Aufgaben:Exercise 4.2Z: MIMO Applications in LTE

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Four MIMO applications in LTE

One of the many innovations of  $\rm LTE$  is the use of multi-antenna concepts.  However, the technology known as  "Multiple Input Multiple Output"  $\rm (MIMO)$  is not an LTE specific development.  For example, WLAN also uses this method.

The basic concept of MIMO is explained in the chapter Multiple Antenna Systems.  Both the transmitter and the receiver are equipped with several antennas.  This means that several data streams can be transmitted simultaneously.  In addition to  "Single Input Single Output"  $\rm (SISO)$, LTE also supports "2x2 MIMO" in the uplink and up to  "4x4 MIMO"  in the downlink.

The advantages of MIMO technology are:

• a  "Diversity Gain"  and a  "Data Rate Gain"  for multiple connections,
• a higher signal-to-noise ratio  $\rm (SNR)$  and a  $\text{higher peak SNR}$  at the same transmission power,
• lower required transmission power for the same quality requirements, or
• more redundancy in system implementation and thus a more robust system.

In most cases, not all advantages can be exploited simultaneously.  Depending on the nature of the channel, it can also happen that you don't even have the choice of which of these advantages you want to use.

Four different multi-antenna methods with different characteristics are combined under the name  "MIMO", which can be useful in certain situations:

• If the largely independent channels of a MIMO system are occupied by a single user, this is called  "Single-User MIMO".  This increases the data rate for this user by a factor of  $\approx 2$  for "2x2 MIMO" and by a factor of  $\approx 4$  for "4x4 MIMO".
• At  "Multi-User MIMO"  one transmits different data streams to different users.  This is especially useful in places with a high demand, such as airports or even football stadiums.  The focus is therefore not on the maximum data rate for one receiver, but on the number of terminal devices that can use the network simultaneously.
• One speaks from  "Beamforming"  when (in the case of poor transmission conditions)  the transmission power of several antennas is bundled and thus data is transmitted to a (particularly important) user in a targeted manner in order to improve his quality of reception.
• With the help of  "Antenna Diversity"  one increases the redundancy and thus makes the transmission more robust against interference.  For example, if there are four channels, they will all transmit the same message in this application.  If one channel fails at some point, there are still three channels that can carry the message.

Notes:

• This task refers to the chapter  Technical Innovations of LTE.
• In the adjacent diagram, the MIMO applications listed above are illustrated by highly simplified diagrams.
• In the subtask  (1)  you should assign the individual applications to the sketches.

Solution

1

Which sketches represent which of the MIMO methods used in LTE?

	Sketch  $\rm A$  illustrates multi-user MIMO.
	Sketch  $\rm B$  illustrates antenna diversity.
	Sketch  $\rm C$  illustrates multi-user MIMO.
	Sketch  $\rm D$  illustrates antenna diversity.

2

What types of diversity are there in general?

	Interference diversity,
	spatial diversity,
	time diversity,
	Rayleigh Diversity,
	frequency diversity.

3

What immediate advantages are there of using MIMO?

	A diversity gain,
	reduction of intercell interference,
	reduction of intersymbol interference,
	a higher signal-to-noise ratio (SNR),
	a more robust system implementation.

4

What (direct) advantages does MIMO have for LTE?

	Higher data rates for individual users,
	longer ranges of the base stations,
	lower energy consumption at the base stations,
	a lower energy consumption of the end devices,
	improved quality of service (QoS),
	a larger number of simultaneous users.

Solution

Solution

(1)  Correct are the answers 1 and 2.  The following sketches should explain the following MIMO methods:

• Sketch  $\rm A$:  "Multi–User MIMO",
• Sketch  $\rm B$:  "Antenna Diversity",
• Sketch  $\rm C$:  "Single–User MIMO",
• Sketch  $\rm D$:  "Beamforming".

(2)  Correct are the answers 2, 3 and 5.  Diversity can always be exploited if the transmission conditions show useful differences:

• In different places   ⇒   "Spatial Diversity",
• at different times   ⇒   "Time Diversity",
• for different frequencies   ⇒   "Frequency Diversity".

The other two terms used here are purely fictional.

(3)  Correct are the answers 1, 4 and 5:

• As described in detail in the theory section, MIMO technology is used to achieve a diversity gain.
• This indirectly results in a better SNR and an improved transmission quality through more robust systems.
• Interference, whether between cells or symbols, cannot be reduced by MIMO.

(4)  Correct are the answers 1, 2 and 6:

• Single-user MIMO enables higher data rates for the individual user.
• Multi-user MIMO allows more simultaneous users to be served.
• Beamforming increases the range of the base stations.

Energy consumption is by no means reduced by MIMO technology, it even increases at the base stations as well as at the terminal:

• The more antennas need to be powered, the higher the power consumption.
• For this reason, mobile phones are currently limited to a maximum of two antennas – otherwise the battery life would be too short.
• The higher power consumption is of course less important at the base stations than with the large number of terminals.
• It is quite possible that MIMO and appropriate controlling will lead to improvements with regard to suggestions 3, 4 and 5. 
  However, these improvements are indirect.