Chapter: Mobile Computing
Fixed Assignment Schemes
FIXED
ASSIGNMENT SCHEMES
TDMA
Time Division Multiple Access (TDMA) is a digital
wireless telephony transmission technique. TDMA allocates each user a different
time slot on a given frequency. TDMA divides each cellular channel into three
time slots in order to increase the amount of data that can be carried.
TDMA technology was more popular in Europe, Japan
and Asian countries, where as CDMA is widely used in North and South America.
But now a days both technologies are very popular through out of the world.
Advantages of TDMA:
·
TDMA can easily adapt to transmission of data as
well as voice communication.
·
TDMA has an ability to carry 64 kbps to 120 Mbps of
data rates.
·
TDMA allows the operator to do services like fax,
voice band data, and SMS as well as bandwidth-intensive application such as
multimedia and video conferencing.
·
Since TDMA technology separates users according to
time, it ensures that there will be no interference from simultaneous
transmissions.
·
TDMA provides users with an extended battery life,
since it transmits only portion of the time during conversations.
·
TDMA is the most cost effective technology to
convert an analog system to digital.
Disadvantages of TDMA
·
Disadvantage using TDMA technology is that the
users has a predefined time slot. When moving from one cell site to other, if
all the time slots in this cell are full the user might be disconnected.
·
Another problem in TDMA is that it is subjected to
multipath distortion. To overcome this distortion, a time limit can be used on
the system. Once the time limit is expired the signal is ignored.
CDMA
Code Division Multiple Access (CDMA) is a digital
wireless technology that uses spread-spectrum techniques. CDMA does not assign
a specific frequency to each user. Instead, every channel uses the full
available spectrum. Individual conversations are encoded with a pseudo-random
digital sequence. CDMA consistently provides better capacity for voice and data
communications than other commercial mobile technologies, allowing more
subscribers to connect at any given time, and it is the common platform on
which 3G technologies are built.
Advantages of CDMA
·
One of the main advantages of CDMA is that dropouts
occur only when the phone is at least twice as far from the base station. Thus,
it is used in the rural areas where GSM cannot cover.
·
Another advantage is its capacity; it has a very
high spectral capacity that it can accommodate more users per MHz of bandwidth.
Disadvantages of CDMA
·
Channel pollution, where signals from too many cell
sites are present in the subscriber. s phone but none of them is dominant. When
this situation arises, the quality of the audio degrades.
·
When compared to GSM is the lack of international
roaming capabilities.
·
The ability to upgrade or change to another handset
is not easy with this technology because the network service information for
the phone is put in the actual phone unlike GSM which uses SIM card for this.
·
Limited variety of the handset, because at present
the major mobile companies use GSM technology.
FDMA
FDMA is the process of dividing one channel or bandwidth into
multiple individual bands, each for use by a single user. Each individual band
or channel is wide enough to accommodate the signal spectra of the
transmissions to be propagated. The data to be transmitted is modulated on to
each subcarrier, and all of them are linearly mixed together.
FDMA divides the shared medium
bandwidth into individual channels. Subcarriers modulated by the information to
be transmitted occupy each sub channel.
The best example of this is the cable
television system. The medium is a single coax cable that is used to broadcast
hundreds of channels of video/audio programming to homes. The coax cable has a
useful bandwidth from about 4 MHz to 1 GHz. This bandwidth is divided up into
6-MHz wide channels. Initially, one TV station or channel used a single 6-MHz
band. But with digital techniques, multiple TV channels may share a single band
today thanks to compression and multiplexing techniques used in each channel.
This technique is also used in fibre
optic communications systems. A single fibre optic cable has enormous bandwidth
that can be subdivided to provide FDMA. Different data or information sources
are each assigned a different light frequency for transmission. Light generally
isn‘t referred to by frequency but by its wavelength (λ). As a result, fiber
optic
FDMA is called wavelength division multiple access (WDMA) or
just wavelength division multiplexing (WDM).
One of the older FDMA systems is the original analog telephone
system, which used a hierarchy of frequency multiplex techniques to put
multiple telephone calls on single line. The analog 300-Hz to 3400-Hz voice
signals were used to modulate subcarriers in 12 channels from 60 kHz to 108
kHz. Modulator/mixers created single sideband (SSB) signals, both upper and
lower sidebands. These subcarriers were then further frequency multiplexed on
subcarriers in the 312-kHz to 552-kHz range using the same modulation methods.
At the receiving end of the system, the signals were sorted out and recovered
with filters and demodulators.
SDMA
Space-division multiple access (SDMA) is a channel
access method based on creating parallel spatial pipes next to higher capacity
pipes through spatial multiplexing and/or diversity, by which it is able to
offer superior performance in radio multiple access communication systems. In
traditional mobile cellular network systems, the base station has no
information on the position of the mobile units within the cell and radiates
the signal in all directions within the cell in order to provide radio
coverage.
This results in wasting power on transmissions when
there are no mobile units to reach, in addition to causing interference for
adjacent cells using the same frequency, so called co-channel cells. Likewise,
in reception, the antenna receives signals coming from all directions including
noise and interference signals. By using smart antenna technology and differing
spatial locations of mobile units within the cell, space-division multiple
access techniques offer attractive performance enhancements.
The radiation pattern of the base station, both in
transmission and reception, is adapted to each user to obtain highest gain in
the direction of that user. This is often done using phased array techniques.
In GSM cellular networks, the base station is aware of the distance (but not
direction) of a mobile phone by use of a technique called "timing
advance" (TA). The base transceiver station (BTS) can determine how
distant the mobile station (MS) is by interpreting the reported TA.
This information, along with other parameters, can
then be used to power down the BTS or MS, if a power control feature is
implemented in the network. The power control in either BTS or MS is
implemented in most modern networks, especially on the MS, as this ensures a
better battery life for the MS. This is also why having a BTS close to the user
results in less exposure to electromagnetic radiation.
This is why one may actually be safer to have a BTS
close to them as their MS will be powered down as much as possible. For
example, there is more power being transmitted from the MS than what one would
receive from the BTS even if they were 6 meters away from a BTS mast. However,
this estimation might not consider all the Mobile stations that a particular
BTS is supporting with EM radiation at any given time.
In the same manner, 5th generation mobile networks
will be focused in utilizing the given position of the MS in relation to BTS in
order to focus all MS Radio frequency power to the BTS direction and vice
versa, thus enabling power savings for the Mobile Operator, reducing MS SAR
index, reducing the EM field around base stations since beam forming will
concentrate rf power when it will be actually used rather than spread uniformly
around the BTS, reducing health and safety concerns, enhancing spectral
efficiency, and decreased MS battery consumption.
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