LTE Physical Layer Interview Questions and Answers

Q45. Explain why mobile cannot be connected to LTE in more than 300 km/hour speed. What is the limit?

Ans. Mobile devices cannot be connected to LTE at speeds greater than 300 km/hour due to the physical limitations of the LTE network and the mobile device. The LTE network uses Orthogonal Frequency Division Multiplexing (OFDM) technology to transmit data over multiple subcarriers. However, at higher speeds, the mobile device moves in and out of range of different cells rapidly, which causes a phenomenon called Doppler shift that distorts the signal and degrades the transmission quality. The Doppler shift becomes significant at speeds over 300 km/hour, making it difficult for the network to maintain a stable connection with the device. While enhancements such as Carrier Aggregation and MIMO can improve performance at higher speeds, there is still a limit to the speed at which a mobile device can be connected to LTE.

Q46. In the case of 4 antennas with 4x4 MIMO, how many maximum layers can be configured?

Ans. In 4x4 MIMO (Multiple-Input Multiple-Output) configuration with 4 antennas, the maximum number of layers that can be configured is 4. Each layer is a separate data stream that is transmitted and received independently on each antenna, resulting in a total of 4 streams in a 4x4 MIMO configuration. Each stream is modulated and demodulated separately, allowing for a higher data throughput and better spectral efficiency. The number of layers that can be configured in a MIMO system depends on the number of antennas and the number of streams that can be transmitted and received simultaneously. The more antennas and streams, the higher the capacity and performance of the system.

Q47. Define CFI, UCI, HI in LTE?

Ans. In LTE (Long-Term Evolution), CFI, UCI, and HI are control channels used for transmitting control information between the base station and the mobile device.
CFI (Control Format Indicator): is a downlink control channel that carries information about the number of OFDM symbols used for transmitting control information on the Physical Downlink Control Channel (PDCCH).
UCI (Uplink Control Information): is an uplink control channel that is used to transmit control information from the mobile device to the base station. It is used for sending feedback information, scheduling requests, and acknowledgement of received data.
HI (Hybrid ARQ Indicator): is a downlink control channel used to carry information about the status of the Hybrid Automatic Repeat Request (HARQ) process. It informs the mobile device whether the received data is error-free or needs retransmission.

In summary, CFI, UCI, and HI are important control channels in LTE that facilitate the transmission of control information between the base station and the mobile device.

Q48. Describe in detail about Preamble in LTE and which Layer initiate Preamble and why?

Ans. In LTE (Long-Term Evolution), a preamble is a special sequence of symbols used by the mobile device to synchronize with the base station and request access to the network. It is a part of the Random Access Procedure, which is used by the mobile device to initiate a connection with the network.

The preamble is generated by the Physical Random Access Channel (PRACH) module in the Physical Layer of the mobile device. It is then transmitted over the air to the base station. The base station uses the preamble to synchronize with the mobile device and allocate resources for further communication.

The preamble is initiated by the Medium Access Control (MAC) Layer in the mobile device when it needs to transmit data to the network. The MAC Layer determines the size of the data to be transmitted and selects a suitable preamble format based on the size of the data.

The use of preamble in LTE ensures that the mobile device can access the network efficiently and with minimal delay. It allows multiple mobile devices to access the network simultaneously without causing interference. The preamble-based Random Access Procedure is a key feature of the LTE system, providing efficient access to the network and enabling high-speed data transfer.

Q49. How many different Preamble id, Preamble Format, Zero co-relation Zone, Root Sequence, Frequency Offset are present and how they all are play an important role to generate Preamble id?

Ans. Preamble Format: There are 5 different Preamble Formats are present 0-4, TS 36.321 Table 7.6-1 each with a different length and structure.

Zero Correlation Zone (ZCZ): The ZCZ is a set of subcarriers where the preamble signal has zero correlation with other preambles. There are four different ZCZ configurations, each with a different number of subcarriers.

Root Sequence: The Root Sequence is a sequence of complex numbers that determine the phase of the preamble signal. There are 839 different Root Sequences available for use in LTE.

Frequency Offset: The Frequency Offset is the difference in frequency between the mobile device and the base station. It can be compensated for by adding a cyclic shift to the preamble signal.