Exercise 4.3Z: Multiple-Access Methods in LTE

From LNTwww


Frequency-time assignments

In the case of the 4G mobile radio system LTE, there was a change not only in the type of modulation compared with the 3G system UMTS , but also in the access procedure in particular.  A suitable access method is characterized by the fact that as many users of a radio cell as possible can be supplied with the highest possible data rate.  The total throughput is decisive.

The figure shows two charts  $\rm A$  and  $\rm B$  with exemplary frequency-time assignments.  You should decide which access methods should be explained by these two diagrams.




Notes:


Questionnaire

1

What access methods are used with LTE?

CDMA,
OFDM,
OFDMA,
SC–FDMA.

2

Which of the statements apply to the OFDMA method?

Each frequency block is occupied by only one user.
Several users can occupy one frequency block.
All users share the entire frequency band.
The use of TDMA is also mandatory.

3

Which chart describes OFDMA?

Chart  $\rm A$.
Chart  $\rm B$.

4

What applies to the allocation of resources?

The resource allocation of OFDMA is more flexible than the allocation of OFDM.
In order to use this flexibility, a coordination between base station ("eNodeB") and terminal is necessary.
Preference is always given to a channel with poor conditions.


Solution

(1)  Correct are the solutions 3 and 4.  LTE uses

  • Orthogonal Frequency Division Multiple Access  $\rm (OFDMA)$  in downlink,
  • Single Carrier Frequency Division Multiple Access  $\rm (SC–FDMA)$  in uplink.
  • Code Division Multiple Access  $\rm (CDMA)$  is used in the 3G mobile system  UMTS.
  • In  DSL, Orthogonal Frequency Division Multiple Access  $\rm (OFDMA)$  is used.
  • In this context, however, this is to be understood more as a modulation method than as a multiple access method.


(2)  Correct are the answers 2 and 4:

  • Several users can occupy one frequency block.
  • TDMA is additionally possible.
  • The answer 1 corresponds to "CDMA", and answer 3 applies to "OFDM".


(3)  According to the solution of task  (2) 

  • the second proposal is applicable   ⇒   chart  $\rm B$.


(4)  Correct are the solution proposals 1 and 2 in contrast to proposal 3:

  • Since OFDMA resource allocation is not only limited to the time domain, but also optimally includes the frequency domain, OFDMA is more flexible than OFDM. 
  • Of course, the users with the best boundary conditions for resource allocation are preferred.
  • The rather unsolidary principle "If you're already doing well, you'll be given" is therefore applied. This is often found in politics as well.
  • It is undisputed, however, that in this way the total throughput of the cell is maximised, which is the basis for optimisation for the network operators.
  • If the entire power were to be made available to a user with extremely poor conditions, the total throughput would be close to zero.