NM-1.03

Question 1

What is latency

Answer 1

The time that passes from a user sending a piece of data quest (e.g filge download or loading a webpage), to the time when the user gets a response.

Question 2

Explain spectral flexibility

Answer 2

1. Ability of a system or device to operate across a range of frequencies or wavelengths within the electromagnetic spectrum.
2. Capacity of comm. systems to adapt to difference frequency bands or channels.

.. operation in a wide range of spectrum and in different-sized bands

Question 3

Describe the protocal layers

Answer 3

1. Physical Layer
   - Handles communication impairments
   - Responsible for modulation, frequency selection, signal detection
   - Encryption
2. Medium Access Control
   - Responsible for mediating access to wireless medium
3. Radio link control
   - Handover between different networks
4. Packet data convergence protocol
   - Reliable end-to-end tx
   - Flow and congestion
   - Error recovery

Question 4

Explain the concept of scheduling

Answer 4

A scheduler is a system component in charge of allocating over time the radio resources needed to transmit to the wireless users

Question 5

How does OFDMA work?

Answer 5

Orthogonal Frequency Division Multiple Access:

• OFDMA divides the available frequency spectrum into multiple smaller subcarriers.
• Subcarriers are orthogonal to each other -> don’t interfere with each other.
• Each carrier is capable of carrying data independently.
• Key feature of OFDMA is its ability to dynamically allocate subcarriers to different users based on their data rate requirements and channel conditions.
• OFDMA can adapt the modulation and coding schemes for each subcarrier based on the channel conditions.

Question 6

Explain Multi-Antenna Techniques

Answer 6

1. Transmit diversity
2. Beam-forming
3. Spatial multiplexing
4. Multi-user MIMO

Question 7

Explain some goals of LTE Advanced

Answer 7

Peak data rates: DL - 1 Gbps; UL - 500 Mbps

Spectrum efficiency: 3 times greater than LTE

Cell edge user throughput: twice that of LTE

Average user throughput: 3 times that of LTE

Question 8

What is mobile broadband?

Answer 8

When you use the internet at home.

Question 9

What is the issue of carrier aggregation evolution?

Answer 9

wavelength = c/f; if we have a lower wl, then we must have to have large antennas and can travel further.

A higher wl can cause smaller antennas; add more antennas -> MIMO. But higher wl can’t travel as far.

Question 10

What benefits do we get from LTE

Answer 10

1. Connect the IOT
2. New ways to connect and interact -> vehicle communication
3. Latency-ctritical control

Question 10

For which of the following devices the battery lifetime is not a metric of major interest?

1. Wireless sensors
2. Mobile phones
3. Cellular base stations

Answer 10

Base stations as it’s connected to the grid

Question 11

What is round robin?

Answer 11

Schedules users cyclically i.e. schedules users independently of their channel conditions.

Question 12

Give the equation for the Proportional Fair schedule and what happens to r_n

Answer 12

n_max = argmax{d(t)/rn(t)}

Question 13

Explain the max C/I

Answer 13

Max throughput;

Question 14

How can latency be measured?

Answer 14

The latency can be measured by the time it takes for an IP packet to travel from the terminal through the network to the Internet server, and back

Question 15

What are the LTE features

Answer 15

1. OFDM/OFDMA
2. All-IP architecture; simplified network management
3. Spectrum flexibility

Question 16

Why does the broadcast channel use QPSK?

Answer 16

It uses QPSK as since it only transmits two bits per time, it’s a lot less error-prone than modulation schemes using more.

Question 17

Where is the scheduler located?

Answer 17

The scheduler is in the eNodeB

Question 18

Explain the PHY layer in detail

Answer 18

1. OFDMA with cyclic prefix in downlink
2. SC-FDMA with cyclic prefix in uplink
3. 10 ms radio frames, containing 20 slots of 0.5ms duration
4. Maximum information block size: 6144 bits
5. CRC-24 for error detection

Question 19

In scheduling, slots consist of either 6 or 7 OFDM symbols. What are longer cyclic prefixes are used for?

Answer 19

Longer cyclic prefixes are used to address longer multipath fading channel length.

Question 20

In OFDM what does the number of available subcarrier change depend on?

Answer 20

Number of available subcarriers changes depending on transmission bandwidth

Question 21

For which is the following devices is the battery lifetime not a metric of major interest?

• Wireless sensors
• Mobile phones
• Cellular base stations

Answer 21

Base stations; connected by cable

Question 22

Which of these access
techniques is the one adopted in
4th generation cellular systems
(LTE/LTE-A)?

• TDMA
• OFDMA
• CDMA

Answer 22

OFDMA

TDMA -> 2G
CDMA -> 3G

Question 23

What are the LTE performance objectives?

Answer 23

1. 100 Mbps peak DL, 50 Mbps peak UL
2. Backwards compatibility with GSM/EDGE/UMTS
3. Wide application; FDD and TDD paired operation, Mobility up to 350 km/hr

Question 24

What is the eNodeB?

Answer 24

It is the hardware that is connected to the mobile phone network that communicates directly wirelessly with mobile handsets.

Question 25

Give details of OFDMA on: 1. Duplexing types 2. radio frames 3. Supported modulation schemes 4, Maximum information block size 5. Which type of error detection.

Answer 25

1. FDD and TDD 2. 10 ms radio frames; 20 slots of 0.5ms 3. QPSK, 16-QAM, 64-QAM 4. 6144 bits 5. cyclic redundancy check

Question 26

The basic OFDM LTE downlink physical resource can be seen as a time-frequency grid. What is the: 1. The subcarrier spacing. 2. Symbol duration time. 3. Cyclic prefix and its role

Answer 26

1. 15 kHz 2. Symbol duration time: 1\ss + CP 3. Cyclic prefix is used to avoid ISI and allows use of one-tap equaliser.