Exercise 4.4: Cellular UMTS Architecture

UMTS cellular architecture

In order to enable a nation-wide network with low transmission power and sufficient frequency economy,  radio cells are set up for UMTS  as for&Nbbsp; "GSM".

For simple system studies,  hexagonal cells are assumed as shown in the diagram.  In reality,  the radio cells vary in size and are also adapted to the existing topology and infrastructure of the coverage area.

There are three different types of radio cells with a hierarchical structure:

1. »Macrocells«:  $\text{4 km}$  to  $\text{6 km}$  in diameter,  speeds allowed up to  $\text{500 km/h}$.
2. »Microcells«:  $\text{1 km}$  to  $\text{2 km}$  indiameter,  maximum speed allowed  $\text{120 km/h}$.
3. »Picocells«:  Diameter up to  $\text{100 m}$,  many participants,  extremely high data volume.

A macrocell can potentially overlay a large number of microcells and picocells.

• For a microcell,  the maximum data rate is  $\text{384 kbit/s}$  for the maximum speed of  $\text{120 km/h}$.
• Subtask  (1)  deals with the maximum data rates allowed in a macrocell and a picocell.

In connection with the cellular structure of UMTS,  terms are often used which will be explained in this task:

1. Intracell interference and intercell interference,
2. cell breathing,
3. handover.

Hints:

Questions

1

Which of the following statements are true?

 UMTS  $($carrier frequency around  $2 \ \rm GHz)$  has larger radio cells than GSM  $($carrier frequency around  $900 \ \rm MHz)$. The data rate is greater in a macrocell than in a microcell. The data rate is greater in a picocell than in a microcell.

2

Are the following statements correct regarding  "interference"?

 Intracell interference occurs when multiple participants in the same cell use the same frequency channel. Intercell interference occurs when the same frequency is used by subscribers in different cells. In the scenario shown,  intracell interference occurs when the frequencies  $f_{1}$  and  $f_{2}$  are the same. In the scenario shown,  intercell interference occurs when frequencies  $f_{3}$  and  $f_{4}$  are equal.

3

What is meant by  "cell breathing"?

 If the number of active participants in the cell increases   ⇒   greater interference power,  the cell radius is reduced. The supply of subscribers at the edge of a busy cell is taken over by a less busy neighboring cell. Cell breathing describes a mechanism for best matching the data rate to the current traffic.

4

Which of the following statements about  "handover"  are true?

 The aim is for the transition of a mobile subscriber from one cell to another to appear uninterrupted. "Hard Handover"  is when the connection is abruptly switched to another connection. With  "Soft Handover",  the handover of a subscriber from one base station to another is gradual.

Solution

Solution

(1)  Only the  last statement  is correct:

• Since the range of radio signals decreases with increasing frequency at the same transmission power,  the radio cells at UMTS  $($carrier frequency around  $2 \ \rm GHz)$  are significantly smaller than at GSM  $($in the D-network around  $900 \ \rm MHz)$.
• The maximum data rates for UMTS  $($which are more theoretical limits$)$ are as follows for
• a macro cell up to  $144 \ \rm kbit/s$,
• a picocell up to  $2 \ \rm Mbit/s$.

(2)  Correct are  all statements:

• The two definitions for intracell and intercell interference are correct.
• Applying these to the scenario at hand  $($graph on the information page$)$,  we see.
• Intracell interference for $f_{1} = f_{2}$,
• Intercell interference for $f_{3} = f_{4}$.
• Intracell interference is more severe than  Intercell interference because of the usually smaller distance of the interferers from the receiver,  that is,  it causes a smaller CIR  $($"carrier-to-interference ratio"$)$.

(3)  Correct are the  statements 1 and 2:

• Cell breathing is accurately described by the first two statements.
• With fewer participants transmitting on a frequency,  the negative influence of intracell interference is reduced.
• The participants in neighboring cells transmit on other frequencies and thus interfere much less.
• An alternative to cell breathing would be to maintain the cell size but reduce the transmitted power of all participants within the cell,
but this also means a reduction in the reception quality.
• The last proposed solution is simply wrong.

(4)  Correct are  all statements:

• For example,  the downlink data is split in theRNC,  broadcast over the participating  "Node Bs",  and reassembled in the mobile station.
• With  "Soft Handover",  a mobile can communicate with up to three base stations simultaneously.
• This variant is of course superior to  "Hard Handover",  but also involves greater effort.