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Software Defined Radio (SDR) techniques are expected to be among the key technologies of heterogeneous cognitive radio networks for realizing efficient and convenient wireless communications by providing multiple radio services to users and decreasing development costs. In this paper, in order to evaluate the feasibility of SDR modems, we study the amount of computing throughput of a recent wireless system and determine a suitable modem architecture. Firstly, the functions for which SDR techniques provide significant benefits are clarified. Secondly, the computing throughputs are measured under the assumption that a dynamically reconfigurable processor, FlexSword™, is employed. Finally, based on a consideration of timing charts, we propose the architecture of an SDR-based modem with FlexSword. The possibility of implementing several wireless systems is also considered.

This paper proposes a preamble structure that raises the throughput of the physical layer of a MIMO-OFDM system that complies with IEEE 802.11n. The proposed preamble is based on two important criteria that have not been considered in conventional preamble structures but have an important bearing on realization of IEEE 802.11n. One is backward compatibility with legacy devices, such as IEEE 802. 11 a or IEEE 802.11g. The other is accurate AGC for MIMO data payload of the packet. Computer simulations and a laboratory test demonstrate that the proposed preamble is superior to the other preambles with respect to those two important criteria. The results were submitted to the task group N under the IEEE 802.11 working group and contributed to development of the current draft of IEEE 802.11n.

Vector coding (VC) is a kind of eigen mode transmission scheme which is typically considered in MIMO systems. In VC systems, several code channels corresponding to the eigenvalues of the channel matrix are created. However, any code channels with low eigenvalues will cause a degradation in performance. In this paper, adaptive modulation and coding (AMC) for a VC system is proposed. In addition to AMC, the number of code channels is adaptively changed by code channel elimination. We show that the BER performance of VC is better than that of MMSE. Secondly, we also show the throughput performance of the proposed scheme is improved compared with the maximum throughput of each individual MCS.

Channel estimation is a key baseband processing task in wireless systems. Filtering or smoothing algorithms can improve the accuracy of channel estimates and the Discrete Cosine Transform (DCT) can be used for this purpose. By using the DCT, performance will be improved compared to the straight-forward approach of per subcarrier estimation (PSE). However, the complexity of the DCT is not negligible. This paper proposes a low-complexity channel estimation scheme using the DCT. Simulation results show that the performance is improved by more than 1dB compared with PSE in MIMO-OFDM system.

Adaptive modulation and coding scheme (AMC) is an effective way to achieve high data rate communication. In AMC system, the key issue is to determine the rule for switching among different Modulation and Coding Schemes (MCSs). In this paper, adaptive threshold controlling scheme for AMC is proposed. The proposed scheme controls switching thresholds according to target block error rate. Simulation results have shown that the throughput performance of the proposed scheme is very close to the performance, which obtained by the optimum SIR thresholds. We also proposed downlink transmission power control (TPC) scheme suitable for AMC. The throughput of the lowest MCS is improved and the tranmission power of the highest MCS can be reduced with the proposed algorithm.