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Transcript

  • 1. CDMA BUITEMS 15-6-2013
  • 2. • FDMA • TDMA • CDMA – Direct sequence – Frequency hopping
  • 3. Attributes of CDMA systems • System Capacity: • QoS: – Soft Handoff – RAKE receiver for multipath signals – Ping pong effect elimination – Hand off noise elimination – Make before break handoff • Economies: – Lower average mobile power – longer battery life
  • 4. RAKE receiver
  • 5. Direct Sequence Spread spectrum • The user data is multiplied by a PN code with higher rate and spread. • spreading factor is the ratio of the chip rate to the data rate, • At the receiver end by multiplying the received baseband signal with a phase-coherent copy of that PN code
  • 6. Power control in CDMA • Most important requirements in a CDMA system • Far-near problem • With out power control – Difficult for BS to detect signal with lower SIR • To over come this power control commands sent by BS to – Increase power of remote MS – Decrease power of nearby MS
  • 7. Power control • One key advantage is design of compact MS with longer battery life. • The system capacity is maximum when the transmitter power of each mobile station is adjusted such that the received SIR from each mobile is just about enough to maintain the desired quality.
  • 8. Power control • Works in both forward and reverse link directions. • MS requests BS to adjust it transmit power • Closed loop power control is based upon direct measurements of the desired signal • Open loop power control where For example, the mobile station could measure the signal received from the base station, estimate the path loss on the forward channel, and accordingly adjust the transmit power of the reverse channel
  • 9. Handoff in CDMA systems • In GSM MS changes its frequency from one cell to other, called Hard handoff – Break before make • Soft handoff in CDMA – Make before break
  • 10. Handoff in CDMA • Handoffs Supported in IS-95 – Soft handoff – Hard handoff – CDMA to analog cellular system handoff
  • 11. Handoff in CDMA • MS continuously monitors pilot channels • It send pilot channel information to BS for initiation of handoff • For the purpose of handoff, a mobile station maintains four sets of pilots: – Active set – Candidate set – Neighbor set – Remaining set
  • 12. CDMA2000 • Traffic types – Traditional Voice and voice over IP – Data traffic- IP based traffic – Circuit-emulated broadband data(Fax etc. dialup etc, circuit emulated audio, video) – SMS
  • 13. CDMA2000 • Bandwidth: – 1.25 and 5 MHz(WCDMA also uses 5MHz) • Quality of service: – MS can request QoS if multiple services are being used • Packet Mode Data Services – if there is a packet to send, the user attempts to establish the dedicated and common control channels using the multiple-access slotted Aloha scheme
  • 14. CDMA2000 • Transmit Diversity • With a 5 MHz, direct-spread CDMA system, the user data may be divided into two or more streams, each spread with an orthogonal code, and then transmitted to mobile stations. Because of multipath diversity, the forward channel performance may improve significantly ,
  • 15. CDMA2000 protocol stack • The link layer consists of the link access control (LAC) and media access control (MAC) layers. • The MAC layer is divided into two sub layers: – the physical layer-independent convergence function (PLICF) – and physical layer-dependent convergence function (PLDCF)
  • 16. CDMA2000 protocol stack • Each traffic type coming from the higher layer has a different QoS requirement in terms of delays, delay variations, and error rates. The function of the LAC is to ensure that various types of traffic are transferred over the air interface according to their QoS requirements.
  • 17. CDMA2000 protocol stack
  • 18. MAC sublayer • A MAC sub layer performs the following functions 1. It controls user access to the physical layer (that is, the medium) by resolving, if necessary, contention among multiple applications from the same user or among multiple users, and scheduling its resources so as to ensure efficient utilization of bandwidth. Resources include buffers, spreading codes and so on.
  • 19. MAC sublayer 2- User data and signaling information from the upper layers (that is, the LAC layer and the higher layers) are multiplexed, mapped into different physical channels, and delivered to the physical layer on a best-effort basis, providing a basic level of transmission reliability.
  • 20. Contd.. • PLCIF: Functions that are independent of the physical layer, such as controlling access to the medium so as transmit packets, are performed by the sub layer called PLICF. • PLDCF: Functions performed at this sub layer when transmitting over the air interface include multiplexing logical channels coming from PLICF, mapping them into physical channels, assigning proper priorities to each according to its QoS requirement, and delivering them to the physical layer.
  • 21. 17-05-2013
  • 22. CDMA 2000(IS-2000) • IS-95 -CDMAone • IxRTT(One time Radio Txn tech.) 1.25 MHz FDD pair • Migration path for next G • Multi carrier vs single carrier • CDMA2000 logical extension of CDMAone • Packet handling capability added to existing architecture.(Packet data serving node)
  • 23. CDMA evolution
  • 24. Spectrum for CDMA evolution
  • 25. Spectrum • CDMA2000 and WCDMA are both based on CDMA technology • Unlike WCDMA(5MHz) spectrum, CDMA2000 can utilize Wide and narrow channels • World wide uniform spectrum allocation • LTE spectrum allocation? Next slide
  • 26. LTE spectrum 2011-2015
  • 27. Capacity of CDMA systems • Eb/No (energy per bit/noise power per bit) relation to capacity of system • Processing gain(ratio of the spread bandwidth to the un spread (or baseband) bandwidth) • Number of Walsh codes • Number of carriers aggregated • Voice activity detection, Saves BW
  • 28. CDMA capacity • Offsets have to be different for different sectors in a cellular system, their allocation must be managed and coordinated in much the same way as the channels for cellular and TDMA systems.