Dr Asad Hussain Wireless and Mobile Computing

1. Dr Asad HussainWireless and Mobile Computing GSM and CDMA

2. Registration • With wireless access to a cellular network each time we turn on the MS we need to establish a new connection and possibly a new registration with the network. • MS synchronizes to the frequency, and frame timings of the closest BS to get ready for information exchange with the BS.

3. Registration • After this step, MS reads the system and cell identity to determine its location in the network. If the current location is not the same as before, the MS initiates registration procedure. • The MS provides its identity in exchange for the identity of the N/W and finally the N/W authenticates the MS.

4. Registration • If MS is turned on in new MSC area that needs changes in the entries of the VLR and HLR

5. Registration procedure

6. Mobile originated call

7. The GSM protocol architecture Um air interface A-bis A CM CM MM MM RRM RRM RRM RRM SCCP SCCP LAPDm LAPDm LAPD LAPD MPT MTP Radio Radio 64kpbs 64kbps 64kbps 64 kbps MS BTS BSC MSC

8. Logical Channels • The functions of signaling for registration, call establishment, synchronization are performed through logical channels. • Logical Channels use TDMA slots to specify an operation in the network

9. GSM Channel Types • Traffic Channels (TCHs) and control channels (CCHs). • Traffic Channels carry digitally encoded user speech or user data and have identical functions and formats on both forward and reverse link. • Control channels carry signaling and synchronizing commands between BS and the mobile station.

10. Traffic Channels • GSM traffic channels may be either full rate or half rate. • When transmitted as full rate, user data is contained within one TS per frame. • Half rate channel users would share the same time slot, but alternately transmit during every other frame.

11. Traffic Channels • The full-rate traffic channel (TCH/F) uses a 13 kbps speech-coding scheme and 9,600 bps, 4,800 bps, and 2,400 bps data. • The half-rate traffic channel (TCH/H): GSM also supports half-rate speech coding traffic channels. The TCH/H channels uses 16 slots per frame that has a gross bit rate of 11.4 kbps. The half-rate TCH supports 4.8 kbps and 2.4 kbps data.

12. Control Channels • The frequency control channel (FCCH) An MS in the coverage area of a BS uses the FCCH to synchronize its internal frequency to the frequency of the BS • The synchronization channel (SCH) used by the BS to broadcast frame synchronization signals to all MSs. Using SCH, MSs will synchronize their counters to specify the location of arriving packets in the TDMA hierarchy.

13. Control Channels • The broadcast control channel (BCCH) is used by BS to broadcast available services, network and cell ID. Once the synchronization between the BS and MS are established, the BCCH informs the MS about the environment parameters associated with the BS covering that area.The BCCH is also is used for signal strength measurements for handoff.

14. CCCH Logical Channels • The paging channel (PCH) is used by the BS to page the MSs in the cell and notifies a specific mobile of an incoming call which originates from the PSTN. PCH may be used to provide text messages to all subscribers as part of the SMS feature of GSM. • The random access channel (RCH) is used by the MS to access the BS for call establishment. The RCH is used for implementation of a slotted ALOHA protocol, which is used by mobile stations to contend for one of the available slots in the GSM traffic frames.

15. CCCH Logical Channels • The access grant channel (AGCH) is used for implementation of the acknowledgement from the BS to the MS after a successful attempt by MS using RCH.

16. DCCH Logical Channels • The stand alone dedicated control channel (SDCCH) is a two-way channel assigned to each terminal to transfer network control information for call establishment and mobility management. It ensures that the MS and the BS remain connected while the BS and MS verify the subscriber identity and allocate resources for the mobile. The physical channel for SDCCH occupies four slots in every 51 control-multiframes with an approximated gross data rate of 2 kbps per channel.

17. DCCH Logical Channels • The slow associated control channel (SACCH) is a two-way channel assigned to each TCH and SDCCH channels. The SACCH is used to exchange the necessary parameters between the BS and the MS to maintain the link. It is used to transmit regularly changing control information to the mobile such as power transmit levels. The reverse SACCH carries information about the received signal strength and quality of the TCH. The gross data rate of the SACCH channel is half of that of the SDCCH.

18. DCCH Logical Channels • The fast associated control channel (FACCH) is a two-way channel used to support fast transitions (urgent) in the channel when SACCH is not adequate. The FACCH is physically multiplexed with the TCH or SDCCH to provide additional support to the SACCH.

19. Call establishment in GSM using logical channels

20. CDMA Design Considerations • Bandwidth – limit channel usage to 5 MHz • Chip rate – depends on desired data rate, need for error control, and bandwidth limitations; 3 Mcps or more is reasonable • Multirate – advantage is that the system can flexibly support multiple simultaneous applications from a given user and can efficiently use available capacity by only providing the capacity required for each service

21. Drawbacks of CDMA Cellular • Self-jamming – arriving transmissions from multiple users not aligned on chip boundaries unless users are perfectly synchronized • Near-far problem – signals closer to the receiver are received with less attenuation than signals farther away • Soft handoff – requires that the mobile acquires the new cell before it relinquishes the old; this is more complex than hard handoff used in FDMA and TDMA schemes

22. Mobile Wireless CDMA Design Considerations • RAKE receiver – when multiple versions of a signal arrive more than one chip interval apart, RAKE receiver attempts to recover signals from multiple paths and combine them • This method achieves better performance than simply recovering dominant signal and treating remaining signals as noise • Soft Handoff – mobile station temporarily connected to more than one base station simultaneously

23. RAKE RECEIVER • In a multipath environment RAKE receiver attempts to recover the signals from multiple paths and then combine them with suitable delays. • The original binary signal to be transmitted is spread by the XOR operation with the transmitter’s chipping code. • The spread sequence is then modulated for transmission over the wireless channel.

24. RAKE RECEIVER • Because of multipath effects, the channel generated multiple copies of the signal, each with a different amount of time delay (1,2,3) and each with a different attenuation factors (a1, a2, a3). • The receiver, the combined signal is demodulated. • The demodulated chip stream is then fed into multiple correlators, each delayed by a different amount.

25. Principle of RAKE Receiver

26. Soft Handoff • The handoff is between two sectors of different cells. • The candidates for handoff include two sectors from the same cell and a third sector from a different cell. • The soft handoff procedure involves several base stations.

27. Soft Handoffs

28. Soft Handoffs • A controlling primary BS coordinates the addition and deletion of other base stations to the call during soft handoff. • At some point of time the primary BS is also changed after handoff. • The signals from multiple BSs are combined in the MSC and processed as a single call using a frame selector join message.

29. Soft Handoffs • The MS detects a pilot signal from a new BS and informs the primary BS. • After a traffic channel is set up with the new BS, the frame selector join message is used to select signal from both BSs at the BSC/MSC. After a while, the pilot signal from the old BS starts falling and MS will requests its removal, which is achieved via a frame selector remove message.

30. Forward/Reverse Channels in CDMA • The air-interface in CDMA systems is by far the most complex of all systems, not symmetrical on the forward and reverse channels unlike TDMA systems. • Spectrum Spreading and error control coding are different for forward and reverse channels.

31. Forward/Reverse Channels in CDMA • In the forward channel, transmissions originate at a single transmitter (the BS) and transmissions for all users are synchronized. • On the reverse channel, mobile terminals transmit whenever they have to.

32. The IS-95 CDMA Forward Channel • The IS-95 forward channel consists of four types of logical channels-pilot channel, synchronization channel, paging channel, and traffic channels. • Modulation scheme used in the forward channel is QPSK. • These channels are separated from one another using different spreading codes

33. IS-95 CDMA Forward Channel • Any information contained in the form of symbols (after coding, interleaving etc) is modulated by Walsh Codes (in the form of Matrices). • Each Walsh code identifies one of the 64 forward channels. • PN (Pseudonoise) codes are used for spreading bits in a spread spectrum system • They are further scrambled in the in-phase and quadrature phase lines by PN-spreading codes • PN spreading codes possess correlation properties to minimize interference among different channels

34. IS-95 CDMA Forward Channel • The orthogonal codes are used used to isolate the transmissions between different channels within a cell, and the PN spreading codes are used to separate the transmissions between different cells. • PN codes are used to differentiate between several BSs in the areas that are all using the same frequency

35. Spreading Procedure on the Forward Channel in IS-95 I Pilot PN codes/sequence Walsh Code Baseband Filter Baseband Filter Q Pilot PN Codes/sequence

36. Types of Channels Supported by Forward Link • Pilot (channel 0) - allows the mobile unit to acquire timing information, provides phase reference and provides means for signal strength comparison • Synchronization (channel 32) - used by mobile station to obtain identification information about cellular system • Paging (channels 1 to 7) - contain messages for one or more mobile stations • Traffic (channels 8 to 31 and 33 to 63) – the forward channel supports 55 traffic channels

37. Forward Traffic Channel Processing Steps • Speech is encoded at a rate of 8550 bps • Additional bits added for error detection • Data transmitted in 2-ms blocks with forward error correction provided by a convolutional encoder • Data interleaved in blocks to reduce effects of errors • Data bits are scrambled, serving as a privacy mask

38. Forward Traffic Channel Processing Steps (cont.) • Power control information inserted into traffic channel • DS-SS function spreads the 19.2 kbps to a rate of 1.2288 Mbps using 64 x 64 Walsh matrix • Digital bit stream modulated onto the carrier using QPSK modulation scheme

39. IS-95 Forward Channel

40. IS-95 Forward Channel

41. IS-95 CDMA Reverse Channel • The CDMA reverse channel is fundamentally different from the forward channel. • It uses OQPSK (closer to constant envelope modulation) rather than QPSK in the forward channel. • Constant envelope modulation techniques provides for a more power efficient implementation of the transmitter at the MS. • No use of Walsh codes.

42. IS-95 Reverse Channel

43. IS-95 Reverse Channel

44. Reference Architecture of 802.11 • The Basic Service Area (BSA) is the coverage area of one access point. • The basic service set (BSS) is a set of stations controlled by one access point. • The distribution system (DS) is the fixed (wired) infrastructure used to connect a set of BSS to create an extended service set.

45. Reference Architecture of 802.11 • The cards in the laptop and the AP support the MAC and PHY layers of the IEEE802.11, the rest of AP device acts as a bridge to convert the 802.11 protocol to MAC and the PHY layer of the backbone DS, that is typically an IEEE802.3 Ethernet LAN.

46. Reference Model

47. Implementation

48. Protocol Entities for 802.11

49. Layered Protocol Architecture • MAC layer is divided into MAC sublayer and MAC management sublayer entities. Responsible for access mech, fragmentation, and reassembly of the packets. • MAC layer management is responsible for roaming in ESS, power mgt, and association and reassociation processes for registration conn mgt.

50. Layered Protocol Architecture • The PHY layer is divided into 3 sublayers: PHY layer convergence protocol (PLCP), PHY medium dependent (PMD) protocol, and the PHY layer mgt sublayer. • The PLCP is responsible for carrier sensing and forming packets for different PHY layers. • PMD sublayer specifies the modulation, and coding technique for signaling with the medium • PHY layer mgt decides on channel tuning to different options for each PHY layer. • Station mgt layer is for the coordination of the interactions between MAC and PHY layers