1. ENGR 100 1 How do multiple users “share” the airwaves? Multiple Access (MA) techniques: –Time Division MA (TDMA) –Frequency Division MA (FDMA) –Code Division MA (CDMA) –Space Division MA (SDMA) Analogies with simultaneous conversations in a classroom

2. ENGR 100 2 Time Division Multiple Access One voice speaks at a time

3. ENGR 100 3 Frequency Division Multiple Access Voices with different “pitch” speak at the same time

4. ENGR 100 4 Code Division Multiple Access Voices speak different languages at the same time and same pitch

5. ENGR 100 5 Space Division Multiple Access Analogy with conversations: Direct voice to location of desired person Space - the final frontier, exploited by “smart antennas” in new wireless systems.

6. ENGR 100 6 Wireless Networks We are looking at “physical layer”: transmission of bits Network issues: –Coding to correct errors –Handoff from cell to cell –Routing calls to mobile users –Applications (voice, data/internet, audio, pictures (maps), video, etc.)

7. ENGR 100 7 A Closer Look at CDMA Developed commercially by Qualcomm Claim: CDMA allows greater capacity (more users) in a wireless system

8. ENGR 100 8 CDMA Codes Consider 4 users Each user wants to transmit a bit: For user 1, for a “0”, for a “1” Codes for 4 users, with 4 “chips” per bit:

9. ENGR 100 9 Each user multiplies its code and bit: All users transmit their 4 chips at the same time Base station antenna receives the sum of the transmissions (the waves add in space!)

10. ENGR 100 10 User signals arrive at base station with different amplitudes. Why? __________________________ Signal amplitudes at base station: Model for four chips received at base station:

11. ENGR 100 11 Example 1 Codes: Amplitudes and bits: