Subject: Wireless Technology

Topic: Fundamentals of Wireless Communication

Difficulty: 5M - Medium, 10 M - Hard

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 sub-carrier, and all of them are linearly mixed together.

FDMA divides the shared medium bandwidth into individual channels. Sub-carriers 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 fiber optic communications systems.

Modulator/mixtures created single side-band (SSB) signals, both upper and lower sidebands. These sub-carriers were then further frequency multiplexed on sub-carriers 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.

Original aerospace telemetry systems used an FDMA system to accommodate multiple sensor data on a single radio channel. Early satellite systems shared individual 36-MHz bandwidth transponders in the 4-GHz to 6-GHz range with multiple voice, video or data signals via FDMA. Today, all of these applications use TDMA digital techniques.

Advantages of FDMA: AS FDMA systems use low bit rates(large symbol time)compared to average delay spread, it offers the following advantages-

• Reduces the bit rate information and the use of efficient numerical codes increases the capacity
• It reduces the cost and lowers the inter symbol interference (ISI)
• Equalization is not necessary
• An FDMA system can be easily implemented. A system can be configured so that the improvements in terms of speech encoder and bit rate reduction may be easily incorporated.
• Since the transmission is continuous , less number of bits are required for synchronization and framing.

Disadvantages of FDMA:

Although FDMA offers several advantages, it has a few drawbacks as well, which are listed below-

• It does not offer significantly from analog systems; improving the capacity depends on the signal-to-interference reduction, or a signal-to-noise ratio (SNR).
• The maximum flow rate per channel is fixed and small.
• Guard bands lead to a waste of capacity.
• Hardware implies narrow-band filters, which cannot be realized in VLSI and therefore increases the cost.

TDMA

TDMA is a digital technique that divides a single channel or band into time slots. Each time slot is used to transmit one byte or another digital segment of each signal in sequential serial data format. This technique works well with slow voice data signals, but it's also useful for compressed video and other high-speed data.

A good example is the widely used T1 transmission system, which has been used for years in the telecom industry.T1 lines carry up to 24 individual voice telephone calls on a single line (Fig. 2). Each voice signal usually covers 300 Hz to 3000 Hz and is digitized at an 8-kHz rate, which is just a bit more than the minimal Nyquist rate of two times the highest-frequency component needed to retain all the analog content.

This T1 digital telephony frame illustrates TDM and TDMA. Each time slot is allocated to one user. The high data rate makes the user unaware of the lack of simultaneity.

At the receiving end, the individual voice bytes are recovered at the 64-kHz rate and passed through a digital-to-analog converter(DAC) that reproduces the analog voice.

The esight time slotes can be voice signals or data such as texts or e-mails. The frame is transmitted at a 270-kbit/s rate using Gaussian minimum shift keying (GMSK), which is a form of frequency shift keying(FSK) modulation

This GSM digital cellular method shows how up to eight users can share a 200-kHz channel in different time slots within a frame of 1248 bits.

Advantages of TDMA:

Here is a list of few notable advantages of TDMA -

• Permits flexible rates (i.e several slots can be assigned to a user, for example, each time interval translates 32Kbps, a user is assigned two 64 Kbps slots per frame).
• Can withstand gusty or variable hit rate traffic. Number of slots allocated to a user can be changed frame by frame (for example, two slots in the frame 1, three slots in the frame 2, one slot in the frame 3, frame 0 of the notches 4, etc.).
• No guard band required for the wideband system.
• No narrowband filter required for the wideband system

Disadvantages of TDMA: The disadvantages of TDMA are as follow- - High data rates of broadband systems require complex equalization - Due to the burst mode, a large number of additional bits are required for synchronization and supervision. - Call time is needed in each slot to accommodate time to inaccuracies (due to instability). - Electronics operating at high bit rates increase energy consumption. - Complex signal processing is required to synchronize within short slots.