Ronald Y. Chang

A Graph Approach to Dynamic Fractional Frequency Reuse (FFR) in Multi-Cell OFDMA Networks

A graph-based framework for dynamic fractional frequency reuse (FFR) in multi-cell OFDMA networks is proposed in this work. FFR is a promising resource allocation technique that can effectively mitigate inter-cell interference (ICI) in OFDMA networks. The proposed scheme enhances the conventional FFR by enabling adaptive spectral sharing per cell load conditions. Such adaptation has significant benefits in a practical environment where traffic load in different cells may be asymmetric and time-varying. The dynamic feature is accomplished via a graph approach in which the resource allocation problem is translated to a graph coloring problem. Specifically, in order to incorporate various versions of FFR in our framework, we construct a graph that matches the specific version of FFR and then color the graph using the corresponding graph algorithm. The performance improvement enabled by the proposed dynamic FFR scheme is further demonstrated by computer simulation for a 19-cell network with asymmetric cell load. For instance, the proposed dynamic FFR scheme can achieve a 12% and 33% gain in cell throughput and service rate over conventional FFR, and render a 70% and 107% gain in cell throughput and service rate with respect to the reuse-3 system.

Multicell OFDMA Downlink Resource Allocation Using a Graphic Framework

A novel practical low-complexity multicell orthogonal frequency-division multiple access (OFDMA) downlink channel-assignment method that uses a graphic framework is proposed in this paper. Our solution consists of two phases: 1) a coarse-scale intercell interference (ICI) management scheme and 2) a fine-scale channel-aware resource-allocation scheme. In the first phase, state-of-the-art ICI management techniques such as ICI coordination (ICIC) and base-station cooperation (BSC) are incorporated in our framework. In particular, the ICI information is acquired through inference from the diversity set of mobile stations and is presented by an interference graph. Then, ICIC or BSC is mapped to the MAX k-CUT problem in graph theory and is solved in the first phase. In the second phase, channel assignment is accomplished by taking instantaneous channel conditions into account. Heuristic algorithms are proposed to efficiently solve both phases of the problem. Extensive simulation is conducted for various practical scenarios to demonstrate the superior performance of the proposed solution compared with the conventional OFDMA allocation scheme. The proposed scheme can be used in next-generation cellular systems such as the 3GPP Long-Term Evolution and IEEE 802.16 m.

New Space Shift Keying Modulation with Hamming Code-Aided Constellation Design

A modulation scheme that maps the information onto the antenna indices, such as space shift keying (SSK) and its generalized form (namely, generalized SSK or GSSK), presents an attractive option for the emerging large-scale MIMO system due to the reduced algorithm and hardware cost. In this letter, we present a new modulation scheme in this category, where we propose use of the Hamming code construction technique to systematically design the constellation. An illustrative example and experimental studies demonstrate that the proposed scheme introduces rich design flexibility and achieves better transmission rate, performance, and power tradeoffs with comparable hardware costs as compared with existing schemes.

Energy Efficient Transmission over Space Shift Keying Modulated MIMO Channels

Energy-efficient communication using a class of spatial modulation (SM) that encodes the source information entirely in the antenna indices is considered in this paper. The energy-efficient modulation design is formulated as a convex optimization problem, where minimum achievable average symbol power consumption is derived with rate, performance, and hardware constraints. The theoretical result bounds any modulation scheme of this class, and encompasses the existing space shift keying (SSK), generalized SSK (GSSK), and Hamming code-aided SSK (HSSK) schemes as special cases. The theoretical optimum is achieved by the proposed practical energy-efficient HSSK (EE-HSSK) scheme that incorporates a novel use of the Hamming code and Huffman code techniques in the alphabet and bit-mapping designs. Experimental studies demonstrate that EE-HSSK significantly outperforms existing schemes in achieving near-optimal energy efficiency. An analytical exposition of key properties of the existing GSSK (including SSK) modulation that motivates a fundamental consideration for the proposed energy-efficient modulation design is also provided.

Symbol and Bit Mapping Optimization for Physical-Layer Network Coding with Pulse Amplitude Modulation

In this paper, we consider a two-way relay network in which two users exchange messages through a single relay using a physical-layer network coding (PNC) based protocol. The protocol comprises two phases of communication. In the multiple access (MA) phase, two users transmit their modulated signals concurrently to the relay, and in the broadcast (BC) phase, the relay broadcasts a network-coded (denoised) signal to both users. Nonbinary and binary network codes are considered for uniform and nonuniform pulse amplitude modulation (PAM) adopted in the MA phase, respectively. We examine the effect of different choices of symbol mapping (i.e., mapping from the denoised signal to the modulation symbols at the relay) and bit mapping (i.e., mapping from the modulation symbols to the source bits at the user) on the system error-rate performance. A general optimization framework is proposed to determine the optimal symbol/bit mappings with joint consideration of noisy transmissions in both communication phases. Complexity-reduction techniques are developed for solving the optimization problems. It is shown that the optimal symbol/bit mappings depend on the signal-to-noise ratio (SNR) of the channel and the modulation scheme. A general strategy for choosing good symbol/bit mappings is also presented based on a high-SNR analysis, which suggests using a symbol mapping that aligns the error patterns in both communication phases and Gray and binary bit mappings for uniform and nonuniform PAM, respectively.

Network Selection in Cognitive Heterogeneous Networks Using Stochastic Learning

Coexistence of multiple radio access technologies (RATs) is a promising paradigm to improve spectral efficiency. This letter presents a game-theoretic network selection scheme in a cognitive heterogeneous networking environment with time-varying channel availability. We formulate the network selection problem as a noncooperative game with secondary users (SUs) as the players, and show that the game is an ordinal potential game (OPG). A decentralized, stochastic learning-based algorithm is proposed where each SUs strategy progressively evolves toward the Nash equilibrium (NE) based on its own action-reward history, without the need to know actions in other SUs. The convergence properties of the proposed algorithm toward an NE point are theoretically and numerically verified. The proposed algorithm demonstrates good throughput and fairness performances in various network scenarios.

Bi-Space Shift Keying Modulation for MIMO Systems

In this letter, we propose a new Space Shift Keying (SSK)-based modulation that maps the information onto the antenna indices. The proposed Bi-Space Shift Keying (BiSSK) scheme extends SSK by employing a dual set of antenna indices, one associated with a real number and the other with an imaginary number. The proposed BiSSK preserves the advantages of low receiver complexity of SSK and yet doubles its transmission rates, with negligible performance penalty. The proposed BiSSK outperforms Generalized Space Shift Keying (GSSK) at the same given target transmission rate, as demonstrated by both error-probability analyses and simulations.

Dynamic fractional frequency reuse (D-FFR) for multicell OFDMA networks using a graph framework†

A graph-based framework is proposed in this paper to implement dynamic fractional frequency reuse (D-FFR) in a multicell Orthogonal Frequency Division Multiple Access (OFDMA) network. FFR is a promising resource-allocation technique that can effectively mitigate intercell interference (ICI) in OFDMA networks. The proposed D-FFR scheme enhances the conventional FFR by enabling adaptive spectral sharing as per cell-load conditions. Such adaptation has significant benefits in practical systems where traffic loads in different cells are usually unequal and time-varying. The dynamic adaptation is accomplished via a graph framework in which the resource-allocation problem is solved in two phases: (1) constructing an interference graph that matches the specific realization of FFR and the network topology and (2) coloring the graph by use of a heuristic algorithm. Various realizations of FFR can easily be incorporated in the framework by manipulating the first phase. The performance improvement enabled by the proposed D-FFR scheme is demonstrated by computer simulation for a 19-cell network with equal and unequal cell loads. In the unequal-load scenario, the proposed D-FFR scheme offers significant performance improvement in terms of cell throughput and service rate as compared to conventional FFR and previous interference management schemes. Copyright

Detection of space shift keying signaling in large MIMO systems

The detection problem of the space shift keying (SSK) signaling and its generalized form (namely, generalized SSK or GSSK) in the emerging large-scale multiple-input multiple-output (MIMO) systems is discussed in this paper. First, we explicitly formulate the tree search and column search detection schemes achieving optimal maximum likelihood (ML) performance, and discuss their pros and cons in the context of large MIMO systems where the size of the GSSK modulation alphabet increases significantly. Secondly, we propose two useful suboptimal detection methods for large MIMO systems and large-alphabet GSSK signaling based on convex relaxation, which induce an approximately 2-4 dB performance penalty as shown through experimental results.

Mandarin tone and vowel recognition in cochlear implant users: Effects of talker variability and bimodal hearing

Objectives: For cochlear implant (CI) users with residual low-frequency acoustic hearing in the nonimplanted ear, bimodal hearing combining the use of a CI and a contralateral hearing aid (HA) may provide more salient talker voice cues than CI alone to handle the variability of talker identity across trials. This study tested the effects of talker variability, bimodal hearing, and their interaction on response accuracy and time of CI users’ Mandarin tone, vowel, and syllable recognition (i.e., combined Mandarin tone and vowel recognition in this study). Design: Fifteen prelingually deafened native Mandarin-speaking CI users (at age 20 or lower) participated in this study. Four talkers each produced six Mandarin single-vowel syllables in four lexical tones. The stimuli were presented in quiet via a single loudspeaker. To study the effects of talker variability, Mandarin tone, vowel, and syllable recognition was tested in two presentation conditions: with stimuli blocked according to talker (blocked-talker condition) or mixed across talkers from trial to trial (mixed-talker condition). To explore the effects of bimodal hearing, two processor conditions were tested: CI alone or CI + HA. The cumulative response time was recorded as an indirect indicator of the cognitive load or listening effort in each condition. The correlations were computed between demographic/hearing factors (e.g., hearing thresholds in the nonimplanted ear) and bimodal performance/benefits (where bimodal benefits refer to the performance differences between CI alone and CI + HA). Results: Mandarin tone recognition with both CI alone and CI + HA was significantly poorer in the mixed-talker condition than in the blocked-talker condition, while vowel recognition was comparable in the two presentation conditions. Bimodal hearing significantly improved Mandarin tone recognition but not vowel recognition. Mandarin syllable recognition was significantly affected by both talker variability and bimodal hearing. The cumulative response time significantly reduced with CI + HA compared with CI alone, but remained invariant with respect to talker variability. There was no interaction between talker variability and bimodal hearing for any performance measure adopted in this study. Correlation analyses revealed that the bimodal performance and benefits in Mandarin tone, vowel, and syllable recognition could not be predicted by the hearing thresholds in the nonimplanted ear or by the demographic factors of the participants. Conclusions: Talker variability from trial to trial significantly degraded Mandarin tone and syllable recognition performance in both the CI alone and CI + HA conditions. While bimodal hearing did not reduce the talker variability effects on Mandarin tone and syllable recognition, generally better Mandarin tone and syllable recognition performance with shorter response time (an indicator of less listening effort) was observed when a contralateral HA was used in conjunction with the CI. On the other hand, vowel recognition was not significantly affected by either talker variability or bimodal hearing, because ceiling effects could not be counted out of the vowel recognition results.