Modulation-Mode Assignment in SVD-Aided Downlink Multiuser MIMO-OFDM Systems

Multicarrier transmission such as OFDM (orthogonal frequency division multiplexing) is an established technique for radio transmission systems and it can be considered as a promising approach for next generation wireless systems. However, in order to comply with the demand on increasing available data rates in particular in wireless technologies, systems with multiple transmit and receive antennas, also called MIMO (multiple-input multiple-output) systems, have become indispensable for future generations of wireless systems. Due to the strongly increasing demand in high-data rate transmission systems, frequency non-selective MIMO links have reached a state of maturity and frequency selective MIMO links are in the focus of interest. In this field, the combination of MIMO transmission and OFDM can be considered as an essential part of fulfilling the requirements of future generations of wireless systems. However, single-user scenarios have reached a state of maturity. By contrast multiple users' scenarios require substantial further research, where in comparison to ZF (zero-forcing) multiuser transmission techniques, the individual user's channel characteristics are taken into consideration in this contribution. The performed joint optimization of the number of activated MIMO layers and the number of transmitted bits per subcarrier shows that not necessarily all user-specific MIMO layers per subcarrier have to be activated in order to minimize the overall BER under the constraint of a given fixed data throughput

Turbo coding performance in OFDM packet transmissions

Abstract: In transmission of the packets using OFDM (Orthogonal Frequency Division Multiplexing) system recommended by IEEE.802.11 [1] there is a constraint on the interleaving depth due to the maximum allowable packet size and processing delay requirement. This results in a non-ideal interleaving which effectively limits the maximum achievable diversity from the channel. In such a channel, fading occurs in block wise manner, i.e., all transmitted symbols of the same block sense the same fade. Also there is considerable correlation among the fade blocks. We call such a channel as Correlated Block Fading (CBF) channel. With the excellent performance of turbo code over AWGN, and ideally interleaved fading channels in mind, we tried to get similarly good performance by using turbo code in OFDM packet transmission. We observed that the system severely suffers from the block fading behavior of the channel. A performance analysis is presented for CBF systems considering the practical case of channel estimation error. Simulations for the OFDM system using rate 1/2 convolutional code and turbo code are performed. While the turbo code has not any gain over the convolutional code with K=7 in BER (Bit Error Rate) performance, it brings about 1.3 dB gain in PER (Packet Error Rate) performance which is more important in packet transmission