Global System for Mobile Communications (GSM) Technology

Global schema for Mobile Communications (GSM) Technology probe on the physical layer technologies employed in the GSM SystemAbsyarie Syafiq bin ShahrinAbstractBasic all toldy in this paper, we intend to ease up a rundown on GSM (Global System for Mobile Communications) specifically on the technologies employed at the physical layer in the GSM dodge. The GSM governance is a very fire topic as it revolutionized the way we communicate and it is compose being employ till this day. It is in reality the 2nd propagation (2G) wireless system as it uses digital instead of running(a) and it also deploys succession course treble Access (TDMA) that is implemented on multiple frequence subbands. Frequency Division Multiple Access (FDMA). GMSK inflexion and demodulation proficiency will also be discussed to pay backher with how it works and what their advantages/disadvantages atomic number 18. The problems with ISI (Intersymbol Interference) in GSM systems will also be intercommu nicate together with how to mitigate ISI victimization post equalization. With that, we will also give a simple explanation on how talk coding is civil in GSM transceivers.Keywords Gaussian Pulse, GMSK, ISI, channel equalizer, ISI equalizer, public lecture codingI. IntroductionGSM is a standard developed by the ETSI (Europe Telecommunication Standards Institute) to describe the protocols of the 2nd Generation (2G) communication technology used by restless ne 2rks and cell phones. It was first off launched in Finland with a data speed of up to 64kbps. The GSM is given the status 2G because it is something completely new comp atomic number 18d to the first generation (1G) with the usage of digital foreshadow emblems instead of analog. It was designed from scratch with no backward compatibility with the previous 1G technology. exploitation 124 channels per cell, it can accommodate up to 8 users by exploitation a combination of TDMA and FDD contrivance 1, though some of its channels are used for control mansions. It also introduces the SIM (Subscriber identity module) card which allows for roaming calls. At first, it was solo designed for operation in the 900 MHz band and later(prenominal) it was adapted for 1800Mhz. GSM is a very popular standard used directly with oer 90% market share, with availability in over 219 countries and territories worldwide. in the beginning the GSM was developed with the intention that it will replace the first generation analog ne dickensrks by having digital, circuit-switched networks which are optimized for full-duplex voice telephony. However as time passes, the GSM system was further developed to include data communications by first off having it on circuit-switched transport, and therefore changing it later to packet-switched transport via GPRS ( General portion Radio Service) and EDGE ( Enhanced Data Rates for GSM Evolution) .In GSM, Gaussian pulse shaping is used and Gaussian Minimum transubstantiation Ke ying (GMSK) as a modulation/demodulation technique with a modulation index of 0.5 2. This modulation system however gives rise to inter symbol arrest. Inter image Interference (ISI) in the GSM system are usually caused by two factors Multipath propagation and Bandlimited channels. An ISI equalizer is used to solve this problem by implementing the maximum Likelihood Sequence Estimation (MLSE) via vertibri algorithm. To make things easier to understand, embark 1 is affiliated to relate how the GSM system can relate to the OSI (Open System Interconnection) model. We will however, direction more on the Physical Layer of the GSM system. fancy 1 How the GSM is complete in the famous OSI model 7.Pulse ShapingIn digital telecommunications systems, we strive to achieve broad dispersed spectrum with significant low-frequency content. This in return, requires a lowpass channel that has a bandwidth sufficient enough to accommodate the essential frequency content in the data stream. Gauss ian function fits this requirement perfectly. The specialism of this waveshape is that, the pulses rise and small smoothly until it settles to a value 14. This is a expensive asset as it gives a solution to problems such as precursors, go-around and ringing in a pulse signal 14. This problems cause uncertainty to the actual value so it is very troublesome. Besides that, it also addresses the two required fatalitys of communication systems which are band-limited channels and reduced Inter-symbol interference (ISI) by doning a Gaussian filtrate symbol-by-symbol. It is nearly impossible to get the perfect sinc spectrum in the time domain as the bandwidth needs to be infinity. We can only nominate an approximation or near the alike sinc spectrum. ISI can also still happen if control is non exercised over the pulse shaping. conformation 2 An impulse response of a Gaussian Filter 15In GSM, we engage Gaussian tense uping for Gaussian Filtering Minimum skid keying (GMSK) a modu lation technique. Basically it is similar as the Minimum Shift Keying (MSK) but the data stream must first go by pulse shaping via Gaussian drip before being apply to the modulator. MSK is already a good modulation scheme as it be in possession of constant envelope and maintains phase continuity. GMSK allows for reduced sideband power which entrusts in the trim of out-of-band interference amid the signal carriers in adjacent frequency channels. The GMSK technique has an advantage of being able to carry data while still maintaining an efficient usage of spectrum. The reduce power in the GMSK is very useable especially for mobile phones as lower battery consumption is postulate for operation 16. The drawback of GMSK is that, it requires more modulation memory in the system and causes ISI.We have two ways to generate GMSK modulation. The most basic way is to apply Gaussian filter on the input signal and then apply a frequency modulator with a modulation index of 0.5 2 16. The problem with this rule is that it must have an exact modulation index of 0.5. In the veritable world, this is impossible as component tolerance drift can vary16.Figure Flow chart of GMSK modulation using a Gaussian filter and Voltage controlled oscillatorThe second method is more realistic and widely used. This GMSK method uses the Quadrature (I-Q) modulator. The operation starts by having the Gaussian filtered data separated into two parts, in-phase I and quadrature phase (Q). The I and Q components will then be mixed up to the frequency of the RF carrier to have a modulated RF signal. This kind of modulator can maintain 0.5 modulation index without having any modifications. The instruction execution of this quadruple modulation depends on the accurate creation of I and Q components. For demodulation, this scheme can be used in reverse 16.X mixer or multiplierLO Local oscillatorFigure 3 Block plat of I-Q modulatorInter symbol interference and channel equalizationISI in the GS M system is mainly caused by multipath propagation. Multipath propagation is a result when signals arrive at different times ( stick around) because it is does not travel in line of piling (LOS). In reality, connection will never be in LOS all the time so the signals will go through different paths by being reflected or refracted from different objects to r to each one the destination. When the signals travel through multiple paths, they will arrive at different times depending on the path they used. It is also possible for reflected signals to overlap with the subsequent signals 13. This in addition, results in strain to the received signals because all the signals have different delay. This situation happens either from mobile move to innovation station or vice versa. Since the delay spread is more than the symbol time, frequency selective fading occurs.Figure 4 An example of multipath propagationFigure 5 ISI as a result of multipath distortion 13To combat the problem with mu ltipath propagation, we use and ISI equalizer. This equalization technique is establish on the MLSE which uses the Viterbi algorithmic rule 3 10. Figure below shows the occlusive diagram of the ISI equalizer.Figure 6 Block diagram on how ISI equalizer is used in GSM environmentWhen the base station or the mobile station transmits a TDMA irrupt, not all of is user data. Instead, 26 splinterings are allocated for the training rate and they are known by their receivers (either mobile station or base station). Each of the known sequence bits unique for a certain sender is unique for a certain transmitter and this sequence bits is also repeat in every transmission burst. The figure below shows the normal burst structure in the GSM burst.Figure 7 GSM Normal Burst bodily structureA channel estimator is needed because to perform MLSE, we require schooling on the CIR ( crease Impulse Response). The channel estimator will estimate the CIR for each of the bursts by comparing the transm itted bits with the received signal to produce he(t) 10. canalize estimation in GSM uses additive MMSE (Minimum mean square actus reus) 11. Since the match filter is in time domain, r(t) will be convoluted with the signal obtained from the channel estimation, he(t) to create a model signal Y(t). The output model signal obtained can then be used to estimate the transmitted bits base on the bits received by performing MLSE. The last wait on uses Viterbi Algorithm hence the process, Viterbi equalisation 2 9.Speech coding in GSM transceiversSpeech is originally analog in nature and GSM is a digital system. In set out to use the speech entropyrmation, we need to run to a series of process known as speech processing. Figure shows how the speech processing is do in a GSM system.In speech coding, the GSM system has used a variety of ways to fit in 3.1 kHz audio into between 6.5 and 13 kbit/s. The first two codecs used was called Half Rate (5.8 kbit/s) and intact rate (13 kbit/s) 4. Both of this codecs use LPC (Linear expectancy Function) where voice signals need to be digitized, and secured using encryption over a narrow voice channel. As time passes, the GSM system was further developed to use the Enhanced abounding Rate (EFR) codec which is a 12.2 kbit/s codec and it uses a full-rate channel.Figure 8 Flow-diagram on GSM speech processing 8 rich rate speech coder is actually part of the Regular Pulse Excitation Long Term Prediction (RPE-LTP) coders 4. Firstly the speech encoder will take an input of 13 bit uniform PCM signal from either the audio part of the mobile station (MS) or the Public Switched Telephone Network (PSTN) side by using 8 bit/A-law to 13 bit uniform PCM con variant. The encoded speech is then delivered to the channel coding function which will then produce an encoded block having 456 bits with a gross bit rate of 22.8 kbps 4 5. The remaining 9.8 kbps is used for error protection purposes. The reverse action is performed for decoding. Wh en encoding, 160 frames in 1 standard is encoded to a block of 260 bits with a sampling rate of 8000 samples/s, hence the bitrate of 13kbps 5. On the decoding part, 260 bits of encoded blocks is mapped back to the 160 frames output reconstructed speech sample.EFR (Enhanced Full Rate) is a newer version of the speech codec which uses ACELP (Algebraic Code Excited Linear Prediction) algorithm. The motif for this development is because of the mediocre / poor fictional character of the GSM-Full Rate codec. This codec is a turn out from the previous FR because it provides speech quality equivalent or adjacent to wireline telephony which uses 32 kbps ADPCM (Adaptive Pulse Code Modulation) 6. This codec can provide wireline quality in both error and error-free conditions 6. EFR which is also a form of handicraft channel is bi-directional and can transmit both speech and data 9.Figure 9 shows how error correction is done at layer 1 of the GSM air interfaceConclusionAll in all, this pa per has helped me to fail understand the GSM system and how it works in the physical layer. GSM has many sources including but not limited to, books, journals, application notes, lecture notes, documentation as good as survey papers. After variation from various sources, I in condition(p) to read efficiently and think critically as the papers create verbally are quite hard and requires a meticulous reading to thoroughly understand what is being presented. I acquired basic research and development (RD) skills and adept writing skills after almost a month of heavy reading and research. How the physical layer in the GSM system works is also understood. The acquired signal must first be shaped through a Gaussian filter in the GMSK modulator. The Quadruple modulator scheme is used as it does not require modifications to maintain 0.5 modulation index. ISI in the GSM are mostly caused by multipath propagations in which gives frequency selective fading. Frequency selective fading hap pens when the delay time is spread because symbols arrive at different times. To address the problems with ISI, we need to have an ISI equalizer. ISI equalizer consists of many components such as match filter and MLSE by Viterbi algorithm. I also learned that we have two speech coding options full rate speech coder and EFR. All this components are essential when building a GSM system.References1 Guifen Gu, Guili Peng The Survey of GSM Wireless Communication System International league on Computer and Information Application (ICCIA) , 20102 B. Baggini, L. Coppero, G. Gazzoli, L. Sforzini, F. Maloberti, G. Palmisano combine Digital Modulator and Analog Front-End for GSM Digital Cellular Mobile Radio System, Proc. IEEE 1991 CICC vol. 31, pp.7.6.14, Mar. 1991.3 M. Drutarovsk, GSM Channel Equalization Algorithm Modern DSP Coprocessor Approarch Radioengineering Vol. 8, No 4, December 1999.4 Besacier, L. Grassi, S. Dufaux, A Ansorge, M. Pellandini, F., GSM speech coding and speaker reco gnition,Acoustics, Speech, and Signal Processing, 2000. ICASSP 00. 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