1. Frequency reuse
The increased capacity in a cellular network, comparing to a network with a single
transmitter, comes from the fact that the same radio frequency can be reused in a
different area for a completely different transmission. If there is a single plain
transmitter, only one transmission can be used on any given frequency. Unfortunately,
there is inevitably some level of interference from the signal from the other cells which
use the same frequency. This means that, in a standard FDMA system, there must be
at least a one cell gap between cells which reuse the same frequency.
The frequency reuse factor is the rate at which the same frequency can be used in the
network. It is 1/n where n is the number of cells which cannot use a frequency for
transmission.
Code division multiple access based systems use a wider frequency band to achieve
the same rate of transmission as FDMA, but this is compensated for by the ability to
use a frequency reuse factor of 1. In other words, every cell uses the same frequency
and the different systems are separated by codes rather than frequencies.
Depending on the size of the city, a taxi system may not have any frequency reuse in
its own city, but certainly in other nearby cities, the same frequency can be used. In a
big city, on the other hand, frequency reuse could certainly be in use.
Frequency Division Multiple Access or FDMA is a channel access method used in multiple-
access protocols as a channelization protocol. FDMA gives users an individual allocation of one or
several frequency bands, or channels. It is particularly commonplace in satellite communication.
FDMA, like other Multiple Access systems, coordinates access between multiple users. Alternatives
include TDMA, CDMA, or SDMA. These protocols are utilized differently, at different levels of the
theoreticalOSI model.
Disadvantage: Crosstalk may cause interference among frequencies and disrupt the transmission.
 In FDMA all users share the satellite simultaneously but each user transmits at single frequency.
 FDMA can be used with both analog and digital signal.
 FDMA requires high-performing filters in the radio hardware, in contrast to TDMA and CDMA.
 FDMA is not vulnerable to the timing problems that TDMA has. Since a predetermined frequency
band is available for the entire period of communication, stream data (a continuous flow of data
that may not be packetized) can easily be used with FDMA.
 Due to the frequency filtering, FDMA is not sensitive to near-far problem which is pronounced
for CDMA.
 Each user transmits and receives at different frequencies as each user gets a unique frequency
slot

2. FDMA is distinct from frequency division duplexing (FDD). While FDMA allows multiple users
simultaneous access to a transmission system, FDD refers to how the radio channel is shared
between the uplink and downlink (for instance, the traffic going back and forth between a mobile-
phone and a mobile phone base station). Frequency-division multiplexing (FDM) is also distinct from
FDMA. FDM is a physical layer technique that combines and transmits low-bandwidth channels
through a high-bandwidth channel. FDMA, on the other hand, is an access method in the data link
layer.
FDMA also supports demand assignment in addition to fixed assignment. Demand
assignment allows all users apparently continuous access of the radio spectrum by assigning carrier
frequencies on a temporary basis using a statistical assignment process. The first FDMA demand-
assignment system for satellite was developed byCOMSAT for use on
the Intelsat series IVA and V satellites.
There are two main techniques:
 Multi-channel per-carrier (MCPC)
 Single-channel per-carrier (SCPC)

3. Time division multiple access (TDMA) is a channel access method for shared medium networks. It
allows several users to share the same frequency 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 radio systems,
and PON networks for upstream traffic from premises to the operator. For usage of Dynamic TDMA
packet mode communication.
TDMA is a type of Time-division multiplexing, with the special point that instead of having
one transmitter connected to one receiver, there are multiple transmitters. In the case of
the uplink from a mobile phone to abase 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 in 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
with Frequency 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 a guard
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 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.

4. 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 CDMA[citation needed], time 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
Code division multiple access (CDMA) is a channel access method used by various radio
communication technologies. It should not be confused with the mobile phone
standards called cdmaOne, CDMA2000 (the 3G evolution of cdmaOne) and WCDMA (the 3G
standard used by GSM carriers), which are often referred to as simply CDMA, and use CDMA as an
underlying channel access method.
One of the basic concepts in data communication is the idea of allowing several transmitters to send
information simultaneously over a single communication channel. This allows several users to share
a band of frequencies (see bandwidth). This concept is called multiple access. CDMA
employs spread-spectrum technology and a special coding scheme (where each transmitter is
assigned a code) to allow multiple users to be multiplexed over the same physical channel. By
contrast, time division multiple access (TDMA) divides access bytime, while frequency-division
multiple access (FDMA) divides it by frequency. CDMA is a form of spread-spectrum signalling,
since the modulated coded signal has a much higher data bandwidth than the data being
communicated.
An analogy to the problem of multiple access is a room (channel) in which people wish to talk to
each other simultaneously. To avoid confusion, people could take turns speaking (time division),
speak at different pitches (frequency division), or speak in different languages (code division). CDMA
is analogous to the last example where people speaking the same language can understand each
other, but other languages are perceived as noise and rejected. Similarly, in radio CDMA, each
group of users is given a shared code. Many codes occupy the same channel, but only users
associated with a particular code can communicate. The technology of code division multiple access

5. channels has long been known. In the USSR, the first work devoted to this subject was published in
1935 by professor D.V. Aggeev in the "CDMA". It was shown that through the use of linear methods,
there are three types of signal separation: frequency, time and compensatory. The technology of
CDMA was used in 1957, when the young military radio engineer Leonid Kupriyanovich in Moscow,
made an experimental model of a wearable automatic mobile phone, called LK-1 by him, with a base
station. LK-1 has a weight of 3 kg, 20-30 km operating distance, and 20-30 hours of battery life
("Nauka i zhizn", 8, 1957, p. 49, "Yuniy technik", 7, 1957, p. 43-44). The base station, as described
by the author, could serve several customers. In 1958, Kupriyanovich made the new experimental
"pocket" model of mobile phone. This phone weighs 0,5 kg. To serve more customers,
Kupriyanovich proposed the device, named by him as correllator. ("Nauka i zhizn", 10, 1958, p.66,
"Technika-molodezhi", 2, 1959, 18-19) In 1958, the USSR also started the development of the
"Altay" national civil mobile phone service for cars, based on the Soviet MRT-1327 standard. The
main developers of the Altay system were VNIIS (Voronezh Science Research Institute of
Communications)and GSPI (State Specialized Project Institute). In 1963 this service started in
Moscow and in 1970 Altay service was used in 30 USSR cities.
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 calledco-
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 arraytechniques.
In GSM cellular networks, the base station is aware of the mobile phone's position 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 and thus a better user
experience (in that the need to charge the battery becomes less frequent). This is why it may

6. actually be safer to have a BTS close to you as your MS will be powered down as much as possible.
For example, there is more power being transmitted from the MS than what you would receive from
the BTS even if you are 6 m away from a mast. However, this estimation might not consider all the
MS's that a particular BTS is supporting with EM radiation at any given time.
Advanced Mobile Phone System (AMPS) was an analog mobile phone system standard
developed byBell Labs, and officially introduced in the Americas in 1983, Israel in 1986,
and Australia in 1987. It was the primary analog mobile phone system in North America (and other
locales) through the 1980s and into the 2000s. As of February 18, 2008, carriers in the United States
were no longer required to support AMPS and companies such as AT&T and Verizon have
discontinued this service permanently. AMPS was discontinued in Australia in September 2000