Ti-wen Tang

Bio: Ti-wen Tang is an academic researcher from National Chiao Tung University. The author has contributed to research in topics: MIMO & Decoding methods. The author has an hindex of 2, co-authored 4 publications receiving 16 citations.

Improving the DMT Performance for MIMO Communication With Linear Receivers

Abstract: Multiple-input-multiple-output (MIMO) linear receivers are often of more practical interest than maximum-likelihood (ML) receivers due to their low decoding complexity but at the cost of worse diversity gain performance. Such a statement on performance loss is due to the assumption of using an independent identically distributed complex Gaussian vector as channel input. By removing this assumption, we find that the diversity performance of MIMO linear receivers can be significantly improved. In an extreme case, it can be the same as that of ML receivers. Specifically, in this paper, we investigate the diversity-multiplexing tradeoff (DMT) performance of MIMO linear receivers with colored and possibly degenerate Gaussian channel inputs. By varying the rank of the covariance matrix of the channel input vector and by allowing temporal coding across multiple channel uses, we show that the MIMO linear receiver can achieve a much better DMT performance than the currently known one. Explicit optimal code constructions are provided, along with simulation results, to justify the above findings. For the case of (2 × 2) and (3 × 3) MIMO linear receivers, simulation results show that the newly proposed codes provide significant gains of 10 and 12.08 dB in Eb/N0 at bit error rate 10-4 compared to the conventional schemes, respectively.

Selection and Rate-Adaptation Schemes for MIMO Multiple-Access Channels With Low-Rate Channel Feedback

Abstract: In this paper, two selection schemes are proposed for coded transmission over multiple-input multiple-output (MIMO) multiple-access channels (MAC) to yield a much higher diversity-multiplexing gain tradeoff (DMT) performance. These schemes require a channel feedback, but at an extremely low rate. The first scheme is based on user selection and can be easily implemented in the existing MIMO-MAC systems. Upper bounds on the minimal computational complexity required by sphere decoders to decode DMT-optimal codes for this scheme as well as for MIMO MAC without feedback are given. It is shown that this scheme can offer both a much larger DMT and an exponential reduction on decoding complexity, compared with the latter. The second scheme selects jointly the users and their transmit antennas. It requires an additional design of rate assignments for performance optimization. A very general framework on the design of optimal rate assignments is thus provided. It is shown that this scheme can yield DMT performances far superior to the optimal MIMO-MAC DMT without channel feedback. The simulation results confirm that in some cases, this scheme can provide an astonishing SNR gain of 14.64 dB at outage probability $10^{-6}$ compared with the optimal MIMO-MAC coding schemes without feedback.

Coding schemes with constant sphere-decoding complexity and high DMT performance for MIMO multiple access channels with low-rate feedback

Abstract: In a MIMO MAC where the base station has fewer receive antennas than the transmit antennas of all users, sphere (lattice) decoding for the existing MIMO-MAC codes requires an exhaustive search of exponentially large size before processing the root of a sphere-decoding tree. In this paper, two coding schemes are proposed and are shown to yield a constant sphere-decoding complexity, independent of the numbers of users and transmit antennas. The schemes require a channel feedback, but only at an extremely low rate. The first scheme is based on user selection, and the second scheme selects jointly users and transmit antennas, using a fast antenna selection algorithm recently proposed by Jiang and Varanasi. It also involves a design of rate-assignments that maximizes the overall DMT performance. It is shown that both schemes yield DMT performances far superior to the optimal MIMO-MAC DMT without channel feedback in certain multiplexing gain regime. Simulation results confirm that in some cases the second proposed scheme can provide an astonishing SNR gain of 14:5 dB at outage probability 10-6 compared to the optimal coding schemes without feedback.

Improving the DMT performances of MIMO linear receivers

Abstract: In this paper, we investigate the diversity multiplexing tradeoff (DMT) performance of MIMO linear receivers with general colored Gaussian input. By varying the rank of the covariance matrix of the channel input and allowing temporal coding across multiple channel uses, we find the DMT performance of MIMO linear receivers can be significantly improved and be much better than the currently known.

Approximately Universal Codes over Slow Fading Channels

Abstract: Performance of reliable communication over a coherent slow fading channel at high SNR is succinctly captured as a fundamental tradeoff between diversity and multiplexing gains. We study the problem of designing codes that optimally tradeoff the diversity and multiplexing gains. Our main contribution is a precise characterization of codes that are universally tradeoff-optimal, i.e., they optimally tradeoff the diversity and multiplexing gains for every statistical characterization of the fading channel. We denote this characterization as one of approximate universality where the approximation is in the connection between error probability and outage capacity with diversity and multiplexing gains, respectively. The characterization of approximate universality is then used to construct new coding schemes as well as to show optimality of several schemes proposed in the space-time coding literature.

Stackelberg Game for Cognitive Radio Networks With MIMO and Distributed Interference Alignment

Abstract: In this paper, we investigate the problem of spectrum leasing in cognitive radio (CR) networks, while incorporating two advanced physical-layer technologies, i.e., multiple-input multiple-output (MIMO) and distributed interference alignment. We present a cooperative spectrum leasing scheme for primary and secondary users to balance the tension between data transmission and revenue collection/payment. A Stackelberg game is formulated, where the primary user is the leader, and secondary users are followers. With backward induction, we derive the optimal strategies for primary and secondary users that can achieve the unique Stackelberg equilibrium, where no player can gain by a unilaterally changing strategy. We find that spectrum leasing is always beneficial to enhancing the utilities of primary and secondary users. The proposed scheme outperforms a no-spectrum-leasing scheme and a cooperative scheme presented in the literature with considerable gains, which demonstrate the benefits of spectrum leasing and distributed interference alignment and validates the efficacy of the proposed scheme.

Selection and Rate-Adaptation Schemes for MIMO Multiple-Access Channels With Low-Rate Channel Feedback

Abstract: In this paper, two selection schemes are proposed for coded transmission over multiple-input multiple-output (MIMO) multiple-access channels (MAC) to yield a much higher diversity-multiplexing gain tradeoff (DMT) performance. These schemes require a channel feedback, but at an extremely low rate. The first scheme is based on user selection and can be easily implemented in the existing MIMO-MAC systems. Upper bounds on the minimal computational complexity required by sphere decoders to decode DMT-optimal codes for this scheme as well as for MIMO MAC without feedback are given. It is shown that this scheme can offer both a much larger DMT and an exponential reduction on decoding complexity, compared with the latter. The second scheme selects jointly the users and their transmit antennas. It requires an additional design of rate assignments for performance optimization. A very general framework on the design of optimal rate assignments is thus provided. It is shown that this scheme can yield DMT performances far superior to the optimal MIMO-MAC DMT without channel feedback. The simulation results confirm that in some cases, this scheme can provide an astonishing SNR gain of 14.64 dB at outage probability $10^{-6}$ compared with the optimal MIMO-MAC coding schemes without feedback.

Optimal Distributed Codes for Feedback-Aided Cooperative Relay Networks

Abstract: A novel transmission scheme for cooperative relay networks is presented in this paper. The proposed scheme is based on the non-orthogonal selection decode-and-forward protocol with an additional assumption of having a low rate feedback channel from the destination to relays. Benefited from the feedback information, an optimal distributed code that has an extremely short delay equal to four is constructed, and the same code is applicable to networks with the arbitrary number of relays to yield optimal cooperative diversity. The proposed code is sphere decodable with a decoding complexity again independent of the number of relays in high SNR regime. In particular, when operating at multiplexing gain ≥(1/2), the lattice decoder at the destination has a zero complexity exponent, meaning a constant decoding complexity and independent of transmission rate. Analyses for the decoding complexity of other existing diversity-optimal distributed codes are also provided. It is shown that these codes have a linear growth in delay and an exponential growth in decoding complexity as the number of relays increases.

Diversity-Multiplexing Tradeoff in an Interweave Multiuser Cognitive Radio System

Abstract: In an interweave cognitive radio system, the spectrum sensing performance affects the rate and reliability of data transmission. In wireless communications, the tradeoff between the reliability and data rate can be evaluated by diversity-multiplexing tradeoff (DMT) analysis. In this paper, through the analysis of the error probability at the secondary receiver, we derive the DMT for the multiple-access channel and user-selection schemes in an interweave multiuser cognitive radio system considering the spectrum sensing effect on the rate and reliability. We show that the DMT of the system can be improved by cooperative spectrum sensing. Simulations also approve our analytical results.