An OFDM system employs 260 data subcarriers, 40 pilot subcarriers, and 34 guard

Question

An OFDM system employs 260 data subcarriers, 40 pilot subcarriers, and 34 guard
subcarriers to transmit data through a wireless channel. QPSK modulation is employed for
each subcarrier.

a)Determine the system bandwidth if the subcarrier frequency spacing is 15 KHz.

b)To combat multipath, a cyclic prefix of 1/4 of the OFDM symbol is employed. Find the
total transmission time of an OFDM symbol and calculate the data rate of the system.

c)If the delay spread of the wireless channel is 20 ?s, find the maximum data rate that can
be supported by the system.

Employing a cyclic prefix of 4.7 ?s, is it possible to maintain the data rate achieved in b)
while keeping the same number of pilot and guard subcarriers, the same system
bandwidth, but with BPSK modulation on each subcarrier? Show your calculations.

Explanation / Answer

contrast to the classical cyclic prefix (CP)-OFDM, the time domain synchronous (TDS)-OFDM employs a known pseudo noise (PN) sequence as guard interval (GI). Conventional channel estimation methods for TDS-OFDM are based on the exploitation of the PN sequence and consequently suffer from intersymbol interference (ISI). This paper proposes a novel data-aided channel estimation method which combines the channel estimates obtained from the PN sequence and, most importantly, additional channel estimates extracted from OFDM data symbols. Data-aided channel estimation is carried out using the rebuilt OFDM data symbols as virtual training sequences. In contrast to the classical turbo channel estimation, interleaving and decoding functions are not included in the feedback loop when rebuilding OFDM data symbols thereby reducing the complexity. Several improved techniques are proposed to refine the data-aided channel estimates, namely one-dimensional (1-D)/two-dimensional (2-D) moving average and Wiener filtering. Finally, the MMSE criteria are used to obtain the best combination results and an iterative process is proposed to progressively refine the estimation. Both MSE and BER simulations using specifications of the DTMB system are carried out to prove the effectiveness of the proposed algorithm even in very harsh channel conditions such as in the single frequency network (SFN) case.

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