LTE Physical layer Interview Questions and Answers 1-10

Q01. Define Frame Structure?

Ans. To know all about lte frame structure click here and to know more about 5G Frame Structure click here.

Q02. How many Sub-frame present in single Frame-Structure, and size of each Sub-Frame in Time-domain?

Ans. In 1 Radio Frame (Single frame structure) have 10 Sub-frame, each sub-frame have equal size of 1ms in time-domain.

Q03. How many Time-Slot are present in Sub-Frame?

Ans. 2 time slot (Tslot) are present in 1 sub-frame.

Q04. What is Cyclic Prefix(CP) in Frame-Structure?

Ans. Cyclic Prefix are used to avoid interference of OFDMA symbols.

Q05. What is Sampling rate in LTE? What is the Sampling rate of Normal CP and Extended CP?

Ans. The sampling rate use in LTE:

LTE Bandwidth support	Sampling rate in lte	Sampling rate of Normal CP/Extended CP
1.4 Mhz	128	10/9
3 Mhz	256	20/18
5 Mhz	512	40/36
10 Mhz	1024	80/72
15 Mhz	2048Ts	160/144
20 Mhz	2048Ts	160/144

Q06. How many OFDM symbols are available in 1 Time slot in Normal CP and Extended CP? In Frequency-domain or Time-domain? in Sub-Frame and Frame?

Ans. Following Information are available in sub-frame, radio frame:

• In 1 Time slot 7 OFDM symbols in Normal CP and 6 OFDM in Extended CP symbols are available.
• In frequency domain 84RE Normal CP and 72 RE in Extended CP are available.
• In 1 sub-frame 14 OFDM symbols in Normal CP and 12 OFDMA in Extended CP symbols are present respectively.
• 1 Radio frame have 140 OFDMA symbols in Normal CP and 120 OFDMA in Extended CP symbols are present.

Q07. Difference between OFDMA and SC-FDMA? What is the reason that OFDMA using in downlink and SC-FDMA using in uplink direction?

Ans. In OFDM (Orthogonal Frequency Division Multiplexing) structures the unique bandwidth is subdivided into more than one sub-carriers. Each of this sub-carriers can them be in my opinion modulated. Typically in OFDM structures we are able to have loads of sub-carriers with a content spacing between them (15KHz on the LTE case). Since the multiple sub-carriers in OFDM are transmitted in parallel, it’s possible for every one to transmit with a lower image rate. That improves robustness at the technology for mobile propagation conditions.

      It is not possible to use OFDMA on the uplink because, as told before, it offers a high Peak-to-average Power Ratio. SC-FDMA (Singe Carrier FDMA) presents the advantage of a single service multiplexing of getting a decrease Peak-to-average Power Ratio. On SC-FDMA before making use of the IFFT the symbols are pre coded via a DFT (Discrete Fourier Transform). This manner each sub-carrier after de IFFT will contain part of each symbol. Looking to the determine bellow it's miles viable to look the distinction among SC-FDMA and OFDMS. Also, it's miles feasible to be aware that the inter symbol interference may be reduced for the reason that all sub-carriers on a time frame represent the identical symbol.

Q08. Define Sub-Carrier? How many Sub-Carrier are present in Resource Block(RB)?

Ans. Sub-carrier spacing is decided through a situation of orthogonality between the sub-carriers, which allows to decode every one without interference form its neighbors.

      I think if we note to general sub-carrier in distinct LTE Bandwidth we are able to identified the answer. 20MHz->1200subcarrier, 15MHz->900, 10MHZ->600, 5MHz->300, 3MHz->180, 1.4MHZ->72 (with 10% protect band for all), 12 is the greatest common divisor of them.

Q09. What is the Smallest unit of Sub-Carrier?

Ans. In normal or extended it’s 15 Khz. But in MBSFN operations it’s 7.5 Khz.

Q10. In extended CP. How many resource elements present in a Resource Block(RB)?

Ans. Total 72 Resorce Elements are present in single Resorce Block.