cdma_lec - ECE 598 Fall 2006 Lecture 18: CDMA What is...

Info iconThis preview shows pages 1–5. Sign up to view the full content.

View Full Document Right Arrow Icon
1 Lecture 18: CDMA ECE 598 Fall 2006 What is Multiple Access? Multiple users want to communicate in a common geographic area Cellular Example: Many people want to talk on their cell phones. Each phone must communicate with a base station. Imagine if only one person could talk on their cell phone at a time! Problem : How should we share our resources so that as many users as possible can communicate simultaneously?
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
2 Freq. Division Multiple Access (FDMA) AMPS (analog), the First Generation (1G) used 30 KHz for each user. Pros Very Simple to design Narrowband (no ISI) Synchronization is easy No interference among users in a cell Cons Narrowband interference Static spectrum allocation Freq. reuse is a problem High Q analog filters or large guard band required Time Division Multiple Access (TDMA) IS-54 (2G) used same 30 KHz channels, but with three users sharing them (3 slots) GSM has 8 slots/270 KHz Pros Better suited for digital Often gets higher capacity (3 times higher here) Relaxes need for high Q filters Cons Strict synchronization and guard time needed Still susceptible to jamming, other-cell interference Often requires equalizer
Background image of page 2
3 Alternative to FDMA and TDMA? What if we could allow users to share time and frequency? Eliminates need for tight synchronization among many different users Eliminates need for expensive analog filters May have favorable impact on capacity (?) But: How do we separate the users? Won’t they interfere with each other? Code Division Multiple Access (CDMA) b k (t) : bits for user k C k : spreading code J : “spreading factor” s k ( t ): transmitted signal for user k y(t) : received signal for all users h k (t) : channel impulse response for user k n(t) : noise ) ( ) ( ) ( ) ( 1 t n t s t h t y K k k k + ! = " =
Background image of page 3

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
4 Spreading and Scrambling? Spreading means increasing the signal bandwidth
Background image of page 4
Image of page 5
This is the end of the preview. Sign up to access the rest of the document.

Page1 / 10

cdma_lec - ECE 598 Fall 2006 Lecture 18: CDMA What is...

This preview shows document pages 1 - 5. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online