Mobile Computing COE 446 Wireless Multiple Access

1. Mobile Computing COE 446Wireless Multiple Access Tarek Sheltami KFUPM CCSE COE http://faculty.kfupm.edu.sa/coe/tarek/coe446.htm Principles of Wireless Networks K. Pahlavan and P. Krishnamurth

2. Outline • CDMA capacity calculations • With/without sectorization gain, voice of active interference reduction factor and interference increase factor • Comparison of the Capacity of different 2G Systems

3. How many users can simultaneously use a CDMA System before the system collapses? • CDMA systems are implemented based on the spread spectrum technology • A spread spectrum transmitter spreads the signal power over a spectrum N times wider than the spectrum of the message • An information BW of R occupies a transmission of BW W W= NR(1) • The spread spectrum receiver processes the received signal with a processing gain of N

4. How many users can simultaneously use a CDMA System before the system collapses?.. • During the processing at the receiver, the power of received signal having the code of that particular receiver will be increased N times beyond the value before processing • Let us consider a single cell with M simultaneous users on the uplink channel • Let us assume that we have an ideal power control enforced on the channel, so that the received power of signals from all MTs has the same value of P • Then, the received power from the target user after processing at the receiver is NP, and the received power interference from M-1 other MTs is (M-1)P

5. How many users can simultaneously use a CDMA System before the system collapses?.. • Let us further assume that a cellular system is limited and the background noise is dominated by interference noise from other users, then, the receiver Signal-to-interference ratio for a target receiver:  (2)

6. How many users can simultaneously use a CDMA System before the system collapses?.. • All users always have a requirement for the acceptable error rate of the received data stream • For a given modulation and coding specification of the system, the error rate requirement will be supported by minimum of Sr requirements • From EQs #1 & 2  (3)

7. Problem 2: Capacity of One Carrier in a Single-Cell CDMA System • Using QPSK modulation and convolution coding, the IS-95 digital cellular systems require 3 dB < Sr < 9 dB. The bandwidth of the channel is 1.25 MHz, and the transmission rate is R = 9600 bps. Find the capacity of a single IS-95 cell. • Solution: • Using Equation (4.3) we can support:

8. Practical Consideration • In practice, design of digital cellular systems, there are other parameters affect the number of users that can be supported by the system • Number of Sectors: • The use of sectored antenna is an important factor in maximizing BW efficiency • Cell sectorization using directional antennas reduces the overall interference and increasing M • Sectorization gain = GA

9. Practical Consideration.. • With ideal sectorization the user in one sector of a BS antenna do not interfere with the users operating in other sectors • GA = Nsec, Nsec is the number of sectors in the cell • In practice antenna patterns can not be designed to have ideal characteristics, and due to multipath reflection, users in general communicate with more than one sector • The sector BS gain assumed to be GA = 2.5 (4 dB) • The Voice of activity interference reduction factor (Gv): • The ratio of the total connection time to the active talkspurt • On the average, in two-way conversation, each user talks roughly 50% of the time

10. Practical Consideration.. • The short pause in the flow of natural speech reduce the activity factor further to about 40% of connection time in each direction • As a result, the typical number used for Gv = 2.5 (4 dB) • The Interference Increase Factor (Ho) • Accounts for the users in other cells in CDMA system • Because all neighboring cells in a CDMA cellular network operate at the same frequency, they will cause additional interference • This interference is relatively small due to the processing gain and the distance involved • Ho = 1.6 (2 dB), is commonly used in industry

11. Practical Consideration.. • From EQs 1, 2 and 3  (4) The last part of EQ.# 4 is called Performance improving factor: K=(GAGV/Ho) = 4 (6 dB)

12. Problem 3: Capacity of One Carrier in a Multi-Cell CDMA System with Correction Factors Determine the multi-cell IS-95 CDMA capacity with correction for sectorization and voice activity. Use the numbers from Problem 2. Solution: If we continue the previous example with the new correction factor included, the range for the number of simultaneous users becomes 64 < M < 260.

13. Problem 4: Comparison of the Capacity of Different 2G Systems Compare the capacity of IS-95 CDMA with AMPS FDMA and IS-136 TDMA systems. For the CDMA system, assume an acceptable signal to interference ratio of 6 dB, data rate of 9600 bps, voice duty cycle of 50 percent, effective antenna separation factor of 2.75 (close to ideal 3-sector antenna), and neighboring cell interference factor of 1.67.

14. Solution • For the IS-95 CDMA using Equation (4.4) for each carrier with W = 1.25 MHz, R = 9600 bps, Sr= 4 (6dB), Gv= 2 (50 percent voice activity). GA = 2.75, and Ho = 1.67 we have: • For the IS-136 with a carrier bandwidth of Wc = 30 kHz, the number of users per carrier of Nu= 3, and frequency reuse factor of K = 4 (commonly used in these systems), each W = 1.25 MHz of bandwidth provides for • For the AMPS analog system with carrier bandwidth of Wc = 30 kHz, and frequency reuse factor of K = 7 (commonly used in these systems), each W = 1.25 MHz of bandwidth provides for

15. Problem 4: Comparison of the Capacity of Different 2G Systems Determine the capacity of GSM for K = 3. Solution: For the GSM system with a carrier bandwidth of Wc = 200 kHz, the number of users per carrier of Nu= 8, and frequency reuse factor of K = 3 (commonly used in these systems), each W = 1.25MHz of bandwidth provides for