Problem 1, LTE 3GPP (20%) During this semester, some of the techniques we studie

Question

Problem 1, LTE 3GPP (20%) During this semester, some of the techniques we studied were proposed and successfully adopted in 4G mobile broadband (a) (6%) In 4G LTE down link which multiple access scheme is used? Which multiple access scheme is used in up-link? why different mutiple access schemes are used in down-link and up-link? (b) (8%) We know the subcarrier bandwidth is 15 KHz, so for 5MHz system, what is the FFT size? Short cyclic shift is 47 s, how to implement it? Long cyclic shift is 16.7 us, how to implement it? At what circumstance shall we use long cyclic shift? Modulation and Coding Scheme (MCS)? If the receiver cannot decode the data packet correctly, does it discard the packet immediately? Why? (c) (6%) We know Adaptive Modulation and Coding (ANC) is used in 4G what information does eNodeB need from UE to decide

Explanation / Answer

A) Modulation scheme in Uplink- SCFDMA

Modulation scheme in Downlink- OFDM

OFDM is used owing to its advantages:

1) Reduced ISI

2) Achieving high data rates

But the main disadvantage of OFDM is it's high peak to average power ratio. This is not a problem for the base station but is a big problem for the mobile.SCFDMA offers the same benefits as OFDM with low peak to average power ratio. It is for this reason why different Modulation schemes are implemented in Uplink and Downlink.

B)The FFT size for a 5MHz system is 512.It is as per the 3GPP specifications.

C)The eNodeB receives the channel state information from the UE.Based on this information the most efficient MCS is implemented to the user.If the receiver could not decode the data it asks for retransmission by sending a negative acknowledgement. It does not discard the packet. Instead it keeps the erroneous packet and also asks for retransmission implementing HARQ process for faster decoding.

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