# Exercise 4.1: General Questions about LTE

Some terms concerning  $\rm LTE$

Since 2011, mobile communications systems have already reached the fourth generation, without serious delays, as was the case with the second generation of mobile communications  $\rm (GSM)$  and even more so with the third generation  $\rm (UMTS)$.

The development of new "releases" is constantly being driven forward by  $\text{3GPP}$.  This international consortium, which includes all major mobile operators and manufacturers worldwide, will continue to play a prominent role in mobile communications in the future.

LTE has been offered commercially since 2011, especially for sparsely populated rural areas without sufficient DSL broadband connections.  This 4G mobile communications standard is gradually becoming more widespread.

Currently (2017)  $\rm 5G$  is the all-dominant buzzword.  However, the author does not allow himself any more precise statements about the time of introduction than "very soon" or "as soon as possible".  More detailed information can be found  here.

Meanwhile (2021), we can say:  It was 2019, almost simultaneously in Korea and the US.

Notes:

### Questionnaire

1

Which important innovations were described by 3GPP in the Release 8?

 A purely packet-oriented transmission, a high spectral efficiency, frequency or bandwidth flexibility, low energy consumption at the base stations.

2

What is the  "duplex gap"  of the LTE frequency band around  $800 \ \rm MHz$  for?

 For the realization of the  "Cyclic Prefix", to avoid interference between uplink and downlink, to avoid overlapping between individual providers. The duplex gap is also used to operate wireless microphones.

3

What methods and techniques does LTE mainly use?

 OFDMA, SC–FDMA, MIMO, CDMA.

4

How do the frequency bands by  $800 \ \rm MHz$  and by  $2.6 \ \rm GHz$  differ?

 By different prices at the auction. In the frequency band around  $800 \ \rm MHz$  there are only paired frequencies. The frequency band around  $2.6 \ \rm GHz$  can only be used for FDD. The areas of application differ fundamentally.

### Solution

#### Solution

(1)  Correct are the answers 1 to 3, which also stand for the most important improvements through LTE:

• Low power consumption at base stations is explicitly wrong, because the aim is to keep power consumption at the terminals as low as possible, for which even increased power consumption at base stations is accepted.

(2)  Correct are the answers 2 and 4:

• The decisive reason for the "Duplex Gap" is to create a safety buffer between uplink and downlink.
• But answer 4 is also correct:   Radio microphones have been and are still operated with frequencies in the range around 800 MHz, i.e. at frequencies that are now needed for LTE.
• When LTE is finally extended to cover entire areas, there will be interference with less powerful radio microphones.
• In addition, the use of event technology on the LTE frequencies will be prohibited for the years after 2015.  Among other things, this is also why there is this duplex gap, but that is not the real reason, it is a useful side effect.

(3)  Correct are the answers 1 to 3:

• OFDMA is used in the downlink of LTE, SC–FDMA in the uplink.
• Multiple-input multiple-output (MIMO) systems are used more frequently in mobile communications, and not only in LTE.
• In fact, CDMA is also used for LTE, but only for control channels.

(4)  Correct are the answers 1, 2 and 4:

• The higher-frequency 2600 MHz range is particularly interesting for large cities due to conditions imposed by the federal government and because of the lower range.
• The 800 MHz range, which consists of paired frequencies only, will be used primarily for rural areas.