Question

a) How do you ensure orthogonality in the OFDM system? b) Explain how the OFDM has minimal Inter Symbol Interference (IST)? c) What is SCFDM vis a. vis. OFDM and which one is recommended for downlink (10 points) (10 points) (10 points) communication and why? What is Code Prefix why is it used and how does it affect the data rate transmission? d)

Answer 1

A.

Orthogonal Frequency Division Multiplexing (OFDM) is a digital multi-carrier modulation scheme that extends the concept of single subcarrier modulation by using multiple subcarriers within the same single channel. Rather than transmit a high-rate stream of data with a single subcarrier, OFDM makes use of a large number of closely spaced orthogonal subcarriers that are transmitted in parallel. Each subcarrier is modulated with a conventional digital modulation scheme (such as QPSK, 16QAM, etc.) at low symbol rate. However, the combination of many subcarriers enables data rates similar to conventional single-carrier modulation schemes within equivalent bandwidths.
The OFDM scheme differs from traditional FDM in the following interrelated ways:

1.   Multiple carriers (called subcarriers) carry the information stream,

2.   The subcarriers are orthogonal to each other, and

3.   A guard interval is added to each symbol to minimize the channel delay spread and intersymbol interference.

B.
Intersymbol interference (ISI) is a form of distortion of a signal in which one symbol interferes with subsequent symbols.
OFDM is multicarrier data transmission modulation scheme. In multipath data transmission ISI is introduces. Therefore, in
OFDM to separate the multisymbols guard bands are added. For reduction of ISI Cyclic prefix (CP) are introduced. CP uses

C.
SC-FDMA, or single-carrier frequency-division multiple access the users are allocted on both time and frequency domain where as in OFDM it is only frequency domain.
for the uplink SC_FDMA will be used and for downlink FDMA due to high peak to average power ratio.

D.
The speed with which data can be transmitted from one device to another is called data transfer rate.
A prefix code is a type of code system (typically a variable-length code) distinguished by its possession of the "prefix property", which requires that there is no whole code word in the system that is a prefix (initial segment) of any other code word in the system.