Lecture 7: Channel capacity and multiplexing

Anders Västberg

vastberg@kth.se

08-790 44 55

Concepts

• Data rate

• Channel capacity

• Signal Bandwidth

• Channel Bandwidth

• Noise

• Error rate

Bandwidth limited channelHartley’s law

MBC 2log2

C Channel capacity (bit/s)M Discrete LevelsB Channel bandwidth (Hz)

Bandwidth limited and noisy channelShannon’s law

)1(log2 SNRBC

C Channel capacity (bit/s)SNR=P/N Signal to Noise ratio (not in dB!)B Channel bandwidth (Hz)

Shannon’s Law

Bandwidth and Capacity

2 4 6 8 1 0B k H z 1 0

2 0

3 0

4 0

5 0

6 0

C k b i t s

Multiplexing• The signals of the different users are multiplexed at the transmitter

and then transmitted as one entity.• Examples of broadcast network:

• Radio and TV broadcast systems• Downlinks in a mobile cellular communication system• Downlinks in a satellite communication system

Multiple Access

Examples of Multiple Access systems:•Uplink in a mobile cellular system•Uplink in a satellite communication system

Multiplexing/Multiple Access

• Frequency division multiplexing (multiple access) (FDM/FDMA)– Channel bandwidth greater than signal

bandwidth

• Time division multiplexing (multiple access) (TDM/TDMA)– Channel capacity greater than signal data rate

FDM/TDM

Frequency divison multiplexing

Guard bandSignal bandwidth

|H(f)|

f

Time divison multiplexing

• Bit interleaving

• Block interleaving

• Synchronous TDM– Time slots fixed and pre-assigned to sources

• Asynchronous TDM– Time given to sources with data to send

1 0 1 1 0 0 1 1

1 0 11 0 01 1

1

1

Homework before F8

• Beskriv principen för DSSS?

• Vad är snabb och långsam FHSS?