GSM Spectrum Allocation

1. GSM Spectrum Allocation • P-GSM Spectrum (Primary GSM) • E-GSM Spectrum (Extended GSM) • DCS-1800 Spectrum • PCS-1900 Spectrum

2. P-GSM Spectrum (Primary GSM) The initial allocation of spectrum for GSM provided 124 carriers with Frequency Division Duplex for uplink and downlink: • Duplex sub bands of width 25 MHz - duplex spacing 45 MHz • Uplink sub band: 890 MHz to 915 MHz • Downlink sub band: 935 MHz to 960 MHz • Frequency spacing between carriers is 200 kHz (0.2 MHz) • One carrier is used for guard bands. • Total number of carriers (ARFCNs) = (25 – 0.2) / 0.2 = 124 • Uplink frequencies: Fu(n) = 890 + 0.2 n () • Downlink frequencies: Fd(n) = Fu(n) + 45 • Where n = ARFCN (ARFCN – Absolute Radio Frequency Carrier Number)

3. E-GSM Spectrum (Extended GSM) • E-GSM allocated extra carriers at the low end of the spectrum. The ARFCN numbers of P-GSM were retained (with 0 now included) and new ARFCNs introduced for the lower end, numbered 975 – 1023. • Duplex sub bands of width 35 MHz - duplex spacing 45 MHz (same as PGSM) • Uplink sub band: 880 MHz to 915 MHz • Downlink sub band: 925 MHz to 960 MHz • Frequency spacing of 200 kHz • One carrier used to provide guard bands • Total number of carriers (ARFCNs) = (35 – 0.2) / 0.2 = 174 • Uplink frequencies: Fu(n) = 890 + 0.2n () Fu(n) = 890 + 0.2 (n – 1024) () • Downlink frequencies: Fd(n) = Fu(n) + 45 0

4. 900 MHz Utilization in Jordan 880 885 890 902.5 915 MHz Zain Orange Zain MHz 925 930 935 947.5 960

5. DCS-1800 Spectrum • Digital Communication System – 1800 MHz introduced a further spectrum range for GSM, typically used for smaller microcells overlaid over existing macrocells. • Duplex sub bands of width 75 MHz - duplex spacing 95 MHz • Uplink sub band: 1710 MHz to 1785 MHz • Downlink sub band: 1805 MHz to 1880 MHz • Frequency spacing of 200 kHz • One carrier used to provide guard bands • Total number of carriers (ARFCNs) = (75 – 0.2) / 0.2 = 374 • Uplink frequencies: Fu(n) = 1710.2 + 0.2 (n – 512) () • Downlink frequencies: Fd(n) = Fu(n) + 95

6. 1800 MHz Utilization in Jordan 1710 1740 1755 1785 MHz Umniah MHz 1805 1835 1850 1880

7. 1800 MHz Utilization in UK

8. PCS-1900 Spectrum • Personal Communication System – 1900 MHz is used in USA and Central America to provide a service similar to GSM. • Duplex sub bands of width 60 MHz - duplex spacing 80 MHz • Uplink sub band: 1850 MHz to 1910 MHz • Downlink sub band: 1930 MHz to 1990 MHz • Frequency spacing of 200 kHz • One carrier used to provide guard bands • Total number of carriers (ARFCNs) = (60 – 0.2) / 0.2 = 299 • Uplink frequencies: Fu(n) = 1850.2 + 0.2 (n – 512) () • Downlink frequencies: Fd(n) = Fu(n) + 80

9. Multiple Access Techniques • Purpose: to allow several users to share the resources of the air interface in one cell • Methods: • FDMA - Frequency Division Multiple Access • TDMA - Time Division Multiple Access • CDMA - Code Division Multiple Access

10. FDMA - Frequency Division Multiple Access Frequency • Divide available frequency spectrum into channels each of the same bandwidth • Channel separation achieved by filters: • Good selectivity • Guard bands between channels • Signallingchannel required to allocate a traffic channel to a user • Only one user per frequency channel at any time • Used in analog systems, such as AMPS, TACS • Limitations on: • frequency re-use • number of subscribers per area Channel BW Time

11. TDMA - Time Division Multiple Access Access to available spectrum is limited to timeslots • User is allocated the spectrum for the duration of one timeslot • Timeslots are repeated in frames Frequency TS0 TS1 TS2 TS3 TS4 TS5 TS6 TS7 TS0 TS1 TS2 TS3 TS4 TS5 TS6 TS7 Time Frame Time slot

12. CDMA - Code Division Multiple Access • Each user is assigned a unique digital code (pseudo - random code sequence) • Code is used at Mobile Station and Base Station to distinguish different user’s signals • Many users’ communications can be transmitted simultaneously over the same frequency band • Advantages: • very efficient use of spectrum • does not require frequency planning • Used in IS - 95 (cdmaOne) • Not used in GSM • Wideband CDMA techniques used in UMTS Frequency Code Time

13. higher GSM frame structures TS4 TS6 TS7 TS0 TS1 TS3 TS5 TS2 4.615 ms 546.5 µs 577 µs GSM - TDMA/FDMA Using FDMA and TDMA techniques, each carrier is divided into 8 Physical channels (timeslots) 935-960 MHz 124 channels (200 kHz) downlink frequency 890-915 MHz 124 channels (200 kHz) uplink time GSM TDMA frame GSM time-slot (normal burst) “Physical Channel” guard space guard space S user data tail tail user data S Training 1 3 1 57 bits 3 bits 57 bits 26 bits 156.25 bit periods

14. Uplink and Downlink Synchronization • TDMA is used to provide a set of 8 physical channels (timeslots) on each carrier • One cycle of 8 timeslots forms the TDMA frame of 4.615 ms duration • Each timeslot lasts for 0.577 ms (156.25 bit periods) and can contain one of several types of data burst • A mobile station cannot transmit and receive simultaneously. • The MS transmit burst is delayed by 3 timeslots after the BTS burst. • This delay allows the MS to compare signal quality from neighboring cells

15. GSM Channels • A timeslot is the basic physical resource (channel) in GSM, which is used to carry all forms of logical channel information, both user speech/data and control signaling. • Logical Channels - the various ways we use the resource- one physical channel may support many logical channels. logical channels are piggybacked on the physical channels • Multiframestructures is used to provide all the logical channels required. • Different structures of data burst are used in the timeslot for different purposes.

16. Logical Channels TCH Traffic Channels TCH/F Traffic Channel (full rate) (U/D) TCH/H Traffic Channel (half rate) (U/D) BCH Broadcast Channels FCCH Frequency Correction Channel (D) SCH Synchronization Channel (D) BCCH Broadcast Control Channel (D) CCCH Common Control Channels PCH Paging Channel (D) RACH Random Access Channel (U) AGCH Access Grant Channel (D) CBCH Cell Broadcast Channel (D) NCH Notification Channel (D) DCCH Dedicated Control Channels SDCCH Stand alone Dedicated Control Channel (U/D) SACCH Slow Associated Control Channel (U/D) FACCH Fast Associated Control Channel (U/D) U = Uplink D = Downlink • GSM uses a set of logical channels to carry call traffic, signaling, system information, synchronization etc. • The logical channels are divided into traffic channels and control channels • They can then be further divided as shown:

17. Traffic Channels (TCH) • TCH carries payload data - speech, fax, data- normally time slots 1 - 7 if TS0 is used for control signaling • Connection may be: • Circuit Switched - voice or data or • Packet Switched – data • TCH may be: • Full Rate (TCH/F) • one channel per user • 13 kbps voice, 9.6 kbps data • or • Half Rate (TCH/H) • one channel shared between two users (alternatively from frame to frame) • 6.5 kbps voice, 4.8 kbps data

18. Broadcast Channels (BCH) BCH channels are all downlink and are allocated to timeslot zero some times called BCCH. The RF carrier used to transmit the BCCH is referred to as the BCCH carrier. BCH Channels are: • FCCH: Frequency correction channel sends the mobile a burst of all ‘0’ bits which allows it to fine tune to the downlink frequency • SCH: Synchronization channel, the SCH carries the information to enable the MS to synchronize to the TDMA frame structure and know the timing of the individual timeslots, it sends the absolute value of the frame number (FN), which is the internal clock of the BTS, together with the Base Station Identity Code (BSIC). • BCCH: Broadcast Control Channel sends radio resource management and control messages: • Location Area Identity (LAI). • List of neighboring cells that should be monitored by the MS. • List of frequencies used in the cell. • Cell identity. • Power control indicator. • DTX permitted. • Access control (i.e., emergency calls, call barring ... etc.). • CBCH description. Some messages go to all mobiles, others just to those that are in the idle state. • As the name suggests, the broadcast channels send information out to all mobiles in a cell. These channels are also important for mobiles in neighboring cells which need to monitor power levels and identify the base stations.

19. Common Control Channels (CCCH) CCCH contains all point to multi-point downlink channels (BTS to several MSs) and the uplink Random Access Channel: • CBCH: Cell Broadcast Channel is an optional channel for general information such as road traffic reports sent in the form of SMS. • PCH: Paging Channel sends paging signal to inform mobile of a call, (paging can be performed by an IMSI, TMSI or IMEI). • RACH: Random Access Channel is sent by the MS to request a channel from the BTS or accept a handover to another BTS. A channel request is sent in response to a PCH message. • AGCH: Access Grant Channel allocates a dedicated channel (SDCCH) to the mobile. • NCH: Notification Channel informs MS about incoming group or broadcast calls. • The main use of common control channels is to carry the information needed to set up a dedicated channel. Once a dedicated channel (SDCCH) is established, there is a point to point link between the base station and mobile. Associated control channels carry additional signalling to support dedicated channels. SACCH is associated with either SDCCH or TCH. FACCH is only associated with TCH.

20. Dedicated Control Channels (DCCH) DCCH comprise the following bi-directional (uplink / downlink) point to point control channels: • SDCCH: Standalone Dedicated Control Channel is used for call set up, Authentication, location updating and also point to point SMS. • ACCH:Associated Control Channels can be associated with either an SDCCH or a TCH, they are used for carrying information associated with the process being carried out on either the SDCCH or the TCH. • SACCH: Slow Associated Control Channel conveys power control and timing information in the downlink direction (towards the MS) and Receive Signal Strength Indicator (RSSI), and link quality reports in the uplink direction during a call or operations associated with SDCCH. • FACCH: Fast Associated Control Channel is used (when needed) for signallingduring a call, mainly for delivering handover messages and for acknowledgement when a TCH is assigned.

21. Multiframes • To provide all the logical channel operations with the physical resources (timeslots) available, an additional time frame structure is required in which the logical channels are multiplexed onto the timeslots. This is the concept of multiframes. • Multiframes provide a way of mapping the logical channels on to the physical channels (timeslots). • A multiframe is a series of consecutive instances of a particular timeslot. • GSM uses multiframes of 26 and 51 timeslots.

22. Traffic Channel Multiframe • The TCH multiframe consists of 26 timeslots. • This multiframe maps the following logical channels: • TCH • SACCH • FACCH • TCH Multiframe structure: Frame # T = TCH, S = SACCH, I = Idle FACCH is not allocated slots in the multiframe. It steals TCH slots when required - indicated by the stealing flags in the normal burst. • TCH is always allocated on the 26 frame multiframe structure shown above. • During a call the mobile is continually monitoring power levels from neighboring base stations. It does this in the times between its allocated timeslot. Once each traffic channel multiframe there is a SACCH burst which is used to send a report on these measurements to the current serving base station. • The downlink uses this SACCH burst to send power control and other signals to the mobile.

23. Control Channel Multiframe • The control channel multiframe is formed of 51 timeslots. • CCH multiframe maps the following logical channels: • A basic BCCH multiframe is shown below which use TS0. The main reason for other structures is the allocation of SDCCH/SACCH.

24. Different Control Channel structures TS0 TS1 While TS0 as in the previous slide

25. GSM hierarchy of frames hyperframe 0 1 2 ... 2045 2046 2047 3 h 28 min 53.76 s superframe 0 1 2 ... 48 49 50 traffic 6.12 s 0 1 ... 24 25 control multiframe 0 1 ... 24 25 120 ms traffic 0 1 2 ... 48 49 50 235.4 ms control frame 0 1 ... 6 7 4.615 ms slot The timing of the hyperframe relates to the cycle of frame numbers transmitted on the synchronization channel (SCH). After 26 x 51 x 2048 = 2715648 frames, the frame number (which consists of 22 bits) resets to zero. burst 577 µs

26. Types of Data Burst • The 156.25 bit periods of a timeslot can hold different types of data burst:

27. Timing Advance Timing Advance is needed to compensate for different time delays in the transmission of radio signals from different mobiles. • Signal from MS1 takes longer to arrive at BTS than that from MS2 • Timeslots overlap - collision • Timing Advance signal causes mobiles further from base station to transmit earlier - this compensates for extra propagation delay

28. TA Cont. • The maximum value of Timing Advance sets a limit on the size of the cell. • Timing Advance is calculated from delay of data bits in the RACH burst received by the base station – long guard period allows space for this delay • It is adjusted during the call in response to subsequent normal burst positions. • TA signal is transmitted on SACCH as a number between 0 and 63 in units of bit periods • TA value allows for ‘round trip’ from MS to BTS and back to MS • Each step in TA value corresponds to a MS to BTS distance of 550 metres • Maximum MS to BTS distance allowed by TA is 35 km

29. TA Cont. • Timing Advance value reduces the 3 timeslot offset between downlink and uplink TA Uplink Actual delay • The Timing Advance technique is known as adaptive frame alignment

30. GSM Modulation Technique Gaussian Minimum Shift Keying (GMSK): • Frequencies are arranged so there is no phase discontinuity at the change of bit period. • Data pulses are shaped using a Gaussian filter: • Smoothes phase transitions • Gives a constant envelope • QPSK is used in IS-95 (CDMA). • Comparison of GMSK and QPSK: • GMSK requires greater bandwidth • QPSK reduces interference with adjacent carrier frequencies • GMSK is more power efficient - less battery drain from MS on uplink • GMSK has greater immunity to signal fluctuations

31. Speech over the Radio Interface

32. Speech Coding • GSM transmits using digital modulation - speech must be converted to binary digits • Coder and decoder must work to the same standard • Simplest coding scheme is Pulse Code Modulation (PCM): • Sampling every 1/(2*4k)=125 μs • Assume each sample is mapped to an 8 bit codeword (256 levels of an equalizer) then this requires data rate of 8k*8=64 kbps • This is too high for the bandwidth available on the radio channels

33. Advanced Speech Coding • Several approaches to modeling human speech which requires less data than PCM have been attempted. • Estimates are that speech only contains 50 bits per second of information • Compare time to speak a word or sentence with time to transmit corresponding text • Attempts to encode speech more efficiently: • speech consists of periodic waveforms -so just send the frequency and amplitude • model the vocal tract - phonemes, voiced and unvoiced speech • Vocoder- synthetic speech quality

34. ASC Cont. • Speech obviously contains far more information than the simple text transcription of what is being said. We can identify the person speaking, and be aware of much unspoken information from the tone of voice and so on. • Early vocoders which reduced the voice to just simple waveform information lacked the human qualities which we need to hold a meaningful communication. • Hybrid encoders give greater emphasis to these qualities by using regular pulse excitation which encodes the overall tone of the voice in great detail.

35. GSM Voice Coder • Hybrid model using multi-pulse excitation linear predictive coding • Regular Pulse Excitation - Long Term Prediction (RPE-LTP) • Divides speech into three parts: • Short term prediction • Long term prediction • Residual periodic pulse - sent as coded waveform like PCM - requires more bits than the other two parts , this is to ensure that the characteristic tone of the voice is reproduced well. • Speech is divided into blocks of 20 ms • Each block of 20 ms is represented by a pattern of 260 bits: • 260 bits every 20 ms, gives an output rate of : kbps Sent as frequency and amplitude

36. Error Correction Coding • To reproduce speech, decoder needs bit error rate no more than 0.1% • Radio channel typically gives error rate of 1% - need error correction • Two approaches to error correction: • Backward error correction: Automatic Repeat Request (ARQ) • Forward error correction

37. ARQ • In backward error correction, we assume that if the known check bits have been transmitted correctly, the rest of the data is correct. If the check bits do not match what is expected, the system asks for re-transmission. • Not suitable for speech as the timing could become unintelligible if several repeats were necessary. However, in normal conversation, we naturally apply backward error correction by asking the person to repeat something we have not understood.

38. FEC • Coding is added to the information bits which enable the original to be reconstructed even if there are errors - redundancy • Repeat transmission is not required - suitable for speech • Two types of FEC: • Block codes • Convolutional codes • GSM uses a combination of both code types

39. GSM Error Correction Scheme • The GSM coding scheme is described as ‘concatenated’. It divides the data into three prioritized sections and applies different levels of coding to each, the resultant code is then put together (concatenated) for transmission. • 260 bits from voice coder are divided into 3 classes, according to their importance for speech reproduction: • Rate of coding describes the amount of redundancy in the coded data: • 1/2 rate code transmits twice as many bits as actual data • Data rate is halved

40. Interleaving • The algorithms used to recover the data are based on an assumption that errors will be randomly distributed. • In practice errors tend to clump together as the mobile passes in and out of fade regions. • To overcome this, the data bursts are not sent in their natural order, but are interleaved according to a pseudo-random pattern among a set of timeslots within the multiframe. • Interleaving is applied after error coding and removed at the receiver before the decoding. Thus the coding algorithm has a more random distribution of errors to deal with.

41. Protocol Stack • A protocol is a set of rules, agreed by both sides, to allow meaningful communication to take place • Protocols are needed whenever systems need to pass information from one to another • ISO 7-Layer OSI Reference Model:

42. Vertical vs. Horizontal Communications Horizontal (Peer-to-Peer) Communication Vertical (Entity-to-Entity) Communication

43. Vertical (Entity-to-Entity) Communication • Each layer requests a service from the layer below • The layer below responds by providing a service to the layer above • Each layer can provide one or more services to the layer above • Each service provided is known as a service ‘Entity’ • Each Entity is accessed via a Service Access Point (SAP) or a ‘gate’. • Each SAP has a unique SAP Identifier (SAPI)

44. GSM Protocols • In the OSI Reference Model, the logical channels of the air interface are at the Service Access Point (SAP) of the Physical Layer (Layer 1) • ISDN Reference Model divides the protocol plane into a Control Plane and a User Plane • corresponds to the control and traffic channels of the logical channels • some user data (notably SMS text messages) is carried by the control plane

45. Protocols on the GSM Air Interface

46. User Plane - Speech Transmission • Speech is encoded at the MS by the GSM Speech Codec (GSC) using hybrid encoders to give a data rate of 13 kbps. Then Forward Error Correction (FEC) is applied • At the BSS the FEC and any encryption is decoded by the TRX and the data is converted to the ISDN format (ITU-T A-law) by a Transcoding and Rate Adaption Unit (TRAU). • The A-law format carries data at 64 kbps across the fixed network. • The TRAU may be part of the BTS or part of the BSC. • If the TRAU is located at the BSC, then up to 4 speech channels may be multiplexed at the BTS (MPX in the diagram) onto an ISDN B channel which reduces the bandwidth required across the Abis interface.

47. Control Plane-GSM Signalling Protocols CM: Connection Management MM: Mobility Management RR: Radio Resources Management LAPD: Link Access Procedure D LAPDm: Link protocol adapted for air interface (Um) BTSM: Base Transceiver Station Management BSSMAP: Base Station System Management Application Part DTAP: Direct Transfer Application Part SCCP: Signalling Connection Control Part TCAP: Transaction Capabilities Application Part MTP: Message Transfer Part MAP: Mobile Application Part UP: User Part ITU-T G.703, G705, G.732: Protocols for digital transfer of signalling messages on the Abis and A interfaces at 2048 kb/s or 64 kb/

48. Protocols Functionality • Layer 1 – Physical Layer • On the air interface, the physical layer uses FDMA/TDMA, multiframe structure, channel coding etc. to implement the logical control channels. • Services provided by layer 1 are: • Access capabilities – multiplexing logical onto physical channels • Error protection – error detection / correction coding mechanisms • Encryption • Layer 2 – LAPDm – Link Access Procedure on Dm channels • Data link protocol responsible for protected transfer of signallingmessages between MS and BTS. • LAPDm supports the transport of messages between protocol entities on Layer 3, in particular: BCCH, PCH, AGCH and SDCCH signalling.

49. Cont. • Within Connection Management, Call Control (CC) is responsible for: • Set up of normal calls (MS originated, MS terminated) • Set up of emergency calls (MS originated only) • Terminating calls • DTMF signalling • Call related supplementary services • Service modification during a call (e.g. speech/data, speech/fax) • Layer 3 - Network Sub-layers: • Radio Resource Management (RR) • Mobility Management (MM) • Connection Management – 3 entities: • Call Control (CC) • Supplementary Services (SS) • Short Message Service (SMS) • RR is responsible for: • Monitoring BCCH and PCH • Administering RACH • Requests for and assignments of data and signalling channels • Measurements of channel quality • MS power control and synchronization • Handover • Synchronization of data channel encryption and decryption • MM is responsible for: • TMSI assignment • Location updating • Identification of MS (IMSI, IMEI) • Authentication of MS • IMSI attach and detach • Confidentiality of subscriber identity

50. Enhancing GSM • AMR (Adaptive multi-rate) speech coder • Trade off speech and error correction bits • Fewer dropped calls • DTX — discontinuous transmission • Less interference (approach 0 bps during silences) • More calls per cell • Frequency hopping • Overcome fading • Synchronization between cells • DFCA: dynamic frequency and channel assignment • Allocate radio resources to minimize interference • Also used to determine mobile’s location • TFO — Tandem Free Operation