EE4607: GSM. Ronan Skehill Wireless Access Research E1016. Wireless Access Research. This Lecture covers the introduction to GSM

Transcription

1 EE4607: GSM This Lecture covers the introduction to GSM Ronan Skehill Wireless Access Research E1016. Wireless Access Research

2 2 nd Generation systems Digital systems introduced into Cellular Useful for Voice and Data Services Encryption possible, therefore better security. Better link margins (i.e. better SNR) using digital modulation Techniques. Mobile assisted handoff to reduce probability of dropped calls. Improved spectral efficiency through the used of speech coding, packet data compression. Question: how can we compress data?

3 GSM Architecture land link MSC VLR HLR land link Radio link Base Station VLR MSC MSC Mobile Switching Center VLR Visitor Location Register HLR Home Location Register

4 Architecture of a GSM network

5 Mobile Station (MS) Mobile Equipment (ME) Subscriber Identity Module (SIM) Mobile unit can be in two modes Idle - listening Dedicated sending/receiving data

6 Base Station Subsystem(BSS) Base Transceiver Station (BTS) Controls coverage of a cell Used to communicate to a node or group node via a wireless link Connected to a BSC Always in an Activate State Base Station Controller (BSC)

7 Network Subsystem(NS) Mobile services Switching Center (MSC) Home Location Register (HLR) Visitor Location Register (VLR ) Equipment Identity Register (EIR) Authentication Center (AuC) MxE

8 Network Stacks

9 GSM Stack cont. Radio Resource (RR) Handover Mobility Management (MM) Location Updating HLR VLR Authentication and security EIR

10 GSM Stack cont. Communication Management (CM) Call Control SMS Call Routing

11 GSM Radio Subsystem Two bands of 25 MHz each are set aside for normal GSM use MHz for subscriber-to-base (down-link) MHz for base-to-subscriber (uplink) Frequency bands are divided into 200 KHz Channels, each with eight (8) time slots

12 TDMA In TDMA systems each user gets not the whole time of the used carrier BUT a short time slot at regular intervals (Frames) To send continuous signal over and TDMA system, such as GSM, the data has to be packed and inserted into the slots.

13 TDMA

14 TDMA Contd. TDMA systems have one carrier in the uplink and one in the downlink Frequency multiplexing The Carrier is then time-multiplexed among users Frequency reuse is an open issue with TDMA systems Must ensure that neighbour cells DO NOT use the same frequency. Cells which use the same frequency contribute cochannel interference to one another

15 Example

16 TDMA Contd. TDMA systems require guard times as a result of Variation in propagation delay Variation in delay spread of the channel If the propagation delay is Tp and the Delay is D, then guard time must be at least Tp+D

17 GSM TDMA Frames Burst bits

18 Channels Traffic Channels Control Channels

19 Logical view Channels

20 Signalling Channels Signalling Channels fall under three areas Broadcast BCCH FCCH SCH, one channels from base to mobile Common Control - RACH AGCH PCH, one way channels, some from Mobile to base other Base to Mobile Dedicated - SDCCH, SACCG, FACCH, two way traffic related. Broadcast and common share the one carrier, dedicated has its own traffic carriers

21 GSM Traffic Channel Setup Mobile Interim Connection For Timing & Authentication SACCH & FACCH Broadcast Channel (BCH) Paging Channel (PCH) Random Access Channel (RACH) Access Grant Channel (AGCH) Stand-alone Dedicated Control Channel (SDCCH) Forward Traffic Channel (FTC) Reverse Traffic Channel (RTC) Base SACCH & FACCH

22 Summary of channels Control channels used by idle mode mobiles to exchange signaling information, required changing to dedicated mode. Mobiles in dedicated mode monitor the surrounding Base Stations for handover and other information. The Control channels include: Broadcast Control Channel (BCCH) serves for BS identification, broadcasts, and frequency allocations. Frequency Correction Channel (FCCH) and Synchronization Channel (SCH) used for synchronization, and physical layer definition (time slots, burst time ) Random Access Channel (RACH) used by mobile to request access to the network. Paging Channel (PCH) used for locating the mobile user Access Grant Channel (AGCH) used to obtain a dedicated channel. (Following the request of RACH)

23 Summary Contd.

24 GSM Procedures Acquiring the network Registration Location Update Call Setup (MO and MT) Call Termination Handoff

25 Call Setup

26 Call Setup By MS MS Searches for a BS with adequate Rx level on the BCCH carrier MS waits for frequency correction burst followed by 64 bit synchronization sequence on FCCH and synchronize to it BS allocates a bidirectional signaling channel (SDCCH) BS sets up a link with the MSC via the network MSC uses IMSI from the MS to interrogate the HLR, the routes the information to the HLR Authentication now takes place

27 Call Setup By MS (cont) After authentication, MS provides the destination number BS allocates a traffic channel MSC routes the call to its destination If the MS moves to another cell, a new BS is assigned and a handover takes place If both BS locations are controlled by the same BSC, the BSC controls the handover, otherwise MSC takes this responsibility

28 Mobile Terminating Example

29 Mobile Terminating Example

30 SMS Not requiring the end-to-end establishment of a traffic path Asymmetric: Mobile Originating Short Message trasmission is considered as a different service from Mobile Terminating Short Message trasmission Ultimate destination (identified by some field in the message) is relevant only for the user and the SMSC, not for the GSM infrastructure

31 Example: short message (SM) E D MS1 Submit SM MS2 MSC1 + VLR Ack MS2 switched on Ack Forward MO SM Deliver SM MSC2 + VLR SMSgwy + GMSC C HLR Ack Send routing info Routing info Forward MT SM Nack (MS2 switched off) Report SM waiting Ack Report ready for SM Forward MT SM Ack Ack Alert SMSC Ack

32 GSM Technical Parameters Multiple Access Frequency Mobile to BS BS to Mobile Carrier Spacing Users per Carrier Modulation Voice Codec Coded Bit Rate Tx Rate TDMA Frame Size Bits per Time Slot FDMA & TDMA 800Mhz Mhz Mhz 200 khz 8(16) GMSK RPE-LTP 22.8 kb/s kb/s 4.6ms 148