RF System Design

Outline

• Circuits for Wireless

• Wireless Communications

– duplex, access, and cellular communication systems

– standards

• Receivers:

– heterodyne

– homodyne

– image reject

• Transmitters

– modulation

– up-conversion

• Transceivers

– frequency synthesis

– examples

Circuits for Wireless

Circuits for Wireless - Overview

• Noise limits the smallest signal

• noise figure

• cascade of stages

• Distortion limits the largest signal

– large (interfering) signals:

• compression, blocking, and desensitization

• inter-modulation

• cascade of stages

• Dynamic Range

Wireless Communication Systems

Wireless Communications - Overview

• ‘ether’ is one medium shared by all

• 1st problem: Duplexing

– how to arrange for a two way communication link

• 2nd problem: Multiple Access

– how to arrange for multiple users

Duplexing - Overview

• Establish two way communications:

– Time division duplex:

• same rcv and xmt frequency channel

• alternating in time between rcv & xmt

– Frequency division duplex

• different frequency channel for rcv and xmt

• full duplex posible

TDD design issues

+ mobile units can communicate

+ Switch low loss (<1db)

+ XMT cannot desensitize RCV

- nearby XMT can overload RCV

+ channel leakage from P/A reduction by proper

timing

• packet based communication:

– Synchronization & Buffering needed

– digital implementation

FDD design issues

– duplexer loss (2~3dB)

– adds directly to noise figure

– reduces XMT efficiency

– duplexer isolation < ~50dB

– still desensitization of RCV by XMT possible

+ less sensitive to nearby XMT

– direct XMT antenna connection

– LO transients or P/A switch results in channel

leakage

+ analog implementation

Multiple Access - Overview

• Frequency Division Multiple Access (FDMA)

– divide band in channels & allocate different channel

for each user

• Time Division Multiple Access (TDMA)

– same channel for different users but each user

accesses in a different time-slot

• Code Division Multiple Access (CDMA)

– all users use same channel at same time but have a

different code

• Carrier Sense Multiple Access (CSMA)

– all users use same channel at different (random) Times

CAF