Exercise 4.1: Different Duplex Methods for UMTS

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UTRA-FDD and UTRA-TDD

$\rm UMTS$  $($"Universal Mobile Telecommunications System"$)$  is a so-called  "third-generation mobile communications system",  planned in the early 1990s and available in Europe since 2004.

It uses in both directions,  uplink  and  downlink,  the multiple access method  "CDMA"  $($"Code Division Multiple Access"$)$.  The standardization essentially provides for two different modes:

• $\text{UTRA–FDD}$  $($"UMTS Terrestrial Radio Access Frequency Division Duplex"$)$  with twelve paired frequency bands for the uplink  $(1920 - 1980 \ \rm MHz)$  and the downlink  $(2110 - 2170 \ \rm MHz)$.

• $\text{UTRA–TDD}$  $($"UMTS Terrestrial Radio Access Time Division Duplex"$)$  provides four channels in the frequency band of  $\rm 1900\ –1920\ MHz$  and another at  $2020\ – 2025\ \rm MHz$.

The band at  $2010 - 2020 \rm MHz$  is also reserved for UTRA–TDD.

• The diagram shows schematically the frequency band assignments of UTRA-FDD (top) and UTRA-TDD (bottom).

• The two methods are quite different both in terms of multiple access and of duplex implementation.

Hints:

• This exercise belongs to the chapter  "General Description of GSM".

• Reference is made in particular to the page  "Full Duplex".

Questions

1

Which of the following statements are true?

	UMTS is a second-generation mobile communications system.
	UMTS is a third-generation mobile communications system.
	UMTS is a fourth-generation mobile communications system.

2

How are  "uplink"  and  "downlink"  separated in  UTRA–FDD?

	The data is transmitted separately in time.
	The data is transmitted in the same frequency band.
	The data is transmitted in paired frequency bands.

3

How are  "uplink"  and  "downlink"  separated in  UTRA–TDD?

	The data are transmitted separately in time.
	The data is transmitted in the same frequency band.
	The data is transmitted in paired frequency bands.

4

What is the total bandwidth allocated for  UTRA–FDD?

$B_{\rm total} \ = \ $ $\ \rm MHz $

5

What bandwidth does each user occupy in  UTRA–FDD  $($both uplink and downlink after bandspreading$)$?

$B_{\rm user} \ = \ $ $ \ \rm MHz $

6

What is the bandwidth of each user at  UTRA–TDD?

$B_{\rm user} \ = \ $ $ \ \rm MHz $

7

Which statements are true?

	In Europe,  only the  FDD mode is used.
	The TDD mode is mainly suitable for asymmetric services.

Solution

Solution

(1)  Correct is the  statement 2:

• A representative of second-generation mobile communications is  "GSM"  ("Global System for Mobile Communications"'),  which has been available since the early 1990s and is based on the modulation method "GMSK"  ("Gaussian Minimum Shift Keying").

• In contrast,  UMTS uses as multiple access method  "CDMA"  (Code Division Multiple Access).

• The fourth-generation mobile communications system is  "LTE"  ("Long Term Evolution"),  which is based on the  "OFDM method"  ("Orthogonal Frequency Division Multiplex").  The LTE rollout began in the early 2010s.

(2)  From the graph on the statement page,  you can see that for UTRA–FDD,  the  last statement  is true.

(3)  Correct are the  statements 1 and 2:

• According to the diagram below,  in UTRA–FDD the uplink and downlink are transmitted in the same frequency band.

• The separation is done by time division multiplexing.

(4)  According to the specification,  uplink and downlink occupy  $60 \ {\rm MHz}$  each   $ \Rightarrow \ B_{\rm total}\hspace{0.15cm}\underline{ = 120 \ \rm MHz}$.

(5)  In each case  $B_{\rm user} \hspace{0.15cm}\underline{ = 5 \ \rm MHz}$,  both in uplink and downlink.

• This value is obtained by dividing the respective total bandwidth for uplink and downlink  $(60 \ \rm MHz)$  by the number of channels  $(12)$.

(6) Here again  $B_{\rm user} \hspace{0.15cm} \underline{= 5 \ \rm MHz}$,  but now this bandwidth must be divided between uplink and downlink by TDMA.

(7)  Both statements are correct:

• There are no plans to offer  TDD mode in Europe.

• For asymmetric service,  the data volume in the downlink is significantly larger than in the uplink.

• Here,  the TDD mode would make sense.  Examples:   Surfing,  downloads on the Internet.