1. September 24, 2012 TDM in
cellphones
TDM-time division multiplexing
Prateek Kumar
CSE-D
1031010186
2. MULTIPLEXING
In telecommunications and computer networks, multiplexing (also known
as muxing) is a method by which multiple analog message signals or digital data
streams are combined into one signal over ashared medium. The aim is to share
an expensive resource. For example, in telecommunications, several telephone
calls may be carried using one wire. Multiplexing originated in telegraphy, and is
now widely applied in communications. George Owen Squier is credited with the
development of multiplexing in 1910.
The multiplexed signal is transmitted over a communication channel, which may
be a physical transmission medium. The multiplexing divides the capacity of the
high-level communication channel into several low-level logical channels, one for
each message signal or data stream to be transferred. A reverse process, known
as demultiplexing, can extract the original channels on the receiver side.
A device that performs the multiplexing is called a multiplexer (MUX), and a
device that performs the reverse process is called a demultiplexer (DEMUX).
Inverse multiplexing (IMUX) has the opposite aim as multiplexing, namely to
break one data stream into several streams, transfer them simultaneously over
several communication channels, and recreate the original data stream.
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3. TELECOMMUNICATION MULTIPLEXING
TDM
Time Division Multiplexing (TDM) is a communications process that transmits two
or more streaming digital signals over a common channel. In TDM, incoming
signals are divided into equal fixed-length time slots. After multiplexing, these
signals are transmitted over a shared medium and reassembled into their original
format after de-multiplexing. Time slot selection is directly proportional to overall
system efficiency.
TDM was initially developed in 1870 for large system telegraphy implementation.
Packet switching networks use TDM for telecommunication links, i.e., packets
are divided into fixed lengths and assigned fixed time slots for transmission. Each
divided signal and packet, which must be transmitted within assigned time slots,
are reassembled into a complete signal at destination.
TDM is comprised of two major categories: TDM and synchronous time division
multiplexing (sync TDM). TDM is used for long distance communication links and
bears heavy data traffic loads from end users. Sync TDM is used for high speed
transmission.
During each time slot a TDM frame (or data packet) is created as a sample of the
signal of a given sub-channel; the frame also consists of a synchronization
channel and sometimes an error correction channel. After the first sample of the
given sub-channel (along with its associated and newly created error correction
and synchronization channels) are taken, the process is repeated for a second
sample when a second frame is created, then repeated for a third frame, etc;
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4. andthe frames are interleaved one after the other. When the time slot has
expired, the process is repeated for the next sub-channel.
Examples of utilizing TDM include digitally transmitting several telephone
conversations over the same four-wire copper cable or fiber optical cable in a
TDM telephone network; these systems may be PCM (pulse code modulation) or
PDH (plesiochronous digital hierarchy) systems. Another example involves
sampling left and right stereo signals using RIFF (Resource Interchange File
Format), also referred to as WAV (Waveform Audio File Format), audio standard
interleaves. Also SDH (synchronous Digital Hierarchy) and SONET (synchronous
optical networking) network transmission standards have incorporated TDM; and
these have surpassed PDH.
TDM can also be used within time division multiple access (TDMA) where
stations sharing the same frequency channel can communicate with one another.
GSM utilizes both TDM and TDMA.
TDMA
TDMA technology, which stands for Time Division Multiple Access, is a cell
phone standard that has been incorporated into the more
advanced GSM standard, which is now the world’s most widely used cell phone
technology.
TDMA is used in second-generation (2G) cell phone systems such as GSM. Most
major third-generation (3G) cell phone systems are primarily based upon GSM
rival CDMA. 3G allows for faster data speeds over 2G.
Time division multiple access (TDMA) is a channel access method for shared
medium networks. It allows several users to share the same frequency
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5. channel by dividing the signal into different time slots. The users transmit in rapid
succession, one after the other, each using its own time slot. This allows multiple
stations to share the same transmission medium (e.g. radio frequency channel)
while using only a part of its channel capacity. TDMA is used in the
digital 2G cellular systems such as Global System for Mobile
Communications (GSM), IS-136, Personal Digital Cellular (PDC) and iDEN, and
in the Digital Enhanced Cordless Telecommunications (DECT) standard
for portable phones. It is also used extensively in satellite systems, combat-net
radiosystems, and PON networks for upstream traffic from premises to the
operator. For usage of Dynamic TDMA packet mode communication, see below.
TDMA is a type of Time-division multiplexing, with the special point that instead
of having one transmitter connected to onereceiver, there are multiple
transmitters. In the case of the uplink from a mobile phone to a base station this
becomes particularly difficult because the mobile phone can move around and
vary the timing advance required to make its transmission match the gap in
transmission from its peers.
TDMA frame structure showing a data stream divided into frames and those frames
divided into time slots.
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6. TDMA CHARACTERISTICS
-Shares single carrier frequency with multiple users
-Non-continuous transmission makes handoff simpler
-Slots can be assigned on demand in dynamic TDMA
-Less stringent power control than CDMA due to reduced intra cell
interference
-Higher synchronization overhead than CDMA
-Advanced equalization may be necessary for high data rates if the channel
is "frequency selective" and creates Intersymbol interference
-Cell breathing (borrowing resources from adjacent cells) is more
complicated than in CDMA
-Frequency/slot allocation complexity
-Pulsating power envelope: Interference with other devices
TDMA IN MOBILE PHONE SYSTEMS
2G systems
Most 2G cellular systems, with the notable exception of IS-95, are based on
TDMA. GSM, D-AMPS, PDC, iDEN, and PHS are examples of TDMA cellular
systems. GSM combines TDMA withFrequency Hopping and wideband
transmission to minimize common types of interference.
In the GSM system, the synchronization of the mobile phones is achieved by
sending timing advance commands from the base station which instructs the
mobile phone to transmit earlier and by how much. This compensates for the
propagation delay resulting from the light speed velocity of radio waves. The
mobile phone is not allowed to transmit for its entire time slot, but there is aguard
interval at the end of each time slot. As the transmission moves into the guard
period, the mobile network adjusts the timing advance to synchronize the
transmission.
Initial synchronization of a phone requires even more care. Before a mobile
transmits there is no way to actually know the offset required. For this reason, an
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7. entire time slot has to be dedicated to mobiles attempting to contact the network
(known as the RACH in GSM). The mobile attempts to broadcast at the
beginning of the time slot, as received from the network. If the mobile is located
next to the base station, there will be no time delay and this will succeed. If,
however, the mobile phone is at just less than 35 km from the base station, the
time delay will mean the mobile's broadcast arrives at the very end of the time
slot. In that case, the mobile will be instructed to broadcast its messages starting
nearly a whole time slot earlier than would be expected otherwise. Finally, if the
mobile is beyond the 35 km cell range in GSM, then the RACH will arrive in a
neighbouring time slot and be ignored. It is this feature, rather than limitations of
power, that limits the range of a GSM cell to 35 km when no special extension
techniques are used. By changing the synchronization between the uplink and
downlink at the base station, however, this limitation can be overcome.
3G systems
Although most major 3G systems are primarily based upon CDMAtime division
duplexing (TDD), packet scheduling (dynamic TDMA) and packet oriented
multiple access schemes are available in 3G form, combined with CDMA to take
advantage of the benefits of both technologies.
While the most popular form of the UMTS 3G system uses CDMA and frequency
division duplexing (FDD) instead of TDMA, TDMA is combined with CDMA and
Time Division Duplexing in two standard UMTS UTRA
MF-TDMA
MF-TDMA (Multi-Frequency, Time Division Multiple Access) is the leading
technology for dynamically sharing bandwidth resources in an over-the-air, two-
way communications network. Many variations of MF-TDMA technology
(including simple TDMA) exist and are commonly used in multiple types of
networks, including:
Most two-way communication satellite networks,
The most common cellular telephony networks (e.g., GSM), and
Some metro-wireless data access networks (e.g., WiMax)
Some alternatives to MF-TDMA are Code Division Multiple Access (CDMA)
and Carrier sense multiple access (CSMA). It is also possible to combine MF-
TDMA technology with these other technologies. For satellite networks MF-
TDMA is the dominant technology because it provides the most bandwidth and
the greatest overall efficiency and service quality, while also allowing the
dynamic sharing of that bandwidth among many (tens of thousands) of
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8. transmitters in a two-way communication mode. MF-TDMA networks can have
either a star-topology, fully meshed or partially meshed topologies.
TDMA FRAMES
This image below shows a sequence of two successive TDMA frames passing
through the satellite. The carrier bit rate is 250 kbit/s
Explanation: Site 1 transmits a burst, starting at the beginning of each TDMA
frame. The burst lasts 180 mS, so at a rate of 250kbit/s site 1 sends 45,000 bits
per burst, or 45,000 bits per second. Site 2 transmits a burst, timed to arrive at
the satellite just after the end of burst 1. The red, second, burst lasts 80 mS, so
at a rate of 250kbit/s, site 2 sends 20,000 bits per burst, or 20,000 bits per
second. The diagram shows a fixed time plan, where each VSAT has been
allocated a predetermined portion of the total time.
There is designed in 20mS guard period between each burst. This allows for
slight mistiming in the transmission of the bursts. Severe mistiming would cause
bursts to arrive overlapping or on top of each other, causing loss of service to
both sites involved in the mutual interference. The long 20mS guard period is
illustrative only, so you can see the white space in the figure above. In actual
TDMA systems the guard band may be very much less and there may be very
many more bursts per frame.
The above is just an example. TDMA frame length may be as short as 2000
microseconds or as long as 1 second, as in the example above. The shortest
TDMA frame periods are associated with the highest higher speed TDMA
systems, operating at say 120.832 Mbit/s. On low speed 250kbit/s VSAT return
links, with perhaps 2 to 5 sites sharing, and used for internet browsing and
emails, the TDMA frame period is typically 500Ms.
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