Modulation Methods/Tasks and Classification - Revision history

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2026-03-16T12:28:10Z

Add German interlanguage link

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	{{Display}}		{{Display}}
			[[de:Modulationsverfahren/Aufgaben_und_Klassifizierung]]

Guenter at 15:11, 19 January 2023

2023-01-19T15:11:56Z

← Older revision		Revision as of 17:11, 19 January 2023
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	== # OVERVIEW OF THE FIFTH MAIN CHAPTER # ==		== # OVERVIEW OF THE FIFTH MAIN CHAPTER # ==
	<br>		<br>
	In the last chapter of the book  "Modulation Methods",  two examples of modern digital modulation methods are discussed,  namely  "CDM"  and  "OFDM",  which are also frequently used as multiple access methods under the names  "CDMA"  and  "OFDMA".		In the last chapter of the book  "Modulation Methods",  two examples of modern digital modulation methods are discussed,  namely   '''»CDM«'''   and  '''»OFDM«'''   which are also frequently used as multiple access methods under the names  '''»CDMA«'''  and  '''»OFDMA«'''.

	*  In  $\rm CDMA$  ("Code Division Multiple Access"),  a specific spreading sequence is provided for each subscriber.  Such spreading sequences – and thus also the overall signal – are high-frequency compared to the useful signal,  so this method is also known as  "Band Spreading".   It is used in particular in third-generation cellular networks such as  [[Examples_of_Communication_Systems/Allgemeine_Beschreibung_von_UMTS\|$\rm UMTS$]].		*  In  $\rm CDMA$  $($"Code Division Multiple Access"$)$,  a specific spreading sequence is provided for each subscriber.  Such spreading sequences – and thus also the overall signal – are high-frequency compared to the useful signal,  so this method is also known as  "Band Spreading".   It is used in particular in third-generation cellular networks such as  [[Examples_of_Communication_Systems/Allgemeine_Beschreibung_von_UMTS\|$\rm UMTS$]].

	*  $\rm OFDM$  ("Orthogonal Frequency Division Multiplex")  is a multicarrier modulation method based on the orthogonality of the carriers used,  which are usually very many.  Due to its advantageous properties,  it is frequently used in wired systems  $($example:   [[Examples_of_Communication_Systems/Allgemeine_Beschreibung_von_DSL\|$\rm xDSL)$]]  and since 2010 also in mobile systems.  In particular, it is used in fourth-generation  $\rm (4G)$  cellular networks as a multiple access method  $\rm (OFDMA)$.   More detailed information can be found in the chapter  [[Mobile_Communications/General_Information_on_the_LTE_Mobile_Communications_Standard\|$\rm LTE$]]  of the book  "Mobile Communications".		*  $\rm OFDM$  $($"Orthogonal Frequency Division Multiplex"$)$  is a multicarrier modulation method based on the orthogonality of the carriers used,  which are usually very many.  Due to its advantageous properties,  it is frequently used in wired systems  $($example:   [[Examples_of_Communication_Systems/Allgemeine_Beschreibung_von_DSL\|$\rm xDSL)$]]  and since 2010 also in mobile systems.  In particular, it is used in fourth-generation  $\rm (4G)$  cellular networks as a multiple access method  $\rm (OFDMA)$.   More detailed information can be found in the chapter  [[Mobile_Communications/General_Information_on_the_LTE_Mobile_Communications_Standard\|$\rm LTE$]]  of the book  "Mobile Communications".


	This chapter deals in detail with:		This chapter deals in detail with
	*the  »tasks of multiple access methods«  in general,		#the  »tasks of multiple access methods«  in general,
	*a general description of  »band spreading techniques and CDMA«,		#a general description of  »band spreading techniques and CDMA«,
	*some  »realization aspects and applications of CDMA«,		#some  »realization aspects and applications of CDMA«,
	*a  »general description of OFDM« ,		#a  »general description of OFDM« ,
	*some  »realization aspects and applications of OFDM«.		#some  »realization aspects and applications of OFDM«.

Hwang at 14:08, 11 January 2023

2023-01-11T14:08:52Z

← Older revision		Revision as of 16:08, 11 January 2023
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	»'''Multiplexing'''«,  on the other hand,  is when a multiplexer bundles several signals at the beginning of a transmission path  (as shown in the  [[Modulation_Methods/~~Aufgaben_und_Klassifizierung~~#~~Multiplexer_und_Demultiplexer~~\|$\text{previous diagram}$]])  and a demultiplexer splits this common signal again at the end.  In this case, the abbreviations FDM, TDM and CDM are used instead of FDMA, TDMA and CDMA:		»'''Multiplexing'''«,  on the other hand,  is when a multiplexer bundles several signals at the beginning of a transmission path  (as shown in the  [[Modulation_Methods/Tasks_and_Classification#Multiplexer_and_demultiplexer\|$\text{previous diagram}$]])  and a demultiplexer splits this common signal again at the end.  In this case, the abbreviations FDM, TDM and CDM are used instead of FDMA, TDMA and CDMA:
	#  Frequency Division Multiplexing  $\rm (FDM)$,		#  Frequency Division Multiplexing  $\rm (FDM)$,
	#  Time Division Multiplexing  $\rm (TDM)$, and		#  Time Division Multiplexing  $\rm (TDM)$, and

Hwang at 14:00, 11 January 2023

2023-01-11T14:00:58Z

← Older revision		Revision as of 16:00, 11 January 2023
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	The graphic shows the procedure:		The graphic shows the procedure:
	*The  $K$  source signals  $q_1(t)$, ... , $q_K(t)$  are each suitably modulated,  combined by the multiplexer  $\rm (MUX)$  to form the ~~transmission~~ signal  $s(t)$  and then transmitted via the common physical channel.		*The  $K$  source signals  $q_1(t)$, ... , $q_K(t)$  are each suitably modulated,  combined by the multiplexer  $\rm (MUX)$  to form the transmitted signal  $s(t)$  and then transmitted via the common physical channel.

	*The demultiplexer  $\rm (DEMUX)$  has the task of extracting the information for the  $K$  subscribers  (sinks)  from the received signal  $r(t)$  and forwarding it to the desired subscriber.		*The demultiplexer  $\rm (DEMUX)$  has the task of extracting the information for the  $K$  subscribers  (sinks)  from the received signal  $r(t)$  and forwarding it to the desired subscriber.

Hwang at 13:44, 11 January 2023

2023-01-11T13:44:24Z

Reed at 13:58, 9 April 2022

2022-04-09T13:58:45Z

← Older revision		Revision as of 15:58, 9 April 2022
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	\|Untermenü=Multiple Access Methods		\|Untermenü=Multiple Access Methods
	\|Vorherige Seite=Nonlinear_Digital_Modulation		\|Vorherige Seite=Nonlinear_Digital_Modulation
	\|Nächste Seite=~~PN–Modulation~~		\|Nächste Seite=Direct-Sequence Spread Spectrum Modulation
	}}		}}

Guenter at 15:37, 17 January 2022

2022-01-17T15:37:45Z

← Older revision		Revision as of 17:37, 17 January 2022
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	The  "World Administrative Radio Conference"  (WARC)  coordinates the use of the frequencies available today and in the future worldwide.  Engineers have the task of using the frequency band made available for an application as effectively as possible and thus supplying as many subscribers as possible.		The  "World Administrative Radio Conference"  (WARC)  coordinates the use of the frequencies available today and in the future worldwide.  Engineers have the task of using the frequency band made available for an application as effectively as possible and thus supplying as many subscribers as possible.

	[[File:~~EN_Mod_T_5_1_S1~~.png\|right\|frame\| Schematic diagram for multiplexing ~~'''KORREKTUR''': Channel~~]]		[[File:EN_Mod_T_5_1_S1_neu2.png\|right\|frame\| Schematic diagram for multiplexing]]

	The graphic shows the procedure:		The graphic shows the procedure:

Guenter at 12:10, 28 November 2021

2021-11-28T12:10:47Z

← Older revision		Revision as of 14:10, 28 November 2021
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	~~'''~~Multiple Access Method~~'''~~  indicates that there are several transmitter-receiver pairs that independently share a transmission medium.		"Multiple Access Method"  indicates that there are several transmitter-receiver pairs that independently share a transmission medium.
	*In mobile communications,  this transmission medium is the radio interface,  in simplified terms the  "air”  in the environment of a base station.		*In mobile communications,  this transmission medium is the radio interface,  in simplified terms the  "air”  in the environment of a base station.
	*The allocation is done either with a central instance in the base station or the subscribers work with collision detection.		*The allocation is done either with a central instance in the base station or the subscribers work with collision detection.

Guenter at 16:12, 27 November 2021

2021-11-27T16:12:45Z

Guenter at 15:44, 27 November 2021

2021-11-27T15:44:45Z

Show changes

← Older revision		Revision as of 15:44, 11 January 2023
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	<br>		<br>
	So far, we have always assumed a single message source that transmits information		So far, we have always assumed a single message source that transmits information
	*to a single sink  ('''point-to-point connection'''),  or		*to a single sink  (»'''point-to-point connection'''«),  or
	*supplies several participants at the same time  ('''point-to-multipoint connection''',  "broadcast").		*supplies several participants at the same time  (»'''point-to-multipoint connection'''«,  "broadcast").


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	<br>		<br>
	[[File: EN_Mod_T_5_1_S2.png \|right\|frame\|Diagram of FDMA, TDMA and CDMA multiple access methods ]]		[[File: EN_Mod_T_5_1_S2.png \|right\|frame\|Diagram of FDMA, TDMA and CDMA multiple access methods ]]
	The diagram with the three axes, namely  "time",  "frequency"  and  "power"  illustrates three widely used  '''Multiple Access Methods''', namely		The diagram with the three axes, namely  "time",  "frequency"  and  "power"  illustrates three widely used  »'''Multiple Access Methods'''«, namely

	#  Frequency Division Multiple Access  $\rm (FDMA)$,		#  Frequency Division Multiple Access  $\rm (FDMA)$,
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	'''Multiplexing''',  on the other hand,  is when a multiplexer bundles several signals at the beginning of a transmission path  (as shown in the  [[Modulation_Methods/Aufgaben_und_Klassifizierung#Multiplexer_und_Demultiplexer\|previous diagram]])  and a demultiplexer splits this common signal again at the end.  In this case, the abbreviations FDM, TDM and CDM are used instead of FDMA, TDMA and CDMA:		»'''Multiplexing'''«,  on the other hand,  is when a multiplexer bundles several signals at the beginning of a transmission path  (as shown in the  [[Modulation_Methods/Aufgaben_und_Klassifizierung#Multiplexer_und_Demultiplexer\|$\text{previous diagram}$]])  and a demultiplexer splits this common signal again at the end.  In this case, the abbreviations FDM, TDM and CDM are used instead of FDMA, TDMA and CDMA:
	#  Frequency Division Multiplexing  $\rm (FDM)$,		#  Frequency Division Multiplexing  $\rm (FDM)$,
	#  Time Division Multiplexing  $\rm (TDM)$, and		#  Time Division Multiplexing  $\rm (TDM)$, and
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	The processes shown in the diagram can be characterized as follows:		The processes shown in the diagram can be characterized as follows:
	*In  "Frequency Division Multiple Access",  each of the  $K$  users is allocated only a portion of the total bandwidth  $B$  and can transmit continuously (analog or digital) within it.  As already described in detail in the introductory chapter  [[Modulation_Methods/Zielsetzung_von_Modulation_und_Demodulation\|Objectives of Modulation and Demodulation]],  all users can transmit simultaneously at different carrier frequencies without any deterioration in transmission quality.  The prerequisite is that no user occupies more than the currently permissible bandwidth  $B/K$  and that filters with arbitrarily steep edges are also available for channel separation.  Otherwise,  protective distances between the individual frequency bands must be observed.		*In  "Frequency Division Multiple Access",  each of the  $K$  users is allocated only a portion of the total bandwidth  $B$  and can transmit continuously (analog or digital) within it.  As already described in detail in the introductory chapter  [[Modulation_Methods/Zielsetzung_von_Modulation_und_Demodulation\|"Objectives of Modulation and Demodulation"]],  all users can transmit simultaneously at different carrier frequencies without any deterioration in transmission quality.  The prerequisite is that no user occupies more than the currently permissible bandwidth  $B/K$  and that filters with arbitrarily steep edges are also available for channel separation.  Otherwise,  protective distances between the individual frequency bands must be observed.

	*In  "Time Division Multiple Access",  on the other hand,  all $K$  subscribers use the entire frequency band  $B$, but only for a fraction  $(1/K)$  of the time.  In the case of time-continuous symbols,  time division multiplexing requires intermediate storage at the transmitter,  blockwise transmission at a data rate that is higher by a factor of  $K$,  and a facility at the receiver,  which reassembles the blocks in the individual time slots to form a continuous data stream.		*In  "Time Division Multiple Access",  on the other hand,  all $K$  subscribers use the entire frequency band  $B$, but only for a fraction  $(1/K)$  of the time.  In the case of time-continuous symbols,  time division multiplexing requires intermediate storage at the transmitter,  blockwise transmission at a data rate that is higher by a factor of  $K$,  and a facility at the receiver,  which reassembles the blocks in the individual time slots to form a continuous data stream.

	*In  "Code Division Multiple Access",  the same frequency band of width  $B$  is shared by all subscribers at all times;  they are virtually on top of each other in terms of power.  In order to be able to separate the subscriber signals at the receiver, they are multiplied at the transmitter by a periodic binary sequence with a higher frequency than the useful bandwidth by a factor of  $J ≥ K$,  which corresponds spectrally to a band spreading by this factor  $J$.		*In  "Code Division Multiple Access",  the same frequency band of width  $B$  is shared by all subscribers at all times;  they are virtually on top of each other in terms of power.  In order to be able to separate the subscriber signals at the receiver, they are multiplied at the transmitter by a periodic binary sequence with a higher frequency than the useful bandwidth by a factor of  $J ≥ K$,  which corresponds spectrally to a band spreading by this factor  $J$.
	*This is referred to as  '''PN modulation'''  or  "Direct Sequence Spread Spectrum".  If  $J$  mutually orthogonal spread sequences are found,  then under ideal conditions,  up to  $J$  users can occupy the common channel simultaneously without mutual interference,  but each only with the useful bandwidth  $B/J$  (based on the data rate).  Due to the band spreading,  however,  each user occupies the entire frequency band  $B$  at all times.		*This is referred to as  »'''PN modulation'''«  or  "Direct Sequence Spread Spectrum".  If  $J$  mutually orthogonal spread sequences are found,  then under ideal conditions,  up to  $J$  users can occupy the common channel simultaneously without mutual interference,  but each only with the useful bandwidth  $B/J$  (based on the data rate).  Due to the band spreading,  however,  each user occupies the entire frequency band  $B$  at all times.


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	==Application examples==		==Application examples==
	<br>		<br>
	'''(1)'''  [[Modulation_Methods/Objectives_of_Modulation_and_Demodulation#Time_Division_Multiplex_methods\|Time Division Multiplexing]]  $\rm (TDM)$  has been used for a very long time,  for example in the  "Plesiochronous Digital Hierarchy"  $\rm (PDH)$  standardized in 1972.		'''(1)'''  [[Modulation_Methods/Objectives_of_Modulation_and_Demodulation#Time_Division_Multiplex_methods\|$\text{Time Division Multiplexing}$]]  $\rm (TDM)$  has been used for a very long time,  for example in the  "Plesiochronous Digital Hierarchy"  $\rm (PDH)$  standardized in 1972.
	*Starting from a data rate of  $\text{2.048 Mbit/s}$,  the higher bit rates  $\text{8.448, 34.368, 139.264}$  and  $\text{564.992 Mbit/s}$  are obtained,  in each case by quadrupling,  taking into account a certain overhead.		*Starting from a data rate of  $\text{2.048 Mbit/s}$,  the higher bit rates  $\text{8.448, 34.368, 139.264}$  and  $\text{564.992 Mbit/s}$  are obtained,  in each case by quadrupling,  taking into account a certain overhead.
	*Since 1988,  there is still the  "Synchronous Digital Hierarchy"  $\rm (SDH)$  with the data rates  $\text{155.52, 622.08, 2488.32, 9953.28, 39813.12 Mbit/s ≈ 40 Gbit/s}$  $($in each case exactly the factor $4)$,  which is used in the newer optical systems.		*Since 1988,  there is still the  "Synchronous Digital Hierarchy"  $\rm (SDH)$  with the data rates  $\text{155.52, 622.08, 2488.32, 9953.28, 39813.12 Mbit/s ≈ 40 Gbit/s}$  $($in each case exactly the factor $4)$,  which is used in the newer optical systems.


	'''(2)'''  The oldest multiplexing method is  [[Modulation_Methods/Objectives_of_Modulation_and_Demodulation#Channel_bundling_.E2.80.93_Frequency_Division_Multiplexing\|Frequency Division Multiplexing]]  $\rm (FDM)$,  which has been used since the early days of analog broadcasting technology in the 1930s.		'''(2)'''  The oldest multiplexing method is  [[Modulation_Methods/Objectives_of_Modulation_and_Demodulation#Channel_bundling_.E2.80.93_Frequency_Division_Multiplexing\|$\text{Frequency Division Multiplexing}$]]  $\rm (FDM)$,  which has been used since the early days of analog broadcasting technology in the 1930s.
	*For example,  the "VHF II" range – in Germany better known as "UKW" – covers the frequency range from  $\text{87.5}$  to  $\text{108 MHz}$, i.e. a bandwidth of  $\text{20.5 MHz}$.		*For example,  the "VHF II" range – in Germany better known as "UKW" – covers the frequency range from  $\text{87.5}$  to  $\text{108 MHz}$, i.e. a bandwidth of  $\text{20.5 MHz}$.
	*With the original frequency grid of  $\text{300 kHz}$,  $K = 68$  channels could be provided.  Nowadays,  the channel spacing is usually narrower,  with  $\text{150 kHz}$  intervals.		*With the original frequency grid of  $\text{300 kHz}$,  $K = 68$  channels could be provided.  Nowadays,  the channel spacing is usually narrower,  with  $\text{150 kHz}$  intervals.
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	'''(4)'''   "Orthogonal Frequency Division Multiplex"  $\rm (OFDM)$,  which is described in detail in the  [[Modulation_Methods/Allgemeine_Beschreibung_von_OFDM\|second part of this chapter]],  is of great importance.		'''(4)'''   "Orthogonal Frequency Division Multiplex"  $\rm (OFDM)$,  which is described in detail in the  [[Modulation_Methods/Allgemeine_Beschreibung_von_OFDM\|"second part of this chapter"]],  is of great importance.
	*Both the wired  [[Examples_of_Communication_Systems/Allgemeine_Beschreibung_von_DSL\|$\rm DSL$]]  ("Digital Subscriber Line")  and the 4G mobile communications system  [[Mobile_Communications/General_Information_on_the_LTE_Mobile_Communications_Standard\|$\rm LTE$]]  ("Long Term Evolution")  are based on this.		*Both the wired  [[Examples_of_Communication_Systems/Allgemeine_Beschreibung_von_DSL\|$\rm DSL$]]  ("Digital Subscriber Line")  and the 4G mobile communications system  [[Mobile_Communications/General_Information_on_the_LTE_Mobile_Communications_Standard\|$\rm LTE$]]  ("Long Term Evolution")  are based on this.
	*These are multi-carrier systems in which the individual spectral functions overlap but do not interfere with each other due to orthogonality.		*These are multi-carrier systems in which the individual spectral functions overlap but do not interfere with each other due to orthogonality.

← Older revision		Revision as of 18:12, 27 November 2021
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	==Application examples==		==Application examples==
	<br>		<br>
	'''(1)'''  [[Modulation_Methods/Objectives_of_Modulation_and_Demodulation#Time_Division_Multiplex_methods\|Time ~~division multiplexing~~]]  $\rm (TDM)$  has been used for a very long time, for example in the  ''Plesiochronous Digital Hierarchy''  $\rm (PDH)$ standardized in 1972.		'''(1)'''  [[Modulation_Methods/Objectives_of_Modulation_and_Demodulation#Time_Division_Multiplex_methods\|Time Division Multiplexing]]  $\rm (TDM)$  has been used for a very long time,  for example in the  "Plesiochronous Digital Hierarchy"  $\rm (PDH)$  standardized in 1972.
	*Starting from a data rate of  $\text{2.048 Mbit/s}$,  the higher bit rates  $\text{8.448, 34.368, 139.264}$  and  $\text{564.992 Mbit/s}$ are obtained, in each case by quadrupling, taking into account a certain overhead.		*Starting from a data rate of  $\text{2.048 Mbit/s}$,  the higher bit rates  $\text{8.448, 34.368, 139.264}$  and  $\text{564.992 Mbit/s}$  are obtained,  in each case by quadrupling,  taking into account a certain overhead.
	*Since 1988, there is still the  ''Synchronous Digital Hierarchy''  $\rm (SDH)$  with the data rates  $\text{155.52, 622.08, 2488.32, 9953.28, 39813.12 Mbit/s ≈ 40 Gbit/s}$  $($in each case exactly the factor $4)$, which is used in the newer optical systems.		*Since 1988,  there is still the  "Synchronous Digital Hierarchy"  $\rm (SDH)$  with the data rates  $\text{155.52, 622.08, 2488.32, 9953.28, 39813.12 Mbit/s ≈ 40 Gbit/s}$  $($in each case exactly the factor $4)$,  which is used in the newer optical systems.


	'''(2)'''  The oldest multiplexing method is  [[Modulation_Methods/Objectives_of_Modulation_and_Demodulation#Channel_bundling_.E2.80.93_Frequency_Division_Multiplexing\|~~frequency division multiplexing~~]]  $\rm (FDM)$, which has been used since the early days of analog broadcasting technology in the 1930s.		'''(2)'''  The oldest multiplexing method is  [[Modulation_Methods/Objectives_of_Modulation_and_Demodulation#Channel_bundling_.E2.80.93_Frequency_Division_Multiplexing\|Frequency Division Multiplexing]]  $\rm (FDM)$,  which has been used since the early days of analog broadcasting technology in the 1930s.
	*For example, the "VHF II" range – better known as "UKW" – covers the frequency range from  $\text{87.5}$  to  $\text{108 MHz}$, i.e. a bandwidth of  $\text{20.5 MHz}$.		*For example,  the "VHF II" range – in Germany better known as "UKW" – covers the frequency range from  $\text{87.5}$  to  $\text{108 MHz}$, i.e. a bandwidth of  $\text{20.5 MHz}$.
	*With the original frequency grid of  $\text{300 kHz}$,  $K = 68$  channels could be provided.  Nowadays, the channel spacing is usually narrower, with  $\text{150 kHz}$  intervals.		*With the original frequency grid of  $\text{300 kHz}$,  $K = 68$  channels could be provided.  Nowadays,  the channel spacing is usually narrower,  with  $\text{150 kHz}$  intervals.


	'''(3)'''  The European mobile communications standard  [[Examples_of_Communication_Systems/Allgemeine_Beschreibung_von_GSM\|$\rm GSM$]]  (''Global System for Mobile Communications'') introduced in the early 1990s also uses an FDMA component in addition to a TDMA component (eight time slots per frequency channel).		'''(3)'''  The European mobile communications standard  [[Examples_of_Communication_Systems/Allgemeine_Beschreibung_von_GSM\|$\rm GSM$]]  ("Global System for Mobile Communications")  introduced in the early 1990s also uses an FDMA component in addition to a TDMA component  (eight time slots per frequency channel).
	*For the connection from the mobile station to the base station  ~~$\rm (~~uplink)$,  the frequency range  $890.1$ ...  $914.9 \ \rm MHz$  is used and in the opposite direction  ~~$\rm (~~downlink)$  the range  $935.1$ ...  $959.9 \ \rm MHz$ ~~is used~~.		*For the connection from the mobile station to the base station   ⇒   "uplink",  the frequency range  $890.1$ ...  $914.9 \ \rm MHz$  is used and in the opposite direction  ⇒   "downlink"  the range  $935.1$ ...  $959.9 \ \rm MHz$.
	*The entire bandwidth  $B\text{ = 24.8 MHz}$  available in the so-called "D band" for uplink and downlink is divided by ~~frequency division multiplexing~~ (FDMA) into  $124$  bands, each  $\text{200 kHz}$  wide.		*The entire bandwidth  $B\text{ = 24.8 MHz}$  available in the so-called  "D band"  for uplink and downlink is divided by  "Frequency Division Multiplexing"  $\rm (FDMA)$  into  $124$  bands, each  $\text{200 kHz}$  wide.
	*The maximum possible number of channels is thus (theoretically)  $\text{8 · 124 = 992}$.  However, since neighboring cells must not use the same frequencies, as this would result in a considerable loss of quality at the cell boundaries, the maximum possible number of channels for the GSM system is  $\text{330}$  $($reuse factor $3)$.		*The maximum possible number of channels is thus  (theoretically)  $\text{8 · 124 = 992}$.  However,  since neighboring cells must not use the same frequencies,  as this would result in a considerable loss of quality at the cell boundaries,  the maximum possible number of channels for the GSM system is  $\text{330}$  $($reuse factor $3)$.


	'''(4)'''   ''Orthogonal Frequency Division Multiplex''  $\rm (OFDM)$, which is described in detail in the  [[Modulation_Methods/Allgemeine_Beschreibung_von_OFDM\|second part of this chapter]],  is of great importance.		'''(4)'''   "Orthogonal Frequency Division Multiplex"  $\rm (OFDM)$,  which is described in detail in the  [[Modulation_Methods/Allgemeine_Beschreibung_von_OFDM\|second part of this chapter]],  is of great importance.
	*Both the wired  [[Examples_of_Communication_Systems/Allgemeine_Beschreibung_von_DSL\|$\rm DSL$]]  (''Digital Subscriber Line'')  and the 4G mobile communications system  [[Mobile_Communications/General_Information_on_the_LTE_Mobile_Communications_Standard\|$\rm LTE$]]  (''Long Term Evolution'')  are based on this.		*Both the wired  [[Examples_of_Communication_Systems/Allgemeine_Beschreibung_von_DSL\|$\rm DSL$]]  ("Digital Subscriber Line")  and the 4G mobile communications system  [[Mobile_Communications/General_Information_on_the_LTE_Mobile_Communications_Standard\|$\rm LTE$]]  ("Long Term Evolution")  are based on this.
	*These are multi-carrier systems in which the individual spectral functions overlap but do not interfere with each other due to orthogonality.		*These are multi-carrier systems in which the individual spectral functions overlap but do not interfere with each other due to orthogonality.


			'''(5)'''  Before that, we will look at different variants of band spreading methods and  "Code Division Multiple Access"  $\rm (CDMA)$ in the three following chapters.  This is used,  for example,  in the 3G mobile communications standard  [[Examples_of_Communication_Systems/Allgemeine_Beschreibung_von_UMTS\|$\rm UMTS$]]  ("Universal Mobile Telecommunications System").
	'''(5)'''  Before that, we will look at different variants of band spreading methods and  ''Code Division Multiple Access''  $\rm (CDMA)$ in the three following chapters.
	* This is used, for example, in the 3G mobile communications standard  [[Examples_of_Communication_Systems/Allgemeine_Beschreibung_von_UMTS\|$\rm UMTS$]]  (''Universal Mobile Telecommunications System'').

	==Exercises for the chapter==		==Exercises for the chapter==