Term Paper Synopsis

ECE444

Topic: FDMA

Submitted to: Submitteby:

Ms. Loveleen Kaur Sonali Sood

Asst. Professor Sec-DE135

RDE135A02

10800641

FDMA:

Frequency Division Multiple Access (FDMA) is an analogue transmission technique used

for mobile phone communications, in which the frequency band allocated to a network is

divided into sub-bands or channels. Each frequency channel can carry either a voice

conversation or digital data, and one channel will be assigned to each subscriber for the

duration of a call. Using FDMA in this way, multiple users can share the available band

without the risk of interference between the simultaneous calls.

FDMA is one of the three main multiplexing techniques that enable users to share the radio

spectrum, and in the case of FDMA the receiver will discriminate between the signals by

tuning to the particular frequency channel that carries the desired signal. Alternative multiple

access techniques are TDMA and CDMA, in which the receivers discriminate between

signals by using, respectively, different time slots or different codes. However, in practice

TDMA and CDMA are always found in combination with FDMA, i.e. TDMA or CDMA are

used to increase capacity on a channel within an FDMA system.

The term FDMA has also been used to describe the cellular technology used in the 1G (first-

generation) analogue systems, such as NMT, AMPS, and TACS, because FDMA was the

predominant transmission technique used in these 1G networks.

Features:

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

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 by COMSAT for use on

the Intelsatseries IVA and V satellites.

There are two main techniques:

Multi-channel per-carrier (MCPC)

Single-channel per-carrier (SCPC)

Analog transmissions uses FDMA technology. FDMA stands for "frequency division

multiple access". FDMA is used exclusively for analog cellular systems, even though in

theory FDMA can also be used with digital. Essentially, FDMA splits the allocated spectrum

into many channels. In current analog cell systems, each channel is 30 kHz. When a FDMA

cell phone establishes a call, it reserves the frequency channel for the entire duration of the

call. The voice data is modulated into this channel’s frequency band (using frequency

modulation) and sent over the airwaves. At the receiver, the information is recovered using a

band-pass filter. The phone then uses a common digital control channel to acquire channels.

FDMA analog transmissions are the least efficient cellular networks since each analog

channel can only be used one user at a time. Analog channels don't take full advantage of

band-width. Not only are these FDMA channels larger than necessary given modern digital

voice compression, but they are also wasted whenever there is silence during a cell phone

conversation. Analog signals are especially susceptible to noise and the extra noise cannot get

filtered out. Given the nature of the signal, analog cell phones must use higher power

(between 1 and 3 watts) to get acceptable call quality. Given these analog features, it is easy

to see why FDMA is being replaced by newer digital networks such as TDMA and CDMA.

Advantages & Disadvantages of FDMA:

FDMA, or frequency division multiple access, allows users to access a single channel,

through a shared frequency; this system is advantageous as it is run through a satellite and

offers users the chance to share a channel easily without time delays. The disadvantage of

FDMA is the expense of running the system, which requires costly, custom filters and other

technical equipment.