the pool of available channels
- The fundamentals of trunking theory were developed by Erlang
- One Erlang represents the amount of traffic intensity carried by a channel that is completely occupied (i.e. one call-hour per hour or one call-minute per minute)
- The GOS is a measure of the ability of a user to access a trunked system during the busiesthour
- GOS is typically given as the likelihood that a call is blocked or the likelihood of a call experiencing a delay greater than a certain queuing time
- The traffic intensity offered by each user is equal to the call request rate multiplied by the holding time
- Each user generates a traffic intensity of Au Erlangs given by
- For a system containing U user and an unspecified number of channels, the total
offered traffic intensity is
- In a C channel trunked system, if the traffic is equally distributed among the channels, then the traffic intensity per channel is
- There are two types of trunked systems: blocked calls cleared and blocked calls delayed
- The path loss for free space model when antenna gains are included is given by
- When antenna gains are excluded the antenna is assumed to have unity gain and path loss is given by
- Explain three propagation mechanism which impacts propagation in mobile
communication system
- Reflection, diffraction and scattering are the three fundamental phenomena that cause signal propagation in a mobile communication system, apart from LOS communication.
- The most important parameter, predicted by propagation models based on above three phenomena, is the received power.
- The physics of the above phenomena may also be used to describe small scale fading and multipath propagation
REFLECTION
Reflection occurs when an electromagnetic wave falls on an object, which has very large dimensions as compared to the wavelength of the propagating wave. For example, such objects can be the earth, buildings and walls. When a radio wave falls on another medium having different electrical properties, a part of it is transmitted into it, while some energy is reflected back. If the medium on which the electromagnetic wave is incident on a perfect dielectric, some energy is reflected back and some energy is transmitted. If the medium is a perfect conductor, all energy is reflected back to the first medium. The amount of energy that is reflected back depends on the polarization of the e.m. wave.,anglae of incidence & frequency of propagating wave. The electric field intensity of reflected and transmitted wave can be related to FRESNEL COEFFICIENT gamma.
POLARIZATION
- Electromagnetic waves are polarized ie because of passive reflectors ,they have
instantaneous electromagnetic field components in orthogonal directions in space.
- A Polarized wave can represented as a sum of two spatially orthogonal components
Vertical or horizontal Left hand or Right hand Circularly Polarized
- Polarization can be used as a degree of freedom for frequency planning
Reflection From Dielectrics
- There is an incident wave. Which broken up into two orthogonal components EI and HI.
- EI is in the plane of the paper. HI is orthogonal to the plane of the paper and it impinges on the surface with another dielectric.
- Part of it is reflected back as an Er and Hr and part of it is transmitted as an ET.
- Parallel” refers to the E-field having direction parallel to the plane of incidence (as in
Figure (a));
- “perpendicular” means perpendicular (normal) to the plane of incidence (as in Figure
b)).
- subscripts i, r, and t to refer to the incident, reflected, and transmitted field.
- ǫ1, ǫ2, is the permittivity of medium 1 and 2.
- μ1, μ2 is the permeability of medium 1 and 2.
- The intrinsic impedance of medium is = √Permeability / Permitivity
- We can calculate how much energy is actually reflected back by knowing the permeability and the permittivity of the dielectric.
- the total transmitted and received electric fields are
- the path difference is
- Two propagating waves arrive at the receiver, one LOS wave which travels a distance of d’ and another ground reflected wave, that travels d’’ , when T-R
separation distance is very large
- The method of image is used to find the path difference between LOS line Of sight & Ground Reflected Path.
- Once the path difference is known, the phase difference is
- The time difference,
fc carrier frequency ƛ wavelength
received power Pr is nothing but the transmit power Pt times Gt the gain of the transmit antenna times gain of the received antenna Gr times square of height of transmitter & receiver antenna line of sight propagation times d squared whole squared
DIFFRACTION
- This occurs when the radio path between the transmitter and receiver is obstructed by a surface that has sharp irregularities or edges. Edges, corners, bends, etc. will cause diffraction.
- Diffraction is very important because otherwise without line of sight, it would not be able to receive any signal from my base station.
- Hence this explains how radio signals can travel urban and rural environments without a clear line of sight and diffraction can be explained by Huygens principle.
- IT says that all points on a wave front can be considered as point sources for the production of secondary wavelets.
- When signal transmit form the transmitter T. it radiate in all directions assuming. It‟s an omnidirectional antenna.
- The wave radiates in all direction.
- The one that hits at a point will generate wavelets which will travel from all directions.
- From Huygens secondary source principle, any point on the wave front will generate its secondary wavelets.
- The wave which is diffracted and will be received at the receiver.
FRESNEL ZONE GEOMETRY
- The Concept of diffraction loss as a function of the path difference around an obstruction is explained by Fresnel zones
- Fresnel zone represent successive regions where secondary waves have a path length from transmitter to receiver greater than the total path length of line of sight path.
- The concentric circle on the plane represent secondary wavelete which propagate to the receiver
- These circles are called Fresnel Zones
- The successive Fresnel zone have the effect of alternately providing constructive and destructive interference to the total received signal.
- The radious of n
th (^) Fresnel Zones circle is denoted by r n and can be expressed in terms of n,ƛ,d 1 and d 2
- The difference between direct path and diffracted path is called excess path length Can be explained from the geometry
- The corresponding phase difference is Ø
replace the buildings by two knife edge diffractors.
The position and the height of this equivalent knife edge will depend on the heights and
distances of the first two original knife edges.
SCATTETING
The actual received signal in a mobile radio environment is often stronger than what is
predicted by reflection and diffraction models alone.
This is because when a radio wave impinges on a rough surface the reflected energy is spread
out in all directions due to scattering.
Scattering as we all know occurs when the medium has object, smaller or comparable to the
wavelength.
Small objects, rough surfaces rain drops, other irregularities in the channel, dust dew drops will
cause scattering
Scattering follows the same principle as diffraction. It causes the transmitter energy to be
radiated in many directions. So foliage, street signs, lamp posts, edges can cause scattering
Surface roughness is often tested using Rayleigh criterion which defines critical height hc for an
given angle of incidence Øi is given by
RADAR CROSS SECTION MODEL
RCS Radar Cross Section of a scattering object is defined as the ratio of the power density of
signal scattered in the direction of the receiver to the power density of the radio wave incident
upon the scattering object and has units of square meters,
For urban mobile radio systems models based on the bistatic radar equation may be used to
compute the received power due to scattering in the far field.
As a mobile moves through a coverage area, these three mechanisms have an impact on the
instantaneous received signal strength.
if the mobile does have a clear line of sight then diffraction and scattering will not dominate
the propagation. The line of sight exists.we have a clear signal strength.
if a mobile is at street level without line of sight then diffraction and scattering will probably
dominate the propagation. So this is important models exists for all of this.
So it is possible to figure out theoretically and by stimulation how much is the received power
actually obtained.
- explain concept of multiuser detection
- Idea is proposed in 1980
- Primary Idea of Multiuser detection techniques is to cancel the interference and noise caused by other users
- Total Interference (Includes interference due to other cells also)
- Near Far Problem in CDMA
o Difficulty to implement more sophisticated algorithm at receiver because of limitations of size, cost, weight of handset.
o Solutions to all such problems is Multi-user detection
- It is done by exploiting information of interfering user rather than ignoring the presence of other user like in single user detection technique and help to overcome near far problem
Features
- Reduced interference leads to capacity increase
- Alleviates the near/far problem
- Capability to reject interference created by narrow band
- Uniform distribution Distribution of numbers in the sequence should be uniform Frequence of occurance of each of numbers should be approximately same. For a stream of binary digits two properties are desired
Balance property : in long sequence the number of binary ones always one more than the number of 0’s.
Run property: run is defined as a sequence of all 1-s or a sequence of all 0-s.Among the runs of 1’s and 0’s in each period of sequence one half the runs of each kind are of length one,one fourth of length two,one eigth of length three and so on as long as these fractions represent meaningful number of runs.
Auto Correlation Property : Autocorrelation function of a maximal length sequence is periodic and binary valued. The periodic autocorrelation of a ±1 m-sequence
is
Correlation Property :. the cross-correlation of two m -sequences tends to be large. If the codes which are used are not completely orthogonal, the cross- correlation factor is unequal to zero. In this situation the different users are interferers to each other, hence the near-far problem appears
good auto-and cross-correlation properties
Independence
no one value in sequence can be inferred from the others
7)State Data Rates and application of Bluetooth and Zigbee Bluetooth
Wireless technology standard for exchanging data over short distance from fixed and mobile devices. It connect several devices overcoming problems of synchronization Data rate 1Mbps Operate in a range 10m Range of frequency 2400-2483.5 Mhz and 2.5 Ghz short range operation
Applications
- Wireless Control of communication between mobile phone and handsfree handset
- Communication between mobile and car stereo system
- Wireless networking
- Low bandwidth application
- Transfer file ,contact ,calendar ,remainder between devices
Zigbee
- Can transmit data over a long distance
- Low data rate ,long battery life & secure networking
- Data rate 20-250Kbps
- Range of operation 10-100m
- 2.4GHz frequency band
Applications
- ZigBee is used in applications that require only a low data rate, long battery life, and secure networking.
wireless light switches,
electrical meters with in-home-displays,
traffic management systems, and
ther consumer and industrial equipment that requires short-range wireless
transfer of data at relatively low rates.
Remote sensing & control
- Explain TDMA
Time Division Multiple Access (TDMA)
- Time division multiple access (TDMA) systems divide the radio spectrum into time slots, and in each slot only one user is allowed to either transmit or receive.
- TDMA systems transmit data in a buffer-and-burst method, thus the transmission for any user is non continuous.
- This implies that, unlike in FDMA systems which accommodate analog FM, digital data and digital modulation must be used with TDMA.
- The transmission from various users is interlaced into a repeating frame structure as shown in Figure. It can be seen that a frame consists of a number of slots.
- Each frame is made up of a preamble, an information message, and tail bits.
idle slot in the TDMA frame.
- TDMA uses different time slots for transmission and reception, thus duplex ers are not required.
- Even if FDD is used, a switch rather than a duplexer inside the subscriber unit is all that is required to switch between transmitter and receiver using TDMA.
- Adaptive equalization is usually necessary in TDMA systems, since the transmission rates are generally very high as compared to FDMA channels.
- In TDMA, the guard time should be minimized. If the transmitted signal at the edges of a time slot are suppressed sharply in order to shorten the guard time, the transmitted spectrum will expand and cause interference to adja cent channels.
- High s y n c h r o n i z a t i o n overhead is required in TDMA systems because of burst transmissions.
- TDMA transmissions are slotted, and this requires the receivers to be synchronized for each data burst. In addition, guard slots are necessary to separate users, and this results in the TDMA systems having larger overheads as compared to FDMA.
- TDMA has an advantage in that it is possible to allocate different numbers of time slots per frame to different users. Thus bandwidth can be supplied on demand to different users by concatenating or reassigning time slots based on priority.
EFFICIENCY OF TDMA:
- The efficiency of a TDMA system is a measure of the percentage of transmitted data that contains information as opposed to pro viding overhead for the access scheme.
- The frame efficiency, 'fir• is the percentage of bits per frame which contain transmitted data. Note that the transmitted data may include source and channel coding bits, so the raw end-user efficiency of a system is generally less than 'fir.
- The frame efficiency can be found as follows.
- The number of overhead bits per frame is [Zie921],
where, Nr is the number of reference bursts per frame, N, is the number of traffic bursts per frame, br is the number of overhead bits per reference burst, bP is the number of overhead bits per preamble in each slot, and bg is the number of equivalent bits in each guard time interval. The total number of bits per frame, br, is
- where Tr is the frame duration, R is the channel bit rate.
- The frame efficiency is thus given as
Number of channels in TDMA system
- The number of TDMA channel slots that can be provided in a TDMA system is found by multiplyin g the number of TDMA slots per channel by the number of channels available and is given by
where m is the maximum number of TDMA users supported on each radio chan nel. Note that two guard bands, one at the low end of the allocated frequency band and one at the high end, are required to ensure that users at the edge of the band do not "bleed over" into an adjacent radio service.
