This paper considers an uplink multiuser multiple-input-multiple-output (MU-MIMO) system. In this system, we have presented a sparse-aware minimum-mean-square-error (SA-MMSE) detector which improves an underlying linear detector using the sparsity of a residual error vector (difference from the transmit vector and the detected one by the linear detector). Despite its attractive performance, the conventional SA-MMSE detector is only available for 4-QAM systems. In this paper, we generalize the SA-MMSE detector for a higher-order modulation system in a non-trivial method. This is referred to as generalized SA-MMSE (GSA-MMSE) detector. The key idea of the proposed detector is to exploit the hierarchical structure of a residual error vector. To be specific, the residual error vector can be decomposed into orthogonal sub-error vectors and, leveraging the orthogonality, the sub-error vectors can be decoded using the corresponding SA-MMSE detector in a successive fashion. Via simulation results, we demonstrate that the GSA-MMSE detector significantly outperforms the conventional linear detectors with a comparable complexity.

In this paper, we deal with sharing of a resource block (RB) which is the basic resource allocation unit for scheduling in 3rd-generation partnership project long term evolution (3GPP-LTE) system. In 3GPP-LTE system, a scheduler of evolved node-B (eNB) allocates respectively one or more RBs to user-equipments (UEs) considered, considering their requests, their channel qualities, and the resource availability. In the case of constant-rate service (i.e., the real-time service), some of RBs may not be fully utilized, depending on the channel quality. Especially, in the case of multiple-input multiple-output (MIMO) multiplexing, the degree of resource wastage may become much severer since a much less amount of the resource may be required for the same data rate. In this paper, we introduce the concept of RB sharing where one RB can be shared by multiple UEs depending on their respective required rates and the corresponding channel qualities. Through computer simulations, we evaluate the average sum rates for both cases of RB sharing and non-sharing. From simulation results, we see that the RB sharing scheme can achieve much greater sum rate as compared to the non-sharing one.

We propose an efficient segment assignment strategy for the incremental frequency reuse (IFR) scheme, in which each cell within a cluster of adjoining cells has its own fundamental segment orthogonal to a maximal extent to those of other cells within the cluster, in order to control effectively inter-cell interference (ICI). First, the IFR scheme with maximally orthogonal fundamental segments (MOFS) (hereafter, called as the IFR-MOFS scheme) divides the entire radio spectrum into multiple segments, in which all the cells within the cluster follow the same segmentation rule. Then, it designates a unique MOFS for each cell, distinct from those of other cells within the cluster. Finally, a set of corresponding segment assignment sequences is defined to mitigate ICI within the cluster greatly through systematic segment allocation among adjoining cells. A system-level simulator for an orthogonal frequency division multiple access (OFDMA) cellular system covering surrounding cells up to 3rd-tier has been implemented. Simulation results show that the IFR-MOFS scheme provides quite bett er the cell-edge performance as well as the overall cell capacity a s compared with the original IFR scheme, the soft frequency reuse (SFR) scheme, and the classical universal frequency reuse (UFR) scheme.

In this paper, we propose a simple scheduling algorithm that can arbitrarily control the throughput-fairness tradeoff performance in multiuser communication systems. A new scheduling criterion is introduced through a linear combining of two well-known criteria such as the instantaneous channel capacity and the average throughput, with a control factor. Changing the control factor, we can control arbitrarily the scheduling criterion so that a different throughput-fairness tradeoff performance could be achieved, thus for system needs to be dynamically adapted. With an appropriate control factor, the proposed algorithm can substitute the proportional fair (PF) scheduler in terms of the throughput-fairness tradeoff performance. We evaluate the throughput and fairness performance of the proposed algorithm according to the control factors, assuming independent Rayleigh fading multiuser channels.

Abstract-We consider an asymmetrical two-way relay channel (TWRC) which consists of one relay node and two user nodes in this paper. In TWRCs, we can efficiently use the channel by using the network coding at the relay with the exclusive OR (XOR) operation. However, due to the nature of the XOR, the rate from the relay to a user node should be the same as the rate from the relay to the other user node. When the channel is asymmetrical, this results in the waste of the channel capacity in the channel with a better channel gain. In this paper, we propose using a superposition coding on top of the XOR operation to utilize the better channel. By using the hybrid network-superposition coding, we can achieve more rate region than that of the XOR-based systems.

We consider a three-way relay channel using a network coding in this paper. We propose a network coding scheme for the three-way relay channels and present a Latin square-like condition for the three-way network code. We also propose several schemes to design a new three-way network code by using the cell swapping and applied the methods to obtain a better network code to obtain a fair symbol error rates among the users.

We propose an incremental frequency reuse (IFR) scheme that reuses effectively the radio spectrum through systematic segment allocation over a cluster of adjoining cells. It divides the entire frequency spectrum into several spectrum segments. Based on the above segmentation, a set of cell- specific segment allocation sequences is designed for universal frequency reuse. Here, each sequence defines its own base segment and allocation order. The designed sequences are then assigned to respective cells over the cell cluster. In this scheme, the base segments over the cell cluster are mutually non-overlapped and collectively exhausted, and the added segments are interfered with from surrounding cells, but only in an incremental and coordinated manner. Hence, the proposed scheme can provide better reuse efficiency over the conventional ones such as the classical universal frequency reuse scheme and the soft frequency reuse (SFR) scheme. In addition, the simple and flexible IFR scheme can be easily configured as most existing reuse schemes only by redefining the set of segment allocation sequences. A system-level simulator for an orthogonal frequency division multiple access (OFDMA) cellular system covering surrounding cells up to 3rd-tier has been implemented. Simulation results show that the IFR scheme provides quite better reuse efficiency as compared to the classical universal reuse scheme and the SFR scheme, especially at the cell-edge region.

In this paper, we deal with a per-user power allocation problem in multiuser multiple input multiple output (MU-MIMO) systems based on minimum mean square error (MMSE) preceding. The MMSE- precoding technique provides a reasonable performance due to minimization of a composite interference-plus-noise power through allowing inter-user interference as well as the higher capacity with multiuser spatial multiplexing. The technique also has a reasonable computational complexity with linear transmit processing. The problem of optimizing per-user power allocation under inter-stream interference is not convex. Hence, we invoke some iterative approaches. In this paper, we propose a simplified iterative water-filling (SIWF) algorithm for per-user optimum power allocation in multiuser MMSE-precoded MIMO systems, in order to maximize the downlink sum capacity. This technique can reduce greatly the computational complexity for the iterative water-filling process through accelerating the iteration process, as compared with the existing modified iterative water-filling (MIWF) algorithm (Wei Yu, 2007), without any performance loss. In the proposed algorithm, both the taxation and interference terms are updated at every iteration of the inner loop of iterative water-filling. In addition, per-user power levels at every iteration for the inner loop are normalized so that the total transmit power constraint could be satisfied, prior to the next iteration. From computer simulations and complexity analyses, we show that the proposed algorithm has much lower complexity but the same capacity, as compared with the original MIWF algorithm.

We propose a new space-time code (STC) structure that can achieve generalized optimal diversity (GOD) with a reduced design complexity, and also provide an increased coding gain. As a class of GOD STC structure, the new structure achieves the full diversity and an optimized coding gain with minimum delay irrespective of the number of transmit antennas (Nt) and the rate (R ), if the rate is less than or equal to the rank of the channel matrix. In the new structure, only R transmit antennas at a time slot are engaged in signal transmission, and a different set of R transmit antennas at a different time slot is used. Ntdata symbols are combined together using complex weights for each antenna at a time slot. The same set of data symbols is repeated at all the remaining time slots in a code block, but is transmitted through different transmit antennas using different sets of combining weights at different time slots. In particular, the above structure eliminates cross-code product terms in the determinant of codeword difference matrices when Rt, thus increasing the coding gain. In addition, we propose a simplified code structure to facilitate the code design, in which a higher-rate GOD STC can be obtained by coupling R rate-one codes using different rotations. We apply two design constraints such as the power and orthogonality constraints, and provide some guidelines to reduce further the design complexity. In conclusion, the proposed structure provides a higher coding gain as compared with the original GOD structure [ 1 ] and the existing linear dispersion STCs (LD-STCs), when Rtand can reduce significantly the design complexity when the rate becomes higher and/or the number of transmit antennas gets larger.

We propose a universal frequency reuse (UFR) system that can reuse effectively a given frequency resource in a mobile cellular environment. The key concept is to control mutual interferences among neighboring cells in a systematic manner. The system first assigns the whole frequency resource to all cells, so that the whole resource could be reused without any restrictions in all the cells as in the cellular systems with frequency reuse factor of 1. Then, it defines a cell-specific resource allocation rule for each cell, so as to control mutual interferences among neighboring cells in a cooperative way. In our case, the resource allocation rules are designed so that they can avoid maximally inter-cell interference (ICI) when the loading factor is less than a threshold loading factor, while exceeding the threshold, ICI would be averaged over the whole frequency resource. Hence, the proposed system can maintain ICI always at the minimum level, in turn expecting greater spectrum reuse efficiency. Finally, we perform extensive computer simulations in order to verify the effectiveness of the proposed reuse system, assuming an OFDMA-based cellular system. From simulation results, we show that the UFR system provides quite better spectrum reuse efficiency, regardless of omni-cell and 3-sector cell systems.

We propose a per-user successive minimum mean-squared error (PU-SMMSE) precoding technique suitable for the use in multiuser multiple-input multiple-output (MU-MIMO) downlink systems. Its preceding complexity is much simpler as compared with that of the successive MMSE (SMMSE) technique (V. Stankovic and M. Haardt, 2004), without any performance loss. The new technique also takes both the high capacity benefit achievable with MIMO multiplexing and the benefit of space division multiple access (SDMA), assuming channel state information (CSI) at both the transmitter and receiver sides. In the new technique, the columns of the precoding matrix are obtained by computing successively reduced-rank MMSE precoding solutions on a per-user basis. Then, a per-user virtual channel which consists of the per-user channel through precoding is decomposed into orthogonal transmission modes. Finally, a proper power assignment can be applied to the decomposed modes according to some assignment policy. We compare the proposed technique with the SMMSE technique in terms of the bit error rate (BER) performance. We also analyze the computational complexity of the PU-SMMSE technique in comparison with the SMMSE and BD techniques. From computer simulations and complexity analyses, we see that the PU-SMMSE and SMMSE techniques have the same BER performance in both cases of dominant mode transmission (DMT) and multiple mode transmission (MMT). However, the computational complexity of the proposed technique is much lower than that of the SMMSE technique, and also is lower than that of the simple block diagonalization (BD) technique.

Abstract We deal with power allocation (PA) and call admission control (CAC) under imperfect power control (IPC) in the reverse link of direct sequence‐code division multiple access systems for supporting multi‐class traffic. First, we briefly review the optimum PA scheme under perfect power control (PPC) and the CAC scheme subject to an outage constraint on the total composite received power. Then, we analyze the outage degradation due to the power control error when the optimum reference power levels under PPC are used. In order to mitigate the outage degradation, we would modify the reference power levels by incorporating a call dropping strategy and an outage‐lowering strategy into the optimum PA scheme under PPC. Also, we derive a constraint inequality to determine the reverse link capacity under IPC. Finally, through numerical analyses, we compute the modified reference power levels under IPC and evaluate the reverse link capacity under IPC. Copyright © 2006 John Wiley & Sons, Ltd.

We propose a computationally efficient sphere decoding (SD) algorithm with smart radius reduction control (SRRC). As a baseline algorithm for SD, we consider the modified Schnorr-Euchner (SE) algorithm [1] (hereafter, called as the MSE algorithm). In principle, the radius after zero-forcing decision feedback equalization (ZF-DFE) estimation can be reduced further if we select a new lattice vector closer to the received signal vector than the lattice vector corresponding to the ZF-DFE estimate does. In our case, we obtain such a better lattice vector by performing a sequence of alternating one-dimensional searches, starting from the ZF-DFE estimate. We then develop a novel SRRC algorithm that adopts adaptively the additional radius reduction process according to the estimated signal-to-noise-power ratio (SNR) after ZF-DFE estimation. In addition, we analyze the effect of detection ordering on the complexity for SD. Column-norm ordering of the channel matrix and optimal ordering [1] are considered here. From our analyses, we see that SRRC can reduce greatly the complexity for SD and the degree of complexity reduction gets significant as the SNR decreases, irrespective of detection ordering schemes used.

Cellular service providers in Korea have started WCDMA commercial service since 2004. The initial deployment has been done in highly-crowded urban districts. Therefore, inter-RAT handover from WCDMA to CDMA2000 (hereafter, called as the inter-RAT handover) should be developed to provide seamless service in out-of-service areas. We develop some handover procedures and set conditions for the inter-RAT handover. For the purpose of performance analysis, we build an inter-RAT handover performance simulator. Using the simulator, we determine proper handover conditions and parameter ranges for the inter-RAT handover. Through simulation results, we assess the handover failure rate and the border cell utilization for WCDMA systems. From simulation results, we define the cost function to evaluate performance of handover for each scenario. Based on the values of the cost function, we suggest optimal handover scenarios and parameter sets in the view of users and service-providers, respectively.

We analyze the computational complexity of sphere decoding (SD) for maximum likelihood detection (MLD) according to initial radius selection schemes, and also propose an efficient initial radius reduction scheme that reduces further the initial radius. As the initial radius for SD, we use the Euclidean distance between the received signal vector and the lattice vector corresponding to a suboptimum initial estimate. The proposed initial radius reduction scheme selects a new lattice vector closer to the received signal vector than the initial lattice vector in order to reduce the initial radius further. From our analyses, the reduction in the overall complexity due to further reduction of initial radius gets more significant as the SNR decreases. The ZF-DFE scheme in a combination with the proposed radius reduction scheme has the fewest computations over practical SNR range for communications, and its computations are less than that of the vertical Bell-labs layered space-time (V-BLAST) detection scheme with optimal ordering, even at low SNR values achieving an uncoded bit error rate (BER) of 0.1.

In this paper, we propose a generalized structure for constructing space-time codes (STCs), which achieves generalized optimum diversity (GOD). A GOD class of STCs can achieve the full diversity and an optimized coding gain with minimum delay for any number of transmit antennas (N/sub t/) and any rate (R), if the rate is less than or equal to the rank of the channel matrix. The structure combines mutually exclusive R data symbols with complex weights at each transmit antenna and the same set of data symbols is repeated at every time slot in a code block, but is transmitted through different transmit antennas with different sets of complex weights. In addition, it does not need any restrictions to the complex weights, thus the code can be designed with the maximum degrees of freedom. Optimized codes based on the proposed structure are obtained under the well-known rank and determinant criteria by applying two design constraints in order to reduce the design complexity. Even using two constraints, however, the proposed structure has an unaffordable design complexity when the number of complex weights gets larger. Hence, we provide some guidelines for code design, which can further reduce the design complexity by examining the characteristics of the determinant of codeword difference matrices.

In this paper, we present a multislot-interleaved turbo-coded MC-CDMA/TDD uplink system with pre-equalization using polynomial fitting and extension (PF and E) for uplink channel estimation at a mobile transmitter. The PF and E scheme that extends the channel state information (CSI) polynomial approximated over some downlink interval to the uplink slot interval provides better uplink CSI estimates in a time-varying channel. To improve further the system performance, we propose a two-step multislot interleaving (TS-MSI) scheme that improves the decoding performance by fully randomizing burst errors near the end of the uplink slot interval due to the CSI inaccurateness when the PF and E scheme is used. In the scheme, the permutation is performed in two steps as follows: (1) the interslot group-level permutation to distribute as evenly as possible information bits in a slot over all slots in an interleaver span; and (2) the intraslot bit-level permutation to separate as distant as possible adjacent bits in a slot after group-level permutation. Computer simulations are performed to evaluate the performance of the proposed system under various system conditions and channel conditions. Simulation results show that using the PF and E scheme for uplink channel estimation at the mobile transmitter can track quite well the time variability of the uplink channel, even with some downlink CSI estimation error. We see that the system performance when using TS-MSI can be improved gradually as the interleaver size increases.

In this paper, we propose a new transmission scheme that can improve greatly the performance of turbo-coded orthogonal frequency division multiplexing (OFDM) systems by making all the turbo-coded symbols have the same reliability for OFDM transmission over a frequency selective fading channel. The same reliability, that is, the same fading can be accomplished through multiplexing of turbo-coded symbols using distinct orthogonal codes and spreading over the whole effective subcarriers (hereafter, called the orthogonal code multiplexing (OCM)). This idea of the same fading for all symbols is the key of the performance improvement using the proposed scheme. As for the orthogonal code selection, a set of Walsh codes could be the best selection in terms of the complexity. However, in the fast Fourier transform (FFT) implementation of OFDM, the number of the whole effective subcarriers may be slightly less than the number of FFT points used. Hence, we choose the set of the discrete Fourier transform (DFT) basis sequences as a good substitute for a set of orthogonal codes, since the code set can hold the orthogonality irrespective of the length. We perform computer simulations using the log-maximum-a-posteriori (log-MAP) algorithm for iterative decoding in order to assess the performance of the proposed system and to compare it with that of the conventional turbo-coded OFDM system.

We investigate some implementation factors that affect the performance of interference cancellers for the International Mobile Telecommunications-2000 (IMT-2000) 3/sup rd/-generation partnership project (3GPP) system. We consider the multi-rate parallel interference canceller (PIC) that can remove effectively multiple access interference (MAI) through block-based detection and sample-based cancellation in asynchronous user environments. We evaluate the receiver complexities of the PIC receivers employing the advanced removal scheme. The proposed structure has significantly lower complexity as compared with that of the existing schemes. We analyze the effects of timing error, oversampling rate, unsynchronized users and/or outer-cell interference, and quantization error on the PIC performance through extensive computer simulations. The models for such factors and the optimum parameters are drawn.

We propose a class of multiple input multiple output (MIMO) systems using transmit diversity pre-combining (TDPC) scheme, assuming perfect channel state information (CSI) at a transmitter. In such class of systems, each antenna at the transmitter is weighted Independently according to the channel condition in order to combat the fading impairment through pre-compensation and/or to utilize effectively fading information through selective transmission. We also introduce the optimum space-time trellis code (STTC) for such class of systems, in which the minimum Euclidean distance between code words at the receiver is maximized. Incorporating both the TDPC scheme and the optimum STTC into MIMO systems can improve greatly the performance of the MIMO systems as compared with that of the conventional ones. Computer simulations are performed to evaluate the frame error rate (FER) performance of the proposed systems and to compare it with that of the conventional ones.

We propose parallel and serial code-acquisition schemes using antenna arrays for initial acquisition of direct-sequence spread-spectrum signals, which can lower substantially the range of detectable signal-to-noise ratio (SNR) as compared to the corresponding conventional schemes with a single antenna. The proposed schemes use the sum of all the single-antenna decision samples from antenna arrays, corresponding to an identical subsequence of a given pseudonoise code as a decision variable in order to enhance SNR of the resulting signal. We analyze the mean acquisition time performance under an additive white Gaussian noise channel and spatially correlated time-varying flat Rayleigh fading channels by deriving the expressions for the probabilities of detection, missing, and false alarm. We consider two extreme cases of spatial fading correlation among antenna arrays, the case of uncorrelated diversity antennas and the case of perfectly correlated antennas. From numerical results, we see that the acquisition performance of the proposed schemes using antenna arrays becomes improved continually as the number of antennas increases and the performance improvement depends on the degree of spatial fading correlation.

We propose a power allocation (PA) scheme under imperfect power control (IPC) in the reverse link of direct sequence-code division multiple access (DS-CDMA) systems for supporting multi-class traffic, and analyze the effect of power control error on the system capacity. First, we present an optimum PA scheme under perfect power control (PPC), which can determine dynamically optimum reference power levels for power control under varying traffic and channel conditions. Second, we analyze the outage degradation due to power control error when the optimum levels under PPC are used. In order to mitigate the outage degradation, we introduce two strategies: (1) call dropping (CD) and (2) outage lowering (OL). Using the modified reference power levels, we analyze the reverse link capacity subject to an outage constraint on the total composite power at a cell-site receiver under IPC. Finally, numerical analyses over various traffic and channel conditions are done to compute the reference power levels under IPC and then the reverse link capacity is evaluated under both PPC and IPC.