View source for Mobile Communications/Characteristics of GSM

 
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== System architecture and basic units of GSM ==
<br>
$\rm GSM$&nbsp; $\rm (G$lobal&nbsp; $\rm S$ystem&nbsp; for&nbsp; $\rm M$obile Communication$)$&nbsp; is a strongly hierarchically structured system of different network components.&nbsp; You can see from the graphic:
[[File:P ID2203 Mob T 3 3 S1 v1.png|right|frame|GSM system architecture '''KORREKTUR: Funkschnittstelle'''|class=fit]]
*The mobile station&nbsp; $\rm (MS)$&nbsp; communicates via the radio interface with the nearest&nbsp; base transceiver station&nbsp; $\rm (BTS$, transmit and receive base station).<br>

*Several such BTS's are grouped together area by area and are jointly subordinate to a&nbsp; base station controller</i>&nbsp; $\rm (BSC$, control station$)$.<br>

*The&nbsp; base station sub&ndash;system&nbsp; $\rm(BSS)$&nbsp; consists of a multitude of BTS's and several BSC's.&nbsp; In the graphic such a&nbsp; BSS&nbsp; is framed in blue.<br>

*Each BSC is connected to a mobile switching center&nbsp; $\rm (MSC$, switching computer$)$, whose function is comparable to a switching node in the fixed network.<br>

*The gateway mobile switching center&nbsp; $\rm (GMSC)$&nbsp; is responsible for the connection between the fixed and mobile networks.&nbsp; For example, if a mobile subscriber is called from the fixed network, the GMSC determines the responsible MSC and transfers the call.<br>

*The operation and maintenance center</i>&nbsp; $\rm (OMC)$&nbsp; monitors a part of the mobile network.&nbsp; It also takes on organizational tasks such as traffic flow control, charging, security management, etc.<br>


More detailed information on GSM system architecture and the individual network components can be found in the chapter&nbsp; [[Examples_of_Communication_Systems/General_Description_of_GSM|"General Description of GSM"]]&nbsp; of the book "Examples of Communication Systems".
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== Multiple access with GSM ==
<br>
[[File:EN_Mob_T_3_3_S2.png||right|frame|Realization of FDMA and TDMA with "GSM 900" '''KORREKTUR: Daten'''|class=fit]]

GSM uses two multiple access methods in parallel:
*$\text{Frequency Division Multiple Access}$ &nbsp; $\rm (FDMA)$,<br>
*$\text{Time Division Multiple Access}$ &nbsp; $\rm (TDMA)$.


The graphic and the following description is valid for the original system "GSM 900" (in the following:&nbsp; "GSM&ndash;D").&nbsp; 

For "GSM/DCS 1800"&nbsp; &rArr; &nbsp;  "GSM&ndash;E" comparable statements apply.

*With "GSM&ndash;D", a bandwidth of&nbsp; $25\ \rm MHz$&nbsp; is provided for uplink and downlink respectively&nbsp; $($duplex spacing:&nbsp; $45\ \rm MHz)$.&nbsp; This is called&nbsp; "Frequency Division Duplex"&nbsp; $\rm (FDD)$.&nbsp; For "GSM&ndash;E", the bandwidth is&nbsp; $75\ \rm MHz$&nbsp; and the duplex spacing is&nbsp; $95\ \rm MHz$.<br>

*Uplink and downlink bands are divided into frequency bands of width&nbsp; $200\ \rm kHz$.&nbsp; Taking into account the protection areas at the respective edges, there are&nbsp; $N_{\rm F} = 124$&nbsp; (GSM&ndash;D) or&nbsp; $N_{\rm F} = 374$&nbsp; (GSM&ndash;E) frequency channels.

*Each cell is assigned a subset of the frequencies &nbsp; &#8658; &nbsp; "Cell Allocation".&nbsp; Neighboring cells usually work at different frequencies, for example with the reuse&ndash;factor &nbsp;$3$, as in section&nbsp; [[Mobile_Communications/Similarities_Between_GSM_and_UMTS#Cellular_architecture|"Cellular architecture"]] indicated by the colors white, yellow, and blue.<br>

*The&nbsp; $124$&nbsp; GSM frequency channels are further divided by time division multiplexing&nbsp; $\rm (TDMA)$.&nbsp; Each FDMA channel is divided into TDMA frames, which each comprise&nbsp; $N_{\rm T} = 8$&nbsp; time slots.

*The slots are periodically assigned to the individual GSM users and each contain a so-called&nbsp; [[Mobile_Communications/Characteristics_of_GSM#Data_and_frame_structure_for_GSM|$\text{Burst}$]].&nbsp; Each user has a time slot available in each TDMA frame.&nbsp; A bundling (maximum six per user) is only possible with GPRS/EDGE.<br>

*The TDMA frames of the uplink are sent delayed by three slots compared to the downlink: &nbsp; "Time Division Duplex"&nbsp; $\rm (TDD)$.&nbsp; The hardware of the mobile station can thus be used for sending and receiving a message simultaneously.


== Data and frame structure for GSM ==
<br>
The mapping of logical to physical channels is done using the GSM frame structure.&nbsp; Here we only look to traffic channels and to the mapping in time.&nbsp; In this case, each multi&ndash;frame of &nbsp; $120 \ \rm ms$&nbsp;  duration is divided into&nbsp; $26$&nbsp; TDMA frames (two of them for control channels) of&nbsp; $4.615\ \rm ms$&nbsp;  duration.&nbsp; Thus, the duration of a time slot is approximately&nbsp; $T_{\rm Z} = 576.9\ \rm &micro; s$.<br>

[[File:EN_Mob_T_3_3_S3a.png||right|frame|Data and frame structure for GSM '''KORREKTUR: Daten'''|class=fit]]

You can see from this graphic:
*In each time slot one "burst" is transmitted, whose duration corresponds to&nbsp; $156.25$&nbsp; bits.&nbsp; The bit duration is&nbsp; $T_{\rm B} = 576.9\ \rm &micro; s/156.25 \approx 3.692 \ \rm &micro;s$&nbsp; and for the total gross data rate:

::<math>R_{\rm total} = {1}/{T_{\rm B}}= 270.833\,{\rm kbit/s}\hspace{0.05cm}.</math>

*The&nbsp; &raquo;'''gross data rate'''&laquo; &nbsp;of each user is&nbsp; $R_{\rm gross} = 33.854 \ \rm kbit/s$.&nbsp; But since in every normal burst only&nbsp; $2 &middot; 57 = 114$&nbsp; data bits (highlighted blue in the graphic) are transmitted, it results in the lower &nbsp;&raquo;'''net data rate'''&laquo;&nbsp; $R_{\rm net} = 22.8 \ \rm kbit/s$.<br>

*This net data rate also takes the channel coding into account.&nbsp; In the case of a speech signal, for every&nbsp; $20\ \rm kbit/s$&nbsp; speech frame&nbsp; $456$&nbsp; bits are transmitted, which results in exactly the rate&nbsp; $R_{\rm net} = 22.8 \ \rm kbit/s$. &nbsp; Without channel coding, the data rate would be only&nbsp; $13 \ \rm kbit/s$.<br>

*In addition to the traffic data, a&nbsp; &raquo;'''normal burst'''&laquo;&nbsp; also contains 
:&ndash; &nbsp; twice three tailbits (red, during this time the channel is remeasured), 
:&ndash; &nbsp; two signaling bits (green), 
:&ndash; &nbsp; the "Guard Period"&nbsp; $\rm (GP)$&nbsp; with&nbsp; $8.25$&nbsp; bit duration $($grey, ca.&nbsp; $30.5 \ \rm &micro; s)$, 
:&ndash; &nbsp; $26$&nbsp; bit for a training sequence (for channel estimation and synchronization).  

These bits increase the data rate from&nbsp; $22.8$&nbsp; to&nbsp; $33.854 \ \rm kbit/s$.

''Note:''
*In GSM, besides the "Normal Burst" there are  some other&nbsp; [[Examples_of_Communication_Systems/Radio_Interface#The_different_burst_types_in_GSM|$\text{types of bursts}$]]:&nbsp; Frequency Correction Burst,&nbsp; Synchronization Burst,&nbsp; Dummy Burst,&nbsp; Access Burst, ... . 
*All these bursts have a uniform length of&nbsp; $156.25$&nbsp; bit durations.&nbsp; This is discussed in more detail in the&nbsp; [[Aufgaben:Exercise_3.2:_GSM Data Rates|"Excercise 3.2"]].

== Modulation method for GSM==
<br>
With GSM, only a bandwidth of&nbsp; $B = 200 \ \rm kHz$&nbsp; is available per frequency channel, in which a total data rate (for eight users) of&nbsp; $R_{\rm total} = 270.833 \ \rm kbit/s$&nbsp; must be transmitted.&nbsp; Required is therefore a modulation method with a bandwidth efficiency of at least

::<math>\beta \ge {R_{\rm total}}/{B} \approx 1.35 \,\,{\rm bit/s/Hz}.</math>

GSM uses the very bandwidth-efficient modulation method&nbsp; [[Examples_of_Communication_Systems/Radio_Interface#Gaussian_Minimum_Shift_Keying_.28GMSK.29| $\text{Gaussian Minimum Shift Keying}$]]&nbsp; $\rm (GMSK)$.&nbsp; It should again be expressly mentioned that this modulation method, just like the FDMA/TDMA multiple access, is exclusively based on the air interface between the mobile station&nbsp; $\rm (MS)$&nbsp; and the base transceiver station &nbsp; $\rm (BTS)$, which is highlighted in yellow in the&nbsp; [[Mobile_Communications/Characteristics_of_GSM#System_architecture_and_basic_units_of_GSM|$\text{system architecture graphic}$]]&nbsp; at the beginning of the chapter. <br>

GMSK has already been described in chapter&nbsp; [[Modulation_Methods/Non-Linear_Digital_Modulation#Properties_of_non-linear_modulation_methods| "Properties of nonlinear modulation methods"]]&nbsp; of the book "Modulation Methods".&nbsp; The most important properties are briefly summarized here.
*GMSK is a special form of binary&nbsp; [[Modulation_Methods/Non-Linear_Digital_Modulation#FSK_.E2.80.93_Frequency_Shift_Keying| $\text{Frequency Shift Keying}$]]&nbsp; $\rm (FSK)$.&nbsp; A prerequisite for the orthogonality between the two waveforms is that the modulation index&nbsp; $h$&nbsp; is a multiple of&nbsp; $0.5$&nbsp;. For integer values of&nbsp; $h$&nbsp; the demodulation can also be performed non-coherently.<br>

*For GSM you use the smallest possible modulation index&nbsp; $h = 0.5$.&nbsp; A higher value would require a much larger bandwidth.&nbsp; Such an FSK with&nbsp; $h = 0.5$&nbsp; is also called [[Modulation_Methods/Non-Linear_Digital_Modulation#MSK_.E2.80.93_Minimum_Shift_Keying|$\text{Minimum Shift Keying}$]]&nbsp;&nbsp; $\rm (MSK)$.&nbsp; However, a coherent demodulation is then required.<br>

*A very narrow spectrum, however, is only obtained if the phase values at the symbol boundaries are matched to each other, thus avoiding phase jumps, which is given with MSK.&nbsp; Such methods are referred to as&nbsp; [[Modulation_Methods/Nonlinear_Digital_Modulation#General_Description_of_Continuous_Phase_Modulation|$\text{Continuous Phase Frequency Shift Keying}$]]&nbsp; $\rm (CP&ndash;FSK)$.&nbsp;<br>

*For GSM, a low-pass filter with Gaussian characteristic is inserted before the frequency modulator &nbsp; &#8658; &nbsp; [[Modulation_Methods/Nonlinear_Digital_Modulation#GMSK_.E2.80.93_Gaussian_Minimum_Shift_Keying|$\text{Gaussian Minimum Shift Keying}$]]&nbsp; $\rm (GMSK)$, which further reduces the bandwidth and improves bandwidth efficiency.<br><br>

Regarding the topic treated here&nbsp; (coherent or non-coherent demodulation of FSK)&nbsp; we refer to two exercises in the book "Digital Signal Transmission":
*[[Aufgaben:Aufgabe_4.16:_Binary_Frequency_Shift_Keying|Excercise 4.16: &nbsp; Binary Frequency Shift Keying]],<br>

*[[Aufgaben:Aufgabe_4.18Z:_BER_von_kohärenter_und_nichtkohärenter_FSK| Excercise 4.18Z: &nbsp; BER of coherent and non-coherent FSK]].<br><br>


{{GraueBox|TEXT=  
$\text{Example 1:}$&nbsp; The following graphic is to clarify the previous statements:

[[File:EN_Mob_T_3_3_S4.png|right|frame|Block diagram and signals with GMSK|class=fit]]

*Starting from a Dirac shaped source signal&nbsp; $q_\delta(t)$&nbsp; at point&nbsp; $(1)$&nbsp; you pass through a filter with the rectangular impulse response&nbsp; $g_{\rm R}(t)$&nbsp; to the rectangular signal&nbsp; $q_{\rm R}(t)$&nbsp; at point&nbsp; $(2)$.

*If the Gaussian low-pass  with the impulse response&nbsp; $h_{\rm G}(t)$&nbsp; were to be omitted &nbsp; &#8658; &nbsp; $q_{\rm G}(t) = q_{\rm R}(t)$, a sectionwise linear phase function&nbsp; $\phi(t)$&nbsp; would result at point&nbsp; $(4)$.&nbsp; All phase values would be multiples of&nbsp; $&pi;/2$&nbsp; at multiples of the symbol duration&nbsp; $T$.

*After the phase modulator, a binary FSK signal&nbsp; $s(t)$&nbsp; would appear at point&nbsp; $(5)$&nbsp; with only two frequencies.&nbsp; This signal is at the same time a MSK signal due to the minimum modulation index&nbsp; $h = 0.5$&nbsp; in case of orthogonality.

*Through the Gaussian low-pass&nbsp; $H_{\rm G}(f)$&nbsp; with the cutoff frequency&nbsp; $f_{\rm G}= 0.45/T$&nbsp; (valid for GSM) the frequency impulse&nbsp; $g(t)$&nbsp; is no longer rectangular but corresponds to the rectangular response of&nbsp; $H_{\rm G}(f)$.&nbsp; According to the Fourier transform,&nbsp; $g(t) = g_{\rm R}(t) \star h_{\rm G}(t)$ applies.

*Thus, the phase function&nbsp; $\phi(t)$&nbsp; is no longer linear in sections but the corners are rounded, as can be seen from the phase function at point&nbsp; $(4)$.&nbsp; The violet&ndash;dotted curve applies to the data sequence assumed at point&nbsp; $(1)$.

*The signal&nbsp; $s(t)$&nbsp; at point&nbsp; $(5)$&nbsp; of the block diagram is the GMSK signal.


Note: &nbsp; The GMSK signal contains much more than just two discrete frequencies.&nbsp; Its power-spectral density decreases very fast, see&nbsp; [[Examples_of_Communication_Systems/Radio_Interface#Advantages_and_disadvantages_of_GMSK|$\text{diagram}$]]&nbsp; in the book "Examples of communication systems".&nbsp; From the above time representation at point&nbsp; $(5)$&nbsp; of the block diagram this fact is difficult to recognize.
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== GSM extensions ==
<br>
GSM was designed and developed as a European mobile radio system for telephone calls with the additional option of data transmission, but only at a low data rate&nbsp; $(9.6 \ \rm kbit/s)$.&nbsp; The standardization of the&nbsp; $\text{GSM-Phase 2}$&nbsp; however, from 1995 on, already included first further developments and some new additional services, already known from ISDN and appreciated by the users.<br>

In the years from 1997 to 2000 new data services with higher bit rates were developed, which are attributed to the&nbsp; $\text{GSM-Phase 2+}$&nbsp; $($or&nbsp; $\text{GSM-Phase 2.5)}$:
*[[Examples_of_Communication_Systems/Weiterentwicklungen_des_GSM#High_Speed_Circuit.E2.80.93Switched_Data_.28HSCSD.29| $\text{High&ndash;Speed Circuit&ndash;Switched Data}$]]&nbsp; $\rm (HSCSD)$&nbsp; offers a line-oriented transmission with&nbsp; $14.4 \ \rm kbit/s$&nbsp; $($against $9.6 \ \rm kbit/s)$, if the channel is sufficiently good due to a higher code rate&nbsp; (dotting of the convolutional code).&nbsp; It also enables channel bundling by combining several time slots &nbsp; &#8658; &nbsp; "Multislot Capability".&nbsp; With a bundling of four time slots, this results in a maximum transmission rate of&nbsp; $57.6 \ \rm kbit/s$.<br>

*[[Examples_of_Communication_Systems/Weiterentwicklungen_des_GSM#General_Packet_Radio_Service_.28GPRS.29|$\text{General Packet Radio Service}$]]&nbsp; $\rm  (GPRS)$&nbsp; enables communication with other networks such as the Internet or company intranets.&nbsp; It is packet-oriented&nbsp; (instead of line-oriented)&nbsp; and supports many data transfer protocols, for example the Internet Protocol (IP), X.25 and Datex&ndash;P.&nbsp; The charges for GPRS are not based on the connection duration, but on the amount of data transmitted.&nbsp; A GPRS user benefits from the shorter access times and the higher data rate up to&nbsp; $21.4 \ \rm kbit/s$.&nbsp; By the bundling of six time slots one reaches so maximally&nbsp; $128.4 \ \rm kbit/s$.<br>

*[[Examples_of_Communication_Systems/Weiterentwicklungen_des_GSM#Enhanced_Data_Rates_for_GSM_Evolution|$\text{Enhanced Data Rates for GSM Evolution}$]]&nbsp; $\rm  (EDGE)$&nbsp; used in addition to GSM standard "GMSK" as a further modulation method&nbsp; [[Modulation_Methods/Quadrature_Amplitude_Modulation#Other_signal_space_constellations|$\text{8-PSK}$]], so that with each symbol three data bits are transmitted and in this way the data rate can (theoretically) be tripled. <br><br>

With the combination of GPRS and EDGE &ndash; one speaks then of&nbsp; $\text{E-GPRS}$&nbsp; &ndash; there are nine different&nbsp; [[Examples_of_Communication_Systems/Weiterentwicklungen_des_GSM#Enhanced_Data_Rates_for_GSM_Evolution|$\text{Modulation and Coding Schemes}$]]&nbsp; $\rm (MCS)$, between which the operator can choose:
*with GMSK or with 8&ndash;PSK modulation,<br>
*resulting code rates between&nbsp; $0.37$&nbsp; and&nbsp; $1.00$, and<br>
*data rates between&nbsp; $8.8 \ \rm kbit/s$&nbsp; (for MCS&ndash;1) and&nbsp; $59.2 \ \ \rm kbit/s$&nbsp; (for MCS&ndash;9).<br><br>

In practice, however, MCS&ndash;8&nbsp; $(54.4 \ \rm kbit/s)$&nbsp; and seven time slots are the maximum applicable.&nbsp; With this, one reaches&nbsp; $380.8 \ \rm kbit/s$&nbsp; and thus the order of magnitude of UMTS&nbsp; $(384 \ \rm kbit/s)$.<br>

We should also mention the&nbsp; [[Mobile_Communications/General_Information_on_the_LTE_Mobile_Communications_Standard#Development_of_the_UMTS_mobile_phone_standards_towards_LTE|$\text{EDGE Evolution}$]]&nbsp; or "Evolved EDGE", i.e. the further development of the evolution of GSM in Release 7 (December 2007). For this, the developers specify data rates up to&nbsp; $1 \ \rm Mbit/s$&nbsp; and halved latency times&nbsp; $(10 \ \rm ms$ instead of $20 \ \rm ms)$&nbsp;. These values can be achieved among other things 
*by&nbsp; 32&ndash;QAM or &nbsp;16&ndash;QAM Modulation instead of 8&ndash;PSK,
* an improved error correction through the use of&nbsp; [[Channel_Coding/Grundlegendes_zu_den_Turbocodes| $\text{Turbo codes}$]], and 
* an increase of the symbol rate of&nbsp; $20\%$&nbsp; from&nbsp; $270.833 \ \rm ksymbol/s$&nbsp; to&nbsp; $325\ \rm ksymbol/s$.<br>

==Exercises for the chapter==
<br>
[[Aufgaben:Exercise 3.5: GMSK Modulation]]

[[Aufgaben:Exercise 3.5Z: GSM Network Components]]

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