1. What are TDMA frames,
multiframes, superframes and hyperframes?
2. Why do we need FCCH, SCH
and BCCH?
FCCH: Identifies BCCH carrier by the carrier frequency and
synchronizes with the frequency.
Synchronization Channel (SCH): Transmits information about the TDMA frame structure in a
cell (e.g. frame number) and the BTS identity (Base Station Identity Code
(BSIC)).
BCCH: Broadcasts some general cell information such as
Location Area Identity
(LAI), maximum output power allowed
in the cell and the identity of BCCH carriers for neighboring cells.
3. Why do we need SACCH?
Instructs the MS the
transmitting power to use and gives instructions on timing advance.
4. What is the purpose of PCH and
CBCH?
Paging Channel (PCH): Transmits a paging message to indicate an incoming call or
short message. The paging message contains the identity number of the mobile subscriber
that the network wishes to contact.
Cell Broadcast Channel (CBCH): BS uses this logical channel to transmit short message
service cell broadcast.
5. Do we keep BCCH on
a hopping radio? Give the reason to support your answer.
No, BCCH is a signaling
channel which must be continuously transmitted in a cell.
6. Explain the
structure of a Traffic Multiframe. Why do we need SACCH and Idle bursts in a
traffic multiframe?
Traffic Multiframe
Structures - The 26 traffic
multiframe structure is used to send information on the traffic channel. The 26
traffic multiframe structure is used to combine user data (traffic),
slow control
signaling (SACCH), and idle time
period. The 12th frame (no. 13) in the 26-frame traffic channel multiframe is
used by the Slow Associated Control Channel (SACCH) which carries link control
information to and from the MS–BTS. Each timeslot in a cell allocated to
traffic channel usage will follow this format, that is, 12 bursts of traffic, 1
burst of SACCH, 12 bursts of traffic and 1 idle.
Idle Bursts: The idle time period allows a mobile device
to perform other necessary operations such as monitoring the radio signal
strength level of a beacon channel from other cells. The time interval of a 26
frame traffic multiframe is 6 blocks of speech coder data (120 msec).
7. How is a FACCH
formed? When is a FACCH used?
Fast Associated
Control Channel (FACCH): Transmits handover information.
8. What are bursts?
Explain various types of bursts, Radio Propagation and Antennas?
Bursts
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Radio Propagation
It is how the radio
propagates from the transmitter till reaching the receiver, maybe the signal
encounter obstacles, Knife Edge…etc.
Wave propagation is
difficult, no one can argue against that. Nevertheless, it is necessary, for
example when planning mobile telephony systems, to define methods for
predicting propagation in an area. Different models for estimating signal
strength are used for this purpose. They might be simple expressions, suitable
for rough assessments or complicated algorithms that demand considerable
computation time. The simplest model for wave propagation is the free space
case.
Antennas
Isotropic antenna: An isotropic antenna is a completely
non-directional antenna that radiates equally in all directions. Since all
practical antennas exhibit some degree of directivity, the isotropic antenna
exists only as a mathematical concept. The isotropic antenna can be used as a
reference to specify the gain of a practical antenna.
The gain of an antenna
referenced isotropically is the ratio between the power required in the
practical antenna and the power required in an isotropic antenna to achieve the
same field strength in the desired direction of the measured practical antenna.
The directive gain in relation to an isotropic antenna is called dBi.
Half-wave dipole
antenna: A half-wave dipole
antenna may also be used as a gain reference for practical antennas. The
half-wave dipole is a straight conductor cut to
one-half of the electrical
wavelength with the radio frequency signal fed to the middle of the conductor.
Directive gain in relation to a dipole is expressed in units of “dBd”.
For a dipole and an
isotropic antenna with the same input power, the energy is more concentrated in
certain directions by the dipole. The difference in directive gain between the
dipole and the isotropic antenna is 2.15 dB. Figure illustrates the differences
in gain between the isotropic, dipole, and practical antenna. The vertical
pattern (Figure) of the practical antenna is that of a directional antenna.
9. What is VSWR? Why
do we need it?
Standing wave ratio
(SWR) is the ratio of the
amplitude of a partial standing wave at an antinode (maximum) to the amplitude
at an adjacent node (minimum), in an electrical transmission line. The SWR is
usually defined as a voltage ratio called the VSWR, for voltage standing wave ratio.
SWR is used as an efficiency measure for transmission lines, electrical cables
that conduct radio frequency signals, used for purposes such as connecting
radio transmitters and receivers with their antennas, and distributing cable
television signals.
A problem with
transmission lines is that impedance mismatches in the cable tend to reflect
the radio waves back toward the source end of the cable, preventing all the
power from reaching the destination end. SWR measures the relative size of
these reflections. An ideal transmission line would have an SWR of 1:1, with
all the power reaching the destination and no reflected power. An infinite SWR
represents complete reflection, with all the power reflected back down the
cable. The SWR of a transmission line is measured with an instrument called an
SWR meter, and checking the SWR is a standard part of installing and
maintaining transmission lines.
The voltage standing
wave ratio is then equal to:
Where (ρ) is the
magnitude of reflection coefficient.
10. What do you mean
by EIRP?
Effective isotropic
radiated power (EIRP) is the amount of power that a theoretical isotropic
antenna (that evenly distributes power in all directions) would emit to produce
the peak power density observed in the direction of maximum antenna gain. EIRP
can take into account the losses in transmission line and connectors and
includes the gain of the antenna. The EIRP is often stated in terms of decibels
over a reference power emitted by an isotropic radiator with equivalent signal
strength. The EIRP allows comparisons between different emitters regardless of
type, size or form.
EIRP = P_T - L_c + G_a
where \scriptstyle
EIRP and \scriptstyle P_T (output power of transmitter) are in dBm, cable
losses (\scriptstyle L_c) is in dB, and antenna gain (\scriptstyle G_a) is
expressed in dBi, relative to a (theoretical) isotropic reference antenna.
11. What is
Polarization? What are the types of polarization?
The polarization
indicates the plane in which the wave is vibrating. The polarization plane is
taken to be that of the electric component.
Vertical and
horizontal are the simplest forms of polarization, and they both fall into a
category known as linear polarization. However, it is also possible to use
circular polarization. This has a number of benefits in areas such as satellite
applications, where it helps to overcome the effects of propagation anomalies,
ground reflections and the spin that occur on many satellites. Circular
polarization is a little more difficult to visualize than linear polarization;
however, it can be imagined by visualizing a signal propagating from an antenna
that is rotating. Another form of polarization is known as elliptical
polarization.
12. What is fading?
Fading is the
variation of the received signal with time, it occurs due to propagation
distance.
Short term (fast)
fading: caused by multipath
propagation.
Long term (slow)
fading: caused by shadowing.
13. What is Rayleigh
Fading?
Rayleigh fading is a
reasonable model when there are many objects in the environment that scatter
the radio signal before it arrives at the receiver.
Rayleigh fading models
assume that the magnitude of a signal that has passed through such a
communications channel will vary randomly, or fade, according to a Rayleigh
distribution the radial component of the sum of two uncorrelated Gaussian
random variables.
Rayleigh fading is
most applicable when there is no dominant propagation along a line of sight
between the transmitter and receiver. If there is a dominant line of sight,
Rician fading may be more applicable.
55. What is multipath
fading?
Multipath fading is
receiving multiple copies of the signal at receiver due to reflections. The
copies reach the receiver with different phases, so summation either
constructive or destructive. This affects the quality of received signal in
terms of BER.
14. What is multipath
fading?
Multipath fading is
receiving multiple copies of the signal at receiver due to reflections. The
copies reach the receiver with different phases, so summation either
constructive or destructive. This affects the quality of received signal in
terms of BER.
15. How can we
minimize multipath fading?
By using channel
equalizer.
16. What are the
different types of diversity?
A. Space Diversity.
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The
distance between the antenna must be such relationships in the two-antenna
signal, which is low. Relationships are the equations that describe the
statistical limits of the signal. In practice, the distance should be a few
meters. At 900 MHz is possible strengthening of 3 dB, used a distance of 5 to 6
meters between the two antenna .. At 1800 MHz the distance would be minimized
because a smaller wavelength.
B. Polarization
Diversity.
Antenna
Dual Polarization is: an antenna device with 2 rows with the same physical
unit. Both lines can be arranged and directed in various ways during the second
plan of polarization have the same performance by strengthening and examples of
radiation. Two forms are commonly used together, namely: Vertical and
Horizontal rows and rows in the slope of 45 °.
C. Time Diversity.
D. Frequency Diversity
17. Explain various
types of Antenna Diversity?
Antenna diversity
increases the received signal strength by taking advantage of the natural
properties of radio waves. There are two primary diverstiy methods: space
diversity and polarization diversity.
Space Diversity: Increased received signal strength at the BTS
may be achieved by mounting two receiver antennae instead of one. If the two Rx
antennae are physically separated, the probability that both of them are
affected by a deep fading dip at the
same time is low. At
900 MHz, it is possible to gain about 3 dB with a distance of five to six
meters between the antennae. At 1800 MHz the distance can be shortened because
of its decreased wavelength.
By choosing the best
of each signal, the impact of fading can be reduced. Space diversity offers
slightly better antenna gain than polarization diversity, but requires more
space.
Polarization Diversity: With polarization diversity the two space
diversity antennae are replaced by one dual polarized antenna. This antenna has
normal size but contains two differently polarized antenna arrays. The most
common types are vertical/horizontal arrays and arrays in 45 degree slant
orientation. The two arrays are connected to the respective Rx branches in the
BTS. The two arrays can also be used as combined Tx/Rx antennas. For most
applications, the difference between the diversity gain for space diversity and
polarization diversity is negligible, but polarization diversity reduces the
space required for antenna.
18. Explain Frequency
Diversity.
By using frequency
hopping sequence.
19. Explain Time
Diversity.
By using code
interleave technology.
