Hybrid Multiple Access Technique

By Team Resalders

My Team

• T. Anushuya

Channel Access on links

Multiple Access Techniques

Various multiple access techniques are:• Frequency Division Multiple Access(FDMA)• Time Division Multiple Access (TDMA)• Code Division Multiple Access(CDMA)

Frequency Division Multiple Access

• In frequency-division multiple access (FDMA), the available bandwidth is divided into frequency bands.

• Each station is allocated a band to send its data.• In this method when any one frequency level is kept idle and

another is used frequently leads to inefficiency.

Time Division Multiple Access

• In time-division multiple access (TDMA), the stations share the bandwidth of the channel in time.

• Each station is allocated a time slot during which it can send data.

• The main problem with TDMA lies in achieving synchronization between the different stations.

• Each station needs to know the beginning of its slot and the location of its slot.

Code Division Multiple Access

• CDMA differs from FDMA because only one channel occupies the entire bandwidth of the link.

• It differs from TDMA because all stations can send data at the same time without timesharing.

• CDMA simply means communication with different codes.

• CDMA is based on coding theory. Each station is assigned a code, which is a sequence of numbers called chips.

Code Division Multiple Access

HYBRID MULIPLE ACCESS TECHNIQUES

• 1. Hybrid FDMA/CDMA(FCDMA),• 2. Hybrid Direct Sequence/Frequency Hopped

Multiple Access(DS/FHMA),• 3. Time Division CDMA(TCDMA),

Hybrid FDMA/CDMA(FCDMA)

• The available wideband spectrum is divided into a number of sub spectra's with smaller bandwidths.

• Each of these smaller sub channels becomes a narrowband CDMA system having processing gain lower than the original CDMA system.

Time Division CDMA ( TDCDMA)

• Different spreading codes are assigned to different cells. Within each cell only one user per cell is allocated a particular time slot. Thus at any time only one CDMA user is transmitting in each cell.

Advantages

• It avoids near far effect.

Time division frequency hoping(TDFH)

• The subscriber can hope to a new frequency at the start of a new frequency at the start of a new TDMA frame. In GSM standard, hoping sequence is predefined and the subscriber is allowed to hop only on certain frequencies which are assigned to a cell.

Advantage :

• Avoid a severe fade or erasude event on a particular channel.

Thank You..!!!