Exercise 4.4Z: Physical Channels in LTE

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Frequency/time assignment of the channels  PDCCH  and  PDSCH

The task refers to the two pages 

•  Physikalsiche Kanäle im Uplink,
•  Physikalsiche Kanäle im Downlink.

All information required for the task can be found on these pages.

The diagram shows the assignment of the two channels  PDCCH  and  PDSCH in frequency and time:

• One frequency block comprises  $180 \ \rm kHz$  and is divided into twelve  $15 \ \rm kHz$–subcarriers
• A subframe is one millisecond long and includes  $14$  symbols.
• Red marked are so called reference symbols.

Notes:

• The task belongs to the chapter Bitübertragungsschicht bei LTE.

Questionnaire

1

Which physical channels belong to the LTE uplink?

	PDCCH,
	PDSCH,
	PUCCH,
	PUSCH.

2

Which of the channels transmit user data?

	PDCCH,
	PDSCH,
	PUCCH,
	PUSCH.

3

Which statements apply to the graphic on the information page?

	A transmission block for the downlink is shown.
	A block is the smallest addressable unit for LTE.
	The reference symbols are used to estimate the channel quality.

4

Which statements are correct without considering the reference symbols?

	In the example 6/7 of the symbols contain useful information.
	The statement "6/7 of the symbols carry useful information" applies generally.

5

What does the individual data rate of a user depend on?

	of the total available bandwidth,
	of the channel quality for this user,
	from the requirements of all users.

Sample solution

Solution

(1)  Correct are the solutions 3 and 4:

The assignment to Downlink or Uplink can be recognized by the second letter. It means:

• PDCCH: Physical Downlink Control Channel,
• PDSCH: Physical Downlink Shared Channel,
• PUCCH: Physical Uplink Control Channel,
• PUSCH: Physical Uplink Shared Channel.

(2)  Correct are the solutions 2 and 4:

• A "C"   ⇒   Control as third letter indicates a control channel.
• User data are always transferred to the Shared Channels   ⇒   "S",

(3)  All statements are correct:

• A block in the LTE downlink occupies $180 \ \ \rm kHz$ (twelve subcarriers at $15 \ \ \rm kHz$ each) and has a duration of $1 \ \ \rm ms$.
• The assignment with PDCCH and PDSCH shows that the downlink is being considered.
• The reference symbols are needed to estimate the channel quality and calculate the Channel Quality Indicator (CQI).
• These reference symbols are distributed over different frequencies or symbols (different times) in order to estimate the channel quality as comprehensively as possible.

(4)  The statement 1 is true because only two of the $14$ columns are occupied by the control channel PDCCH.

However, this result cannot be generalized. Rather, the division between PDCCH– and PDSCH symbols is dependent on the user's requirements and therefore dynamic.

• For many users with a low data rate, the PDCCH would include three or four symbols, because this requires more intensive tuning than for a few concurrent users with high data rates.
• The information "How many PDCCH symbols" is communicated to the terminal device via the Physical Control Format Indicator Channel (PCFICH).

(5)  All statementsabove are correct:

• The data rate of each user depends directly on the number of blocks of width $180 \ \rm kHz$ assigned to him.
• The total LTI frequency width is between $1.4 \ \rm MHz$ and $20 \ \rm MHz$.
• In the frequency band of $1.4 \ \ \rm MHz$ six blocks of $180 \ \ \rm kHz$ are placed.
• The overhead is thus $(1.4 - 6 \cdot 0.18)/1.4 \approx 22.8 \%$.
• At $20 \ \rm MHz$ total frequency width, i.e. $100$ blocks, the overhead is $(20 - 100 \cdot 0.18)/26 = 10 \ \ \%$.
• The more blocks are available in total, the more can be allocated to each individual user if his channel is good and if many other users do not also have high demands at the same time.