Haewoon Nam

Wideband channel sounder with measurements and model for the 60 GHz indoor radio channel

A wideband channel sounder and measurement results for the short range indoor 60 GHz channel are presented. The channel sounder is based on a 1 gigasamples/s dual channel arbitrary waveform generator and A/D converter/software demodulator, which synthesize and detect a baseband PN sequence with 500 MHz bandwidth. A heterodyne transmitter and receiver translate the baseband PN sequence to and from the 60 GHz band. Ten channel measurements taken across the 59 GHz to 64 GHz range are concatenated to provide a continuous channel measurement covering 5 GHz of bandwidth, resulting in 0.2 ns time domain channel impulse response resolution. The dynamic range and maximum sensitivity performance of the channel sounder are discussed in detail. Comparisons of results with a vector network analyzer based system are shown to verify the accuracy of the sounder. In an extensive measurement campaign with vertically polarized omnidirectional antennas, several different rooms (offices, labs, conference rooms and others) in four different buildings have been investigated. Over 700 channel measurements are the basis for a comprehensive characterization of the short range 60 GHz indoor radio channel with omnidirectional antennas. Finally, a simple stochastic static multipath channel model is derived from the measurement results.

Low-Complexity PAPR Reduction Scheme Without Side Information for OFDM Systems

This paper proposes a novel peak to average power ratio (PAPR) reduction scheme that requires no side information in orthogonal frequency division multiplexing (OFDM) systems. Unlike the selective mapping (SLM) or the partial transmit sequence (PTS) scheme, the proposed scheme deals with post inverse fast Fourier transform (IFFT) symbols and thus requires only a single IFFT processor in the transmitter. Compared to other conventional schemes implemented with a single IFFT processor, such as the circularly shifted phase sequences (CSPS) or the optimised circularly shifted phase sequences (OCSPS) method, the proposed scheme achieves an even lower complexity since only phase rotation and cyclic shifting of OFDM symbols are performed. More importantly, the proposed scheme significantly outperforms the CSPS and the OCSPS methods in reducing PAPR as shown in simulation results. An added benefit of the proposed scheme is that it employs a linear receiver, such as a maximal likelihood (ML) detector, a minimum mean square error (MMSE) estimator, or a zero forcing (ZF) estimator, to demap quadrature amplitude modulation (QAM) symbols. Especially the ML detector demaps the QAM symbols with no side information. Simulation results also show that the bit error rate (BER) of the proposed scheme has no loss when the ML detector, the ZF or the MMSE estimator is used with hard-decision compared to that of the conventional OFDM system without any PAPR reduction scheme over Rayleigh fading channel.

Performance analysis of joint switched diversity and adaptive modulation

In this paper, a simple and practical system based on a switched diversity scheme with adaptive modulation is presented. This system provides a reduced number of channel estimation while offering the optimum spectral efficiency given by a selection diversity system. In addition, the switching threshold is easily manipulated so as to make an efficient use of the tradeoff between spectral efficiency and channel estimation overhead. An extension of switched diversity into a multiuser scheduling is later also considered. This switch-based multiuser access scheme results in a lower average feedback load than that for the optimal selection-based multiuser scheme. Numerical results show that we can obtain a trade-off between spectral efficiency and the feedback load by choosing the switching threshold appropriately.

A high throughput FFT processor with no multipliers

A novel technique for implementing very high speed FFTs based on unrolled CORDIC structures is proposed in this paper. There has been a lot of research in the area of FFT algorithm implementation; most of the research is focused on reduction of the computational complexity by selection and efficient decomposition of the FFT algorithm. However there has not been much research on using the CORDIC structures for FFT implementations, especially for large, high speed and high throughput FFT transforms, due to the recursive nature of the CORDIC algorithms. The key ideas in this paper are replacing the sine and cosine twiddle factors in the conventional FFT architecture by non-iterative CORDIC micro-rotations which allow substantial (~ 50%) reduction in read-only memory (ROM) table size, and total removal of complex multipliers. A new method to derive the optimal unrolling/unfolding factor for a desired FFT application based on the MSE (mean square error) is also proposed in this paper. Implemented on a Virtex-4 FPGA, the CORDIC based FFT runs 3.9 times faster and occupies 37% less area than an equivalent complex multiplier-based FFT implementation.

Location-based resource allocation for OFDMA cognitive radio systems

In cognitive radio systems, in order for the secondary users to opportunistically share the spectrum without interfering the primary users, an accurate spectrum measurement and a precise estimation of the interference at the primary users are necessary but are challenging tasks. Since it is impractical in cognitive radio systems to assume that the channel state information of the interference link is available at the cognitive transmitter, the interference at the primary users is hard to be estimated accurately. This paper introduces a resource allocation algorithm for OFDMA-based cognitive radio systems, which utilizes location information of the primary and secondary users instead of the channel state information of the interference link. Simulation results show that it is indeed effective to incorporate location information into resource allocation so that a near-optimal capacity is achieved.

Multiuser switched diversity scheduling systems with per-user threshold

A multiuser switched diversity scheduling scheme with per-user feedback threshold is proposed and analyzed in this paper. The conventional multiuser switched diversity scheduling scheme uses a single feedback threshold for every user, where the threshold is a function of the average signal-to-noise ratios (SNRs) of the users as well as the number of users involved in the scheduling process. The proposed scheme, however, constructs a sequence of feedback thresholds instead of a single feedback threshold such that each user compares its channel quality with the corresponding feedback threshold in the sequence. Numerical and simulation results show that thanks to the flexibility of threshold selection, where a potentially different threshold can be used for each user, the proposed scheme provides a higher system capacity than that for the conventional scheme.

Spectral Efficiency Enhancement in Multi-Channel Systems Using Redundant Transmission and Diversity Reception

We first consider conventional multi-channel systems with fixed-power variable-rate transmission of independent data streams as a function of channel variations. In these conventional systems, no data is transmitted through the sub-channels whose received signal strength is below a certain cutoff threshold. We then propose new hybrid adaptive modulation and diversity schemes for multi-channel transmission systems. The proposed hybrid schemes attempt to increase the overall spectral efficiency by (i) transmitting redundant information symbols through the deeply faded sub-channels that are not to be used in conventional multi-channel systems and then (ii) diversity combining these replicas at the receiver end. We evaluate the performance of the newly proposed hybrid schemes and compare their spectral efficiency with that of (i) the conventional scheme and (ii) a benchmark scheme which maximizes the overall spectral efficiency but is too complex for practical implementation. We show using some selected numerical examples that the proposed hybrid schemes offer spectral efficiency very close to that of the optimal scheme with much reduced complexity.

Robust Multidimensional Scaling for Cognitive Radio Network Localization

Localization of primary users (PUs) and secondary users (SUs) is one of the essential features of cognitive radio networks (CRNs). Given that there is no communication between PUs and SUs, localization of the whole network is a challenging task. In this paper, we propose a two-stage localization algorithm that combines multidimensional scaling (MDS) and Procrustes analysis for a CRN with proximity information. In the proposed algorithm, a hybrid-connectivity-and-estimated-distance-based strategy is introduced to get maximum benefit from the information available in the network. Simulations are included to compare the proposed algorithm with weighted centroid localization (WCL) in terms of the root-mean-square-error (RMSE) performance, as well as the Cramér-Rao lower bound (CRLB) for CRN localization. It is proved that the proposed algorithm outperforms the WCL solutions for the CRN localization problem.

Performance Analysis of OT-MRC Over I.I.D. Nakagami and Non-I.I.D. Rayleigh Fading Channels

This paper presents the performance analysis of a conditional diversity combining scheme, which is called output-threshold maximum ratio combining (OT-MRC), over the independent identically distributed Nakagami-m with integer m and independent but not necessarily identically distributed Rayleigh fading channels. Based on the closed-form expressions for the statistics, the authors analyze the performance of the OT-MRC systems and compare it with that of the conventional MRC systems. They exploit the tradeoff between complexity and performance in this scheme over the Nakagami fading channels and see the effect of unbalance of the fading gains over multiple diversity branches on the performance of OT-MRC. They show, from some numerical examples, that the OT-MRC scheme, when compared with the MRC scheme, can reduce the complexity significantly without losing the required performance

Fuzzy C-Means Clustering and Energy Efficient Cluster Head Selection for Cooperative Sensor Network

We propose a novel cluster based cooperative spectrum sensing algorithm to save the wastage of energy, in which clusters are formed using fuzzy c-means (FCM) clustering and a cluster head (CH) is selected based on a sensor’s location within each cluster, its location with respect to fusion center (FC), its signal-to-noise ratio (SNR) and its residual energy. The sensing information of a single sensor is not reliable enough due to shadowing and fading. To overcome these issues, cooperative spectrum sensing schemes were proposed to take advantage of spatial diversity. For cooperative spectrum sensing, all sensors sense the spectrum and report the sensed energy to FC for the final decision. However, it increases the energy consumption of the network when a large number of sensors need to cooperate; in addition to that, the efficiency of the network is also reduced. The proposed algorithm makes the cluster and selects the CHs such that very little amount of network energy is consumed and the highest efficiency of the network is achieved. Using the proposed algorithm maximum probability of detection under an imperfect channel is accomplished with minimum energy consumption as compared to conventional clustering schemes.