Code Division Multiple Access

1. Code Division Multiple Access Eng. MshariAlabdulkarim

3. Spread Spectrum Methods.

4. Technologies based on CDMA.

5. Performance analysis of IEEE 802.11 protocol.

7. Code Division Multiple Access Introduction (Cont.): How does it support more than one group in the same channel? It allows multiple groups to share the same channel at the same time by assigning each group to different code. CDMA Modulation Steps: Generate a local pseudo-random code with a higher rate than the data to be transmitted. XOR the data which need to be transmitted with the generated code.

8. Code Division Multiple Access Introduction (Cont.): Data Signal Pseudo-random Code Transmitted Signal Time

9. Code Division Multiple Access Introduction (Cont.): Input data Output data Channel Encoder Modulator Channel Demodulator Channel Decoder Pseudo-noise Generator Pseudo-noise Generator General Model of Spread Spectrum Digital Communication System

11. The immunity from the multipath distortion.

12. It can be used as an encrypting technique.

14. It must appear random to a listener who does not have a prior knowledge of the code.

15. The cross-correlation between any two codes must be small.

17. The best performance will be when the users’ signals are separated as much as possible.

18. The separation can be done by correlating the received signal with the local generated code.

21. The original data is spread out across a wider frequency band by using the spreading code.

24. The receiver should use the same hopping pattern simultaneously with the transmitter in order to receive the data correctly.

27. It encodes the information by using wideband linear frequency modulated chirp.

28. It uses the entire allocated bandwidth to broadcast the signal, which helps making it robust to channel noise.

29. Also, it is immunized from the multi-path fading, because it utilizes the spectrum broadband.

31. It consumes low power due to the very low duty cycles.

33. Code Division Multiple Access Technologies based on CDMA: WiFi (IEEE 802.11): 802.11b and 802.11g working in the 2.4 frequency band (Industry, Science and Medicine band or ISM). DSSS and FHSS physical layer options have been designed specifically to overcome the interfering with other devices in this band.

34. Code Division Multiple Access 22 MHz Channel 6 2.4370 Channel 11 2.4620 Channel 1 2.4120 2.4000 GHz 2.4835 GHz Three Non-Overlapping DSSS Channels in the ISM Band

35. Code Division Multiple Access

37. It uses Frequency Hoping Spread Spectrum (FHSS) to make it rare for more than one device to transmit on the same time using the same frequency and to avoid any interference.

41. The satellite carrier frequencies are modulated using the ranging signal which is PRN code which is different between satellites.

42. The receiver uses the satellite PRN code to reconstruct the actual message data.

44. Code Division Multiple Access Performance analysis of IEEE 802.11 protocol: There are several assumptions have been considered in presenting this analytical framework: The effect of bit errors introduced by channel noise has been ignored. The only considered errors are the one which caused by collisions due to other simultaneous transmissions. There are no hidden stations. The propagation delays are not considered. There are n stations in the network.

45. Code Division Multiple Access Performance analysis of IEEE 802.11 protocol (Cont.): Every station always has a packet to send (saturated conditions). The collision probability of a transmitted frame is constant and independent of the number of retransmissions that this frame has experienced in the past. RTS/CTS access mechanism.

46. Code Division Multiple Access Performance analysis of IEEE 802.11 protocol (Cont.): The sequence of events in a successful frame transmission using the RTS/CTS access method will be like the following: RTS: Request To Send. SIFS: Short Inter-Frame Space. CTS: Clear To Send. DIFS: Distributed Coordination Function.

47. Code Division Multiple Access t0 DATA RTS Source Destination SIFS CTS SIFS SIFS ACK Other NAV (RTS) DIFS NAV (CTS) NAV (DATA) Defer Access Back-off RTS: Request To Send. SIFS: Short Inter-Frame Space. CTS: Clear To Send. DIFS: Distributed Coordination Function.

48. Code Division Multiple Access Performance analysis of IEEE 802.11 protocol (Cont.): Abbreviations:

49. Code Division Multiple Access Performance analysis of IEEE 802.11 protocol (Cont.): For simplicity, I assumed that the value of τ will be known and we will start by calculating the value of p: Because we have n stations and all of them will transmit with a probability of τ , then the value of will be:

50. Code Division Multiple Access Performance analysis of IEEE 802.11 protocol (Cont.): The probability of successful transmission will be equal to the probability that one station is transmitting while the remaining stations stay silent: If is the frame header, the average time delays for RTS/CTS will be:

51. Code Division Multiple Access Performance analysis of IEEE 802.11 protocol (Cont.): The throughput will be equal to the time needed to transmit the payload information divided by the average length of slot time:

53. It supports frequency division (FDD) and inter-cell asynchronous operation.

54. It employs coherent detection on both the uplink and downlink.

55. It has a Variable mission on a 10 ms frame basis.

56. It supports Multi-code transmission.

58. The signals from different users are separated in both time and code domain.

59. It has a frame length of 10 ms and it is divided into 15 time slots.

60. Each slot has duration of 666 micro second and length of 2560 chips.

62. It is better suited for densely populated areas.

63. It uses time-division duplexing (TDD).

64. It can accommodate asymmetric traffic with different data rates on downlink and uplink.