NM-1.03 Flashcards
What is latency
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.
Explain spectral flexibility
- Ability of a system or device to operate across a range of frequencies or wavelengths within the electromagnetic spectrum.
- Capacity of comm. systems to adapt to difference frequency bands or channels.
.. operation in a wide range of spectrum and in different-sized bands
Describe the protocal layers
- Physical Layer
- Handles communication impairments
- Responsible for modulation, frequency selection, signal detection
- Encryption - Medium Access Control
- Responsible for mediating access to wireless medium - Radio link control
- Handover between different networks - Packet data convergence protocol
- Reliable end-to-end tx
- Flow and congestion
- Error recovery
Explain the concept of scheduling
A scheduler is a system component in charge of allocating over time the radio resources needed to transmit to the wireless users
How does OFDMA work?
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.
Explain Multi-Antenna Techniques
- Transmit diversity
- Beam-forming
- Spatial multiplexing
- Multi-user MIMO
Explain some goals of LTE Advanced
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
What is mobile broadband?
When you use the internet at home.
What is the issue of carrier aggregation evolution?
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.
What benefits do we get from LTE
- Connect the IOT
- New ways to connect and interact -> vehicle communication
- Latency-ctritical control
For which of the following devices the battery lifetime is not a metric of major interest?
- Wireless sensors
- Mobile phones
- Cellular base stations
Base stations as it’s connected to the grid
What is round robin?
Schedules users cyclically i.e. schedules users independently of their channel conditions.
Give the equation for the Proportional Fair schedule and what happens to r_n
n_max = argmax{d(t)/rn(t)}
Explain the max C/I
Max throughput;
How can latency be measured?
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
What are the LTE features
- OFDM/OFDMA
- All-IP architecture; simplified network management
- Spectrum flexibility
Why does the broadcast channel use QPSK?
It uses QPSK as since it only transmits two bits per time, it’s a lot less error-prone than modulation schemes using more.
Where is the scheduler located?
The scheduler is in the eNodeB
Explain the PHY layer in detail
- OFDMA with cyclic prefix in downlink
- SC-FDMA with cyclic prefix in uplink
- 10 ms radio frames, containing 20 slots of 0.5ms duration
- Maximum information block size: 6144 bits
- CRC-24 for error detection
In scheduling, slots consist of either 6 or 7 OFDM symbols. What are longer cyclic prefixes are used for?
Longer cyclic prefixes are used to address longer multipath fading channel length.
In OFDM what does the number of available subcarrier change depend on?
Number of available subcarriers changes depending on transmission bandwidth
For which is the following devices is the battery lifetime not a metric of major interest?
- Wireless sensors
- Mobile phones
- Cellular base stations
Base stations; connected by cable
Which of these access
techniques is the one adopted in
4th generation cellular systems
(LTE/LTE-A)?
- TDMA
- OFDMA
- CDMA
OFDMA
TDMA -> 2G
CDMA -> 3G