Multiple Access Techniques And Cellular Concept-UNIT-I
Part-A
1. What is multiple access?
Multiple
access is a signal transmission situation in which two or more users wish to simultaneously communicate with each other using the same propagation channel.
The multiple access methods are used in
(i) Satellite networks
(ii) Cellular and mobile communication networks
(iii) Military communication and
(iv) Underwater acoustic networks
3. Mention the types of multiple access
techniques.
The types of multiple access techniques
(i) Frequency division multiple access(FDMA) (ii) Time division multiple access
(TDMA)
(iii) Code division multiple access(CDMA) (iv) Space division multiple access (SDMA)
4. What is narrow band system?
The term narrow band is used to relate the bandwidth of a single channel
to
the expected
coherence bandwidth of the channel.
In a narrowband multiple access system, the available radio spectrum is divided into a large number of narrowband channels.
5. What is wideband system?
In wideband systems, the
transmission bandwidth of a single channel is much larger than the coherence bandwidth of the channel. In this system, a large number of transmitters are
allowed to transmit on the same channels.
6. Define FDMA.
In FDMA, the total bandwidth is divided into non-overlapping frequency sub bands. Each user
is allocated a unique frequency sub band (channels) for the duration of the connection, whether the connection is in an active or idle state.
7. What is the need of guard bands in FDMA?
The adjustment frequency bands in the FDMA spectrum are likely to interference with each other. Therefore it is necessary to include the
guard bands between the
adjacent frequency bands.
8. Mention some features of FDMA.
The
features of FDMA
are
(i) FDMA is relatively simple to implement.
(ii) The FDMA channel carries only one phone circuit at a time.
(iii) After the assignment of a voice channel, the base station and the mobile transmit simultaneously and continuously.
(iv) To provide interference-free transmission
between the uplink
and the downlink
channels,
the frequency allocations have
to
be
separated by
a sufficient amount
(guard bands)
9. Write the nonlinear effects in FDMA.
In a FDMA system, many channels share the same antenna
at the base station. The power amplifiers and the power combiners used are nonlinear and tend to generate inter modulation frequencies resulting in inter modulation distortion.
To minimize the effects of inter modulation distortion, stringent RF filters are required to reject inter modulation distortion; RF filters are heavy, cumbersome and costly.
Write the expression for number of channels used in FDMA system.
The number of channels that can be simultaneously supported in a FDMA system is given by
|
B − 2B N = S g
BC
Where, BS==total spectrum allocation (or) system bandwidth
Bguard = guard band allocated at the edge of the
allocated spectrum band and
BC = Channel bandwidth
10. Write the formula for spectral efficiency of FDMA.
The spectral efficiency of FDMA is given by
|
|
Ndata BC
FDMA
BS
Where, Ndata = number of data channels in the system
and
is given by
Ndata = NS – Nctl
Where, Nctl = number of allocated control channels.
11. Mention the advantages of FDMA.
The
advantages of FDMA are
(i) It supports a narrow band, and is not suitable for multimedia communications with
various
transmission rates.
(ii) If an FDMA channel is not is use, then it is idle and cannot be used by other users to
increase or share capacity. It is essentially a wasted resource.
(iii) The
first US analog
cellular
system, the
advanced mobile
phone
system
(AMPS) is based on FDMA/FDD.
(iv) In AMPS, analog narrow band frequency modulation (NBFM) is used to modulate
the carrier.
(v) FDMA is an old and proven system and is used for the analog signal.
12. Define 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.
13. What is W-TDMA?
In wideband TDMA, transmission in each slot
uses the entire frequency band.
Define N-TDMA.
In narrow band TDMA, the whole frequency band is divided into sub bands;
transmission in each slot only uses the frequency width of one sub band.
14. Write the features of TDMA.
The
features of TDMA
are
i.
TDMA shares a single carrier frequency with several users, where each user makes use of non-overlapping time slots.
ii.
Data transmission for users of a TDMA system is not continuous, but occurs in
bursts. This
results
in low
battery
consumption, since the subscriber
transmitter can
be turned off when not
in use.
iii.
Because of discontinues transmissions in TDMA, the handoff process is much simpler for a subscriber unit, since it is able to listen for other base stations during idle time slots.
15. What is frame efficiency in TDMA?
The frame
efficiency
ηf is
the percentage of
bits per
frame which contain transmitted
data.
16. Write the expression for frame efficiency in TDMA?
The frame efficiency ηf is given by
b
|
η = 1 − OH ×100%
bT
17. Write the formula for number of channels in TDMA.
The number of
channel in TDMA is given by
m(B − 2B )
N = tot guard
BC
18. What is spreading signal in CDMA?
In CDMA systems, the narrowband message signal is multiplied by a very large bandwidth
signal called the spreading signal.
19. Define near-far problem in CDMA.
Some of the mobile units are close to the base station while others are far from it. A strong signal received at the base from a near-in mobile unit masks the weak signal from a far-end mobile unit.
This
phenomenon is called the near-far problem.
20. Define soft capacity in CDMA.
Unlike TDMA or
FDMA, CDMA has a soft capacity limit.
As in
practice,
the PN
sequences
are
not
truly
orthogonal,
multiple access
interference (MAI) or co-channel interference (CCI) will degrade the transmission BER performance.
The maximum number of users that can be supported in each cell depends on the required quality of service (QoS) and is limited by MAI.
21. Write a short note on soft hand off in CDMA.
Because
of
the universal frequency reuse, a mobile user
can simultaneously communicate with several nearby base stations using the same frequency band and the same spreading signal in each
link.
When the mobile user is at the cell boundary, it can establish a connection with
the
new base station before terminating the connection with the old base station, this will improve
handoff performance.
22. Define self-jamming problem in CDMA.
Self-jamming is a problem in CDMA system. Self-jamming arises from the fact that the spreading sequences of different users are not exactly orthogonal, hence in the dispreading of a
particular PN code, non-zero contributions to the receiver decision statistic for a desired user arise from the transmissions of other users in the system.
23. Write some features of CDMA.
(i) Many users of a CDMA system share the same frequency. (ii) Channel data rates are very high in CDMA system.
(iii) CDMA has more flexibility than TDMA in supporting multimedia service.
24. Define channel capacity.
Channel capacity for a radio system can be defined as the maximum number of channels or users that
can be provided in a fixed frequency band.
25.What is radio capacity?
Radio capacity is a parameter which measures spectrum efficiency of a wireless
system. This parameter is determined by the required carrier-to-interference radio(C/I) and the channel
bandwidth (BC)
26.What are the types of interference occur in a cellular system?
There are two types of interferences are occur in a cellular system.
i. Reverse
channel
interference and
ii.
Forward channel interference
27. Define reverse and forward channel interference.
Reverse channel interference:
in a cellular system, the interference at a base station receiver
will come from the subscriber units in the surrounding cells.
Forward channel interference: for a particular subscriber unit, the desired base station will provide the desired forward channel while the surrounding c0-channel base stations
will provide the forward channel interference.
28. Define co-channel reuse ratio.
The minimum ratio of D/R that is required to provide a tolerable level of co-channel interference is called the co-channel
reuse ratio and is given by
Q = D R
Where, D-Distance between two co-channel
cells and
R – Cell radius.
|
Write the expression for C/I.
|
− no
|
o
M
I D − nk
∑ K
k =1
Where, n0 – path loss exponent in the desired cell
D0 – Distance from the desired base station to the mobile. DK – Distance of the Kth cell from
the mobile and
nk – path loss exponent
to
the Kth interfering base station.
29.
How bit energy-to-noise ratio is obtained in cellular CDMS?
It
is
obtained by dividing the signal
power by the base band information bit rate R, and the interference power by the total RF bandwidth W.
30. What are the techniques used to improve
capacity of CDMA?
Reducing interference in antenna sectorization. Monitoring of voice activity such that each
transmitter is switched off during periods of no voice activity.
31. Write the cellular concept.
If a given set of frequencies or radio channels can be reused without increasing the interference, then the large geographical area covered by a single high power transmitter can be divided into a number of small areas, each allocated a subset of frequencies. With a small geographical coverage, lower power transmitters with lower antennas can be used.
32. What is cell
cluster?
Cell cluster: A group of cells that
use a different
set of frequencies in each cell
is
Called a cell cluster.
Cluster: The N cells which collectively use the complete set
of available
Frequencies is called a cluster.
33. Define cell.
Each cellular base station is allocated a group of radio channels to be used with small
geographic area called a cluster.
34. Write the frequency reuse concept.
Physical separation of two cells is sufficiently wide; the same subset of frequencies can be
used in both cells. This is the concept of
frequency reuse.
35. Define foot print.
The actual radio coverage of a cell is known as the foot print. It is determined from field measurements or propagation prediction models.
36. What are the rules to determine the nearest co channel neighbors?
The following two-steps rules can be used to determine the location of the nearest co channel cell:
Step1: Move i
cells along any chain of hexagons;
Step2: An
allowance for frequency reuse.
37.
Define dwell
time.
The time over
which a call
may be maintained within a cell, without
handoff, is called the
dwell time.
38. What are the methods used for handoffs?
Depending on the information used and the action taken to initiate the handoff,
the methods
for handoff can be
a)
Mobile Controlled Handoff(MCHO)
b)
Network controlled Handoff(NCHO)
c)
Mobile Assisted handoff(MAHO)
39. What is intersystem
handoff?
During a call if a mobile moves from one cellular system to a different cellular system controlled by a different MSC. This type of handoff is called intersystem handoff.
40. Write the guard channel concepts?
One method for giving priority to handoffs is called the guard channel concepts, i.e., out of total available channels in a cell the no. of channels are reserved for handoff for the outgoing calls.
41. Define hard handoff and soft handoff.
Hard handoff:
If the MSC monitors the strongest
signal base station and transfer the
call
to that base station then it
is
called hard handoff.
Soft handoff:
Mobile communicates with two or more cells at the same time and find which one is a strongest signal base station then it automatically transfer the call to that base station is
called soft handoffs.
42. Write the features of handoff.
a. Fast and lossless
b. Minimal number of control signal
exchanges.
c. Scalable with network size.
d. Capable of recovering from link failures.
e.
Efficient use of resources.
43. What is intracell interference?
Interference from other mobiles at the cell-site (base station) receiver in the same cell is
intracell interference.
44. What are the major types of cellular interference available?
The two major types of system-generated cellular interference are
(i) Co-channel
interference and
(ii) Adjacent channel
interference
45. What is breathing cell
effect?
When specific radio channels are in use, the CDMA system instead has a dynamic,
time
varying coverage region which varies depending on the instantaneous number of users on the CDMA radio channels.
This
effect is known as breathing cell effect.
46. Define the grade of service.
A measure of congestion which is specified as the probability of a call being blocked (for Erlang B) or the probability of a call being delayed beyond a certain amount of time(for Erlang C).
47. What is set-up time?
The time required to allocate a trunked radio channel
to
a requesting user.
48. Define blocked call.
Call which cannot be completed at time of request, due to congestion also referred to as
a lost
call.
49. Define holding time.
Average duration of
a typical call. It is denoted by H (in seconds).
50. Define traffic intensity.
Measure
of channel time
utilization,
which
is the
average
channel occupancy measured in Erlangs. This is a dimensionless quantity and may by used to measure the time utilization
of single or multiple channels. It is denoted by A.
PART- B
1. Explain elaborately about types of handoffs.
2. Explain in detail about
dropped call rate and cell splitting.
3. Explain the different techniques of improving coverage
and capacity in cellular system
4. Explain in detail about usage of repeater for coverage improvement.
5.
Explain in detail about the various trends in personal wireless communication
systems.
6. Narrate wireless
communication systems with the help of paging system.
7.
Write short notes on
(i) Cordless telephone system
(ii) Cellular telephone system
8.
Discuss briefly about Interference and system capacity in cellular systems.
9.
Explain in detail about trunking and grade of service in cellular systems
10. Explain in
detail about the various Multiple Access Schemes.
11. Explain in
detail about the handoff strategies with suitable diagram.
12. Explain in
detail about the umbrella cell concepts.
13. Discuss
briefly about Improving channel capacity in cellular systems.
14. Explain in detail about Co-channel
interference and system capacity.
15. Explain
spread spectrum with its types
16. what is need
for frequency reuse? Explain frequency reuse concept and show that N = i2 +
ij +j2.
Where N is the no of cells
17. derive
expression for signal to noise ratio for 7 cell cluster
18. Explain
channel assignment concept in detail
19. Explain
about CSMA protocols
20 . Explain about TDMA
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