GSM Air Interface
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The document discusses the GSM air interface and its importance in mobile communication systems, detailing the RF spectrum, GSM specifications, and modulation methods. It explains the use of FDMA and TDMA for channel access, digital voice transmission, channel coding, interleaving, and various types of burst formats used in communication. Additionally, it covers the operational aspects of user access, synchronization, and features like discontinuous transmission and diversity to enhance connection quality.
GSM Air Interface
- 1. GSM Air Interface (𝑈 𝑚) PRESENTED BY: NAVEEN JAKHAR, ITS ABHISHEK SINGH, ITS
- 2. Introduction The Air-interface is the central interface of every mobile system and typically the only one to which a customer is exposed. The physical characteristics of the Air-interface are particularly important for the quality and success of a new mobile standard.
- 3. RF Spectrum GSM 900 Mobile to BS (UP-LINK) -890 to 915 MHz BS to Mobile (DOWN -LINK) -935 to 960 MHz Bandwidth - 25 MHz GSM 1800 or PCS 1800 Mobile to Cell (UP-LINK) -1710 to 1785 MHz Cell to Mobile (DOWN -LINK) -1805 to 1880 MHz Bandwidth - 75 MHz
- 4. GSM SPECIFICATIONS Carrier Separation - 200 kHz Duplex Distance - 45 MHz No. of RF Carriers - 124 Access Method - TDMA/FDMA Modulation Method - GMSK Transmission Rate - 270.833 Kbps Speech Coding - Full rate 13 Kbps Half rate 6.5 Kbps
- 5. GSM Air Interface structure (FDMA/TDMA) GSM utilizes a combination of frequency division multiple access (FDMA) and time division multiple access (TDMA) on the Air-interface. That results in a two-dimensional channel structure,
- 6. WHY FDMA and TDMA? In a pure FDMA system, one specific frequency is allocated for every user during a call. That quickly leads to overload situations in cases of high demand. In fullrate configuration, eight time slots (TSs) are mapped on every frequency; in a halfrate configuration there are 16 TSs per frequency.
- 7. in a TDMA system, each user sends an impulse like signal only periodically, while a user in a FDMA system sends the signal permanently. The difference between the two is illustrated in Figure
- 8. DIGITAL VOICE TRANSMISSION Speech Coding In GSM speech coding a block of 20 ms is encoded in one set of 260 bits. This calculates as 50X 260 = 13 kbps. Thus GSM speech coder produces a bit rate of 13 kbps per subscriber. This provides speech quality which is acceptable for mobile telephony and comparable with wire-line PSTN phones.
- 10. Interleaving The process of interleaving smaller packages of 456 bits over a larger time period, that is, distributing them in separate TSs. How the packets are spread depends on the type of application the bits represent. Signalling traffic and packets of data traffic are spread more than voice traffic. The whole process is referred to as interleaving.
- 11. The goal of interleaving is to minimize the impact of the peculiarities of the Air-interface that account for rapid, short- term changes of the quality of the transmission channel. It is possible that a particular channel is corrupted for a very short period of time and all the data sent during that time are lost. Interleaving does not prevent loss of bits, and if there is a loss, the same number of bits are lost. However, because of
- 12. BURST FORMATTING Adds training sequence that helps in the working equalizer to assist in countering the effects of radio channel on the signal. Total of 136 bits added, bringing overall total to 592 bits.
- 13. Each TS of TDMA frame is 0.577 ms long and during this time 156.25 bits are transmitted. One burst contains only 148 bits. Rest of the space, 8.25 bits time, is empty and is called Guard Period ( GP ). GP enables MS/BTS to “ramp up” and “ ramp down”.
- 14. GMSK The modulation method in GSM is GMSK which facilitates the use of narrow bandwidth and coherent detection capability. Rectangular pulses are passed through a Gaussian filter prior to their passing through a modulator. The modulation scheme almost satisfies the adjacent channel power spectrum density requirements of -60dB specified by CCIR.
- 16. FRAME HIERARCHY
- 17. The frame hierarchy is used for synchronization between BTS and MS, channel mapping, and ciphering. Every BTS permanently broadcasts the current frame number over the synchronization channel (SCH) and thereby forms an internal clock of the BTS. There is no coordination between BTSs; all have an independent clock, except for synchronized BTSs. That information is very important,
- 18. Physical Versus Logical Channels Physical channels are all the available TSs of a BTS, whereas every TS corresponds to a physical channel. Two types of channels need to be distinguished, the halfrate channel and the fullrate channel. Logical channels are piggybacked on the physical channels. Logical channels are, so to speak, laid over the grid of physical channels. Each logical channel performs a specific task.
- 19. LOGICAL CHANNELS
- 21. Frequency Correction Burst This burst format is used by FCCH channel only. The whole data space (142 bits) is used for unmodulated carrier (pure sinusoid) or carrier modulated with all zero bits. This pure carrier is the ‘identity’ of a beacon frequency (also called BCCH-frequency or base- frequency) and FCCH slot
- 22. Synchronization Burst This burst format is used by SCH channel only. This channel makes a mobile station time- synchronized with the base station clock. That is why the synchronization training sequence is very large for this burst comparing to other burst types. Only one training sequence is defined for this burst.
- 23. Access Burst This burst format is used by RACH and AGCH channels. When a mobile station sends an RACH message and receives an AGCH reply, neither MS nor the BTS does have the timing- advance information. For that reason, the actual message is relatively short and have a long guard band (GB) in order to make sure that there will be no overlap with the next burst. The length of the guard band in the access burst (68.25 bits x 3.69 = 251.16 ms) is equivalent to 37.5 km propagation delay. The GSM allows a cell radius up to of 35 km. That is, an RACH message from an MS at a distance of up to 35 km from the base station can reach to the base station antenna without overlapping the next burst. The FACCH channel uses this burst during handover operation
- 24. Normal Burst This burst format is used by all other channels (except FCCH, SCH, RACH and AGCH). This normal burst is used by TCH, SDCCH, SACCH, FACCH, BCCH and PCH. A few important features of the burst is stated below. Maximum 57 x 2 = 114 bits of voice/data per burst Flag bit is to indicate if the channel is carrying user traffic (Flag = 0) or control message bits (Flag = 1). That is the flag is 0 for TCH and 1 for others.
- 25. Dummy Burst This is like normal burst but has no meaning of its payload bits
- 27. TRAFFIC MULTIFRAME (FULL RATE) This example shows 2 users using full rate voice traffic channels. (One user uses the slot 2 at every frame and the other user use the slot 4 at every frame). At the center of the 26-frame traffic channel multiframe (i.e, Frame 12) is the Slow Associated Control Channel (SACCH) which carries link control information to and from the MS–BTS. At the last frame is 1 idle frame. All the remaining frame are allocated for Traffic. There is no dedicated FACCH frame or slots. FACCH steals TCH whenever it needs.
- 28. TRAFFIC MULTIFRAME (HALF RATE) This example shows 26- multiframe structure for TCH/HR, showing 2 users using HR voice traffic channel. In this example, the two users shares the slot 2 of every frame in alternating fashion. You would notice that there are two Frames for SACCH. The Frame 12 is for SACCH of user 1 and Frame 25 is for SACCH of user 2.
- 29. CONTROL MULTIFRAME (BROADCAST CHANNELS) An example of control channel multiframe structure for Broadcast Channel (Base Control Channel) which is made up of FCCH, SCH, BCCH, CCCH. It is for Downlink multiframe structure. In Uplink, every frame is for single channel - RACH. Some key facts about DL Base Control Channel are There are five FCCH equally spaced within the 51 multiframe. Each FCCH is followed by a SCH, meaning that there are five SCH as well. Four frames (Frame 2-5) are allocated for BCCH. The last frame (Frame 50) is allocated for Idle. All the remaining Frames are allocated for CCCH(e.g, PCH or AGCH).
- 30. Mapping of Logical Channels Onto Physical Channels
- 31. OPERATIONAL ASPECTS Subscribers are not allocated dedicated channels TCH allocated to users only when needed Hence IDLE MODE & DEDICATED MODE
- 32. IDLE MODE When MS is powered on (active) without being in dedicated mode MS stays continuously in touch with BS Listens to transmissions from BS to intercept Paging Messages ( for incoming calls) • Monitors Radio Environment in order to evaluate Channel Quality & choose the most suitable BS • Listens to BS to avail short message broadcast service
- 33. ACCESS PROCEDURE Access to system ( switch over from IDLE to DEDICATED Mode) MS indicates to BS that it needs a connection BS accepts the request & indicates which traffic channel it may use For above purpose specific transmission is done over “ Common Channels”
- 34. OTHER FEATURES Discontinuous Transmission Synchronization between uplink and downlink Diversity
- 35. Discontinuous Transmission Speech activity only 40% of time. Needs Voice activity detection. Annoying clicks/inefficient DTX. Generation of Comfort Noise at receiver to avoid the feeling of the set being dead.
- 36. SYNCHRONIZATION BETWEEN UPLINK AND DOWNLINK For technical reasons, it is necessary that the MS and the BTS do not transmit simultaneously. Therefore, the MS Is transmitting three timeslots after the BTS. The time between sending and receiving data is used by the MS to perform various measurements on the signal quality of the receivable neighbour cells.
- 37. Depending on the distance between the two, a considerable propagation delay needs to be taken into account. That propagation delay, known as timing advance (TA), requires the MS to transmit its data a little earlier as determined by the
- 38. DIVERSITY Space Diversity Mounting two receiver antenna physically separated a distance. - Probability of both of them being affected by a deep fading dip at same time is low.
- 39. THANK YOU!