A transmission experiment of a multi-user MIMO system with user selection and linear spatial precoding is conducted and the results are reported. As a low-complexity user selection algorithm, the chordal distance-based user selection (CDUS) algorithm is employed. The testbed used in the experiment is implemented by taking full advantage of the software-defined radio. An amplify-and-forward-based simple channel estimation method is adopted. As a result of the experiment in a conference room, the CDUS algorithm improved both the power efficiency and the interference suppression accuracy compared to the round-robin user selection.

In this paper, we analyze the performance of a downlink beamforming system with partitioned vector quantization (P-VQ). A multiuser multiple-input single-output (MU-MISO) downlink beamforming system with accurate channel state information (CSI) at user terminals is considered. The optimization of feedback bits allocation to different partitions and channel direction indicator (CDI), channel quality indicator (CQI) corresponding to each partition has been performed exploiting quantization mean square error (MSE). The impacts of equal and unequal partitioning, and the number of partitions on codebook memory and overall system capacity have also been investigated and depicted through simulation results. We have found that with optimized CQI and CDI bits allocations, the amount of bits allotted to equal and unequal partitions are proportional to the size ratios of the respective partitioned sub-vectors. It has also been observed that for small-sized partitions, the optimal ratio of CDI to CQI is much smaller than that of large-sized partitions.

In this study, an adaptive and variable-rate feedback reduction technique for partitioned vector quantization (P-VQ) in MU-MISO downlink precoding is proposed. Independent feed-back reduction is performed for both channel quality indicator (CQI) and channel direction indicator (CDI) over slowly-varying Rayleigh channel. The users adaptively reduce the feedback over-head for quantized channel state information (CSI) depending on the rate and direction of change in the correlated CQIs and CDIs. Simulation results show that the proposed scheme greatly lessens the communication overhead with very small performance loss and enhances the quantization efficiency.

This paper considers coordinated linear precoding in downlink multicell multiuser orthogonal frequency-division multiple access (OFDMA) network. A less-complex, fast and provably convergent algorithm that maximizes the weighted sum-rate with per base station (BS) transmit power constraint is formulated. We approximate the nonconvex weighted sum-rate maximization (WSRM) problem with a solvable convex form by means of sequential parametric convex approximation (SPCA) approach. The second order cone program (SOCP) formulations of the objective function and constraints of the optimization problem are derived through proper change of variables, first order linear approximation and hyperbolic constraints transformation, etc. The algorithm converges to the suboptimal solution taking fewer number of iterations in comparison to other known iterative WSRM algorithms. Finally, numerical results are presented to justify the effectiveness and superiority of the proposed algorithm.

In order to demonstrate the channel capacity improvement by multiple-input multiple-output (MIMO) transmission techniques in distributed antenna systems (DAS), the channel capacities of both centralized antenna systems (CAS) and DAS are compared assuming spatially correlated shadowing. Many studies deal with these channel capacities assuming spatially independent shadowing. In this paper, we compare the characteristics of MIMO channel capacity in CAS and DAS under the effect of distance-dependent path loss, spatially correlated shadowing loss, Rayleigh fading, and inter-cell interference. Computer simulation results indicate that DAS-MIMO transmission can significantly reduce the spatially correlated shadowing loss, thus improving the outage capacity compared to CAS-MIMO transmission. Furthermore, the simulation results also show a relationship between inter-antenna distance and channel capacity.

A practical and efficient vector quantization called partitioned vector quantization (P-VQ) based non-linear precoder for MIMO downlink broadcasting channels has been analyzed in this paper. P- VQ has been found to be an efficient way of reducing the memory requirements and search complexity in conventional VQ systems, especially for large MIMO. Simulation results reveal that P-VQ technique can enhance the overall quantization performance in terms of achieved capacity, bit error rate (BER) at a cost of reasonable additional complexity under certain feedback budgets. Tomlinson-Harashima precoding (THP), a power and complexity efficient precoding technique along with data dependent vector perturbation (VP) has been employed to meet the power constraint requirements of our system. Least-square (LS) channel estimation is performed at the user terminals to acquire their channel state information (CSI).

In this paper, the end-to-end packet error rate (PER) performance of a space-time block code (STBC) based multi-hop cooperative relaying system is discussed. This system consists of a source, a destination, and two STBC based cooperative relays in each hop. These relays decode their received packets and forward the packets only when no error is found. In this system, there are conditions where the end-to-end PER performance improves with the number of hops. The theoretical end-to-end PER performance of this system have been derived analytically. However, in real-world wireless systems, various practical issues, e.g., timing synchronization and imperfect channel estimation, have a large impact on the performance. To confirm the end-to-end PER performance of this system in actual setup, an in-lab experiment using a fading emulator and four transceivers is performed under Rayleigh fading environment. The experimental results clarify the PER performance of the STBC based multi-hop cooperative relaying system. In addition, the theoretical PER performance is also verified from the experimental results.

Adaptive traffic ratio control (ATRC) with hybrid network coding and cooperative relaying is proposed to improve the overall throughput in an asymmetric data traffic network. The scheme is used in conjunction with adaptive transmission protocol and relay selection based on instantaneous channel state information. Simulation results show that (i) ATRC provides better trade-off between achieved throughput and traffic ratio and (ii) both maximum throughput and guaranteed traffic ratio can be simultaneously achieved by ATRC as offered traffic approaches symmetric.

In this paper, implementation of a multi-user MIMO experimental system is presented. Linear spatial precoding and a simple two- way channel estimation technique are adopted in the experimental system. In-lab and field transmission experiments are carried out and the performance is evaluated. The impact of channel estimation error under average channel gain discrepancy between two mobile stations is analyzed by means of computer simulation. It is shown that channel estimation error has a greater influence on the mobile station with greater average channel gain.

In this paper, we propose a receiver using the concept of a "virtual channel" and Turbo codes for multiuser multiple-input multiple-output (MU-MIMO) systems with cooperating transmitters. In order to reduce feedback information from receivers, a precoding method with reduced channel state information (CSI) is needed. Although signals interfere with each other in such a system, the proposed receiver enables high-speed transmission by detecting signals iteratively. In the proposed iterative scheme, soft information is exchanged between virtual channel detectors and Turbo decoders to improve the bit error rate (BER) performance. We evaluate the performance of the proposed receiver in systems with two transmitters, each equipped with four antennas, and two receivers, each equipped with two antennas. The results show that the proposed receiver can improve the system performance owing to the iteration scheme.

In multi-hop cooperative communications, multiple relay stations are used at each hop in order to enhance transmission performance. In previous studies, transceivers have been used to evaluate the transmission performance of multi-hop cooperative communications involving the use of a space-time block code, in real propagation channels. In the present study, an experiment involving multi-hop cooperative networks is conducted with transceivers. The transceivers are implemented on field programmable gate arrays, which perform real-time signal processing. Synchronization of the timing and carrier frequency among wireless stations is carried out on the basis of received signals. The transmission performance of cooperative communications is compared to the performance of non-cooperative multi-hop communications involving the use of only one relay station. The performances of the relay schemes are measured quasi-simultaneously. The transmission performance is evaluated in terms of the cumulative distribution function (CDF) of the bit error ratio.

This paper proposes a game-theoretic approach for self-organized resource allocation in OFDMA femtocells with closed access configuration. The objective of the proposed scheme is to overcome the interference problem caused by co-channel operation of femtocells in an existing macrocell network using a self-organization capability of femto users. Inspired by the emerging cognitive radio technology, each femto user acts as an autonomous entity and attempts to select the most appropriate subset of resource blocks in a decentralized manner in order to mitigate the cross- and co-tier interference. Such a self-organized resource allocation scheme can be modeled as a potential game, which guarantees the convergence to a Nash equilibrium as long as distributed sequential play based on the best response strategy is adopted. Simulation results show that the proposed scheme improves the capacity of the femtocell network, while minimizing the performance degradation of the macrocell network.

With the purpose of improving the performance of next generation wireless networks, cooperative relaying (CoR) and network coding (NC) are promising techniques for bi-directional or two-way transmissions. Compared to CoR, NC can reduce the number of bi-directional transmission slots, which results in higher total throughput. However, the asymmetric traffic condition has significant impact on the bi-directional throughput of NC. CoR is robust against asymmetric traffic conditions when adaptive transmission direction switching (TDS) based on channel and traffic condition is applied. In order to improve the throughput of NC even under asymmetric traffic condition, two hybrid NC and CoR schemes with opportunistic scheduling are proposed in the present paper. In the proposed schemes, adaptive resource allocation is realized for each transmission protocol, and the impact of the traffic ratio and the relay position on the throughput is investigated. Simulation results reveal that the proposed schemes not only achieve higher throughput than the conventional scheme but are also robust against asymmetric traffic conditions. Moreover, the results can contribute to the development of relay selection and routing schemes for practical networks.

In this paper, we propose a virtual layered space-frequency receiver with an ordered successive detector for multiuser multiple-input multiple-output (MU-MIMO) systems where multiple transmitters cooperate with one another. The proposed receiver enables high-speed transmission in the systems even if the transmitters know only partial channel state information (CSI). Actually, the receiver shows such superior performance when the concept of "virtual channel" is applied to cooperative MU-MIMO systems. Moreover, we propose an iterative detection scheme for the receiver; in this scheme, some signals from the Viterbi decoder are fed back to the ordered successive detector. The proposed iterative scheme helps improve the transmission speed when the number of cooperating transmitters is increased. We evaluate the performance of MIMO systems in which the transmitters have four antennas each and two users have two antennas. The receiver can double the transmission speed at the cost of 3 dB in Eb/N0at a BER of 10-4. Furthermore, the receiver can triple the transmission speed at the cost of 7 dB in Eb/N0at a BER of 10-4. When the system has three transmitters, we observe an error floor. However, the proposed iterative method can suppress the error to a value below BER = 10-7.

In cognitive radios, reliable detection of wireless systems is crucial since these radios share the frequency spectrum with other wireless systems. In a heterogeneous environment, systems with different priorities have different protection requirements and operating parameters.The cognitive radio must guarantee highly reliable communication in the primary systems, but need not necessarily guarantee reliable communication in the secondary systems. Therefore, classification of the signals in a cognitive radio is necessary to fulfill the above mentioned protection requirements. Cyclostationary detection allows for the classification of coexisting cyclostationary signals with different parameters, whereas energy detection cannot be used for distinguishing primary signals from secondary signals. This method has been theoretically investigated, but it has not been applied to an actual field. This paper presents the experimental results obtained from field trials on the use of cyclic-feature-based methods for distributed signal detection and signal classification.

Recently, the concept of self-organization has drawn considerable attention for its possible use in distributed interference management. To realize self-organized interference management, a minority game (MG) is a promising tool because it facilitates self-organized decision-making. Although the existing interference management scheme that involves the use of an MG achieves transmission control aware of the system priority, it cannot actively control the outage probability of a higher-priority system. In the present study, the existing scheme is modified to control the outage probability for a predefined target value. It is proposed that the criterion for deciding whether or not to transmit should be changed adaptively, depending on the interference power in the higher-priority system. In this way, robustness against the location of stations is also achieved. Numerical simulation confirms the above advantage of the proposed scheme.

In multi-hop cooperative communications, multiple relay stations are used at each hop in order to enhance transmission performance. Transceivers for multi-hop cooperative communications using space-time block code are developed in order to evaluate the transmission performance in real propagation channels. In the present paper, an experiment involving multi-hop cooperative networks is conducted with this equipment. The transceivers are implemented on field programmable gate arrays, which perform real-time signal processing. Synchronization of timing and carrier frequency among wireless stations is established based on the received signals. The transmission performance of cooperative communications is compared to the performance of non-cooperative multi-hop communications using one relay station. The performances of the relay schemes are measured quasi-simultaneously. The performance is evaluated in terms of the cumulative distribution function (CDF) of the bit error ratio.

In the present paper, a game theoretic framework of joint channel selection and power allocation for spectrum sharing in distributed cognitive radio networks is proposed. The objective of the proposed scheme is to investigate the performance of the lower- priority system in spectrum sharing where the lower-priority users utilize the spectrum by selecting the channel and transmit power while aware of the interference to the higher-priority users. The utility function that captures the cooperative behavior to minimize the interference and the satisfaction to improve the throughput is defined. The proposed joint channel selection and power allocation game can be formulated as a potential game and it is guaranteed to converge to a Nash equilibrium when the best response dynamic is performed. The simulation results verify the convergence of the proposed potential game and reveal that the proposed utility function with coefficient adjustment improves the throughput performance of the lower-priority system compared to that of without coefficient adjustment, while ensuring the Quality of Service (QoS) constraint at the higher-priority system.

This paper offers a new insight to the fundamental problem of efficient admission control in arbitrary power-controlled wireless networks with an unknown call arrival distribution. Active transmitter-receiver pairs are assumed to (i) communicate simultaneously over shared channels, (ii) define target signal-to-interference and noise ratios (SINRs) by nonlinear functions of channel interference, and (iii) use adaptive power control to maintain the actual SINR at the target level in response to interference variations. Unlike other studies, in this study, power control with limited dynamic range and both the discrete-time and the continuous-time dynamics is explicitly considered, as well as the effects of stochastic radio propagation phenomena. Without relying on a priori assumptions, we first define sufficient conditions for a channel allocation mechanism to ensure the SINR constraints in cooperation with the deployed power control mechanism. We use the concept of Lyapunov stability as a cross-layer optimization criterion. Subsequently, we focus on the widely assumed case of SINR targets being defined by linear functions of interference, and show that such targets can be achieved if hii> |Ai|¿ j¿i hij¿i, where hijis the channel gain between the transmitter of link j and the receiver of link i, and Ai is the slope of the linear definition of the target SINR. This knowledge allows us to propose a simple distributed algorithm for implementing an admission control mechanism that (i) uses interference and pilot signal measurements as its only decision-making input, and (ii) allows links to adaptively adjust the SINR targets within the system stability bounds. This mechanism is shown to outperform the carrier sensing approach (CSMA/CA) for admission control.

Adaptive radio resource management policy aware of the demand for wireless services is termed dynamic spectrum access (DSA). In order to evaluate the effectiveness of DSA, dynamics of the demand should be considered. In the present paper, dynamics of the spectrum market, which is a way to realize DSA, are evaluated from the economic perspective. Modeling the spectrum market system by using game theory, the convergence characteristics of the system are analyzed from two points of view: allocation interval and prediction period. The available spectrum is allocated at regular time intervals and the length of this allocation interval naturally has a significant impact on the dynamics of the system. Also, available spectrum is allocated based on the prediction of the time variation of the demand for wireless services. This prediction thus influences the dynamics of the system. Simulation results reveal that short allocation interval and long prediction period are required to improve the convergence property of the system. Simulation results also reveal that long prediction period leads to the increase in the revenue of spectrum holder.

The introduction of adaptive array antennas to wireless ad hoc networks is effective for increasing the overall network throughput because of its interference suppression capability. The network throughput can be further improved by controlling antenna-pattern, channel, and power simultaneously. In this paper, the asynchronous distributed pricing (ADP) and multi-channel ADP (MADP) algorithms, which are joint channel selection and power control schemes, are modified to also select antenna-pattern. Simulation results show that the proposed scheme achieves high network throughput.

Reliable detection of other wireless systems is crucial for the systems that share the same frequency band. In wireless communication channels, there is uncertainty in the received signal level due to multipath fading and shadowing. Cooperative sensing techniques in which wireless stations share their sensing information can improve the detection probability of other systems. This paper presents the field trial results of a cooperative sensing scheme with energy detection and hard decision cooperation. The results demonstrate that the cooperation of wireless stations can relax the stringent sensitivity requirement of energy detection.

The present paper focuses the application of the base station cooperation (BSC) technique in fractional frequency reuse (FFR) networks. Fractional frequency reuse is considered to be a promising scheme for avoiding the inter-cell interference problem in OFDMA cellular systems, such as WiMAX, in which the edge mobile stations (MSs) of adjacent cells use different subchannels for separate transmission. However, the problem of FFR is that the cell edge spectral efficiency (SE) is much lower compared to that of the cell center. The BSC technique, in which adjacent BSs perform cooperative transmission for one cell edge MS with the same channel, may improve the cell edge SE. However, since more BSs transmit signals for one cell edge MS, the use of BSC can also increase the inter-cell interference, which might degrade the network performance. In the present paper, with a focus on this tradeoff, we propose an adaptive BSC scheme in which BSC is only performed for the cell edge MSs that can achieve a significant capacity increase with only a slight increase in inter-cell interference. Moreover, a channel reallocation scheme is proposed in order to further improve the performance of the adaptive BSC scheme. The simulation results reveal that, compared to the conventional FFR scheme, the proposed schemes are effective for improving the performance of FFR networks.

This paper investigates the performance of a multiuser distributed antenna system (DAS) with spatial precoding in which mobile stations (MSs) just send back the received signals along with their training sequences. In this method, base stations (BSs) can obtain the channel state information of both the uplink and downlink without estimating the channels in MSs. This two-way multiple-input multiple-output (MIMO) channel estimation technique have been proposed and mathematically analyzed in the literature. However, implementation aspects of this method, such as forward gain and dynamic range, are not studied yet. In this paper, these aspects are investigated in order to confirm its feasibility in a multi-user DAS where a precoder is employed in BSs. Computer simulation results are presented to demonstrate the required hardware specs for an amplify and forward (AF) operation in MSs by examining the bit error rate (BER) performance. It is shown that an 8-bit A/D converter together with a fixed gain amplifier brings almost no degradation to the BER performance. Furthermore, a performance comparison with a vector feedback method is given.

A distributed power, channel, and route selection scheme for multi-hop radio networks is proposed by extending multichannel asynchronous distributed pricing (MADP) algorithm, which is a power and channel selection scheme for single-hop radio networks. MADP algorithm achieves suboptimal system throughput in single-hop radio networks, however, since MADP algorithm allocates more resources to links with good quality, there may be a large difference in throughput among communication links. Particularly in multi-hop radio networks, MADP algorithm might not necessarily achieve high system throughput because the link with the lowest throughput limits the end-to-end throughput of a given multi-hop route. In order to achieve high system throughput in multi-hop radio networks, a modification of MADP algorithm named end-to-end throughput aware MADP (ETA-MADP) algorithm is proposed. It is proved that all links in a given multi-hop route have the same throughput through the use of ETA-MADP algorithm. Simulation results reveal that the introduction of the proposed algorithm achieves high spectral efficiency in multi-hop radio networks even with lower transmit power compared to that in single-hop radio networks.

Multi-hop relaying can extend communication range using limited transmit power, and improve area spectral efficiency. However, they might cause considerable degradation of end-to-end error performance due to error accumulation. Cooperative relaying, in which multiple stations forward the received signal using transmit diversity techniques, can recover the errors unless all the relay stations do not receive the signal correctly, and improve the transmission performance. In this paper, the FER performance of multi-hop cooperative networks is analyzed by using state transition. Theoretical analysis reveals that the FER performance can be kept almost constant, or even improved with increasing the number of hops. The computer simulation results closely agree with the analysis.

Cooperative relaying is a promising technique that improves the end-to-end error performance in multihop vehicular networks. When cyclic delay diversity (CDD) is employed as transmit diversity at each hop, the control of the delay time introduced at each transmitting station improves the diversity gain. In this paper, a novel distributed delay time selection algorithm for cooperative multi-hop vehicular networks with CDD is proposed. In this algorithm, neither dedicated control packets nor an external controller for the delay time selection is required since the delay time is selected by overhearing the transmission in the next hop. Simulation results show that the proposed algorithm can achieve the end-to-end FER performance close to that of fixed delay time allocation, and is not sensitive to vehicles movements.

Cooperative relaying is a promising technique for multihop wireless networks to exploit spatial diversity. Most of the studies of multihop relaying assumed a simple i.i.d. Rayleigh fading model. In this case, there is no correlation of phase and amplitude among the received signals. However, this assumption is not always fulfilled in practice. For instance, the transmission distance in multihop wireless transmission is supposed to be a close range and the shadowing effect needs to be taken into account. The ray launching method based on geometrical optics is a technique for estimating a deterministic propagation channel. In this paper, 3D ray launching method is used for wireless propagation prediction. By computer simulations, the performance of 2-hop cooperative relaying with two relay stations is investigated. The simulation-based performance analysis confirms that the cooperative relaying scheme has an advantage of diversity gain thus improving the bit error ratio performance.

In this paper, the performance of decentralized adaptive route selection problem in cooperative relaying networks in which each source independently chooses a route to enhance its own throughput is studied. Cooperative relaying is a new technique to enhance spectral efficiency by using multiple stations. However, when multiple stations transmit simultaneously, it also increases the number of interference signals. For this reason, if each source independently and selfishly transmits, it is not clear whether cooperative relaying always increases the network capacity. In order to investigate the capacity of cooperative relaying under the decentralized adaptive route selection, the game-theoretic approach is applied. Simulation results reveal that the introduction of the cooperative relaying always increases the capacity under centralized control, however that it may reduce the capacity under decentralized control due to the increase in the number of interference signals.

This paper analyzes distributed asynchronous power and rate control for wireless ad hoc networks. Importantly, all network transmitters are considered to be independent of any management infrastructure and to have the freedom to choose their own arbitrary control rules, using as input only information on local interference and achieved carrier signal-to-interference ratio (CIR). Such an approach respects diverse user preferences of on quality of service (QoS) and allows them to adapt to local network conditions in contrast with conventional cellular systems, whose users must follow centralized control commands from serving base stations. For this purpose, we develop a general non-cooperative game-theoretic framework and characterize the resulting power and rate allocation dynamics in terms of its convergence to network-wide acceptable equilibrium states under stochastic communication channels. Chief among the attractive features of our proposed framework is the fact that it is developed in an entirely abstract way without any particular technological or architectural assumptions, which are typically made in related works. Numerical simulations prove the potential of our approach to provide for fair, robust and comparably better CIR allocation in ad hoc networks with varying topology and user density.

This paper theoretically analyzes cross-layer optimized design of transmit power allocation in distributed interference-limited wireless networks with asynchronously acting links and stochastic communication channels, whereby the network link's architecture is abstracted into three layers - a physical, a data link and a network layer. We treat the transmit power allocation process as a result of coupled interaction, in which all the three layers try to satisfy their individual requirements on power control, admission control and routing respectively. Using a best-response approach for system modeling and the notion of network's stability for its cross-layer optimization, we present simple power control and admission control algorithms for convergent iterative allocation of equilibrium transmit powers, which optimally balance the network-wide trade-off between allocated transmit powers and resulting interference. Numerical simulations evaluate the achievable stability of our scheme with theoretical bounds.

In cognitive radio, reliable detection of the primary system is crucial for the secondary system. However, there is an uncertainty in the received power level due to shadowing. The cooperative sensing where the stations in the secondary system share their sensing information can improve the detection accuracy of the primary system. In this paper, a new secondary system for reducing interference to the primary system is proposed. In the proposed system, a secondary transmitter receives sensing information by using multi-hop transmission from all secondary stations which detect the primary system, and makes decision of its transmission based on the received sensing information. Moreover, it controls the transmit power by the received CINR information from the secondary sensing stations. Simulation results assuming spatially independent shadowing and ideal multi-hop transmission in the secondary system reveal that the proposed system is helpful to protect the primary receivers which have no transmission ability.

This paper analyzes distributed asynchronous power and rate control for wireless ad hoc networks with stochastic channels. In contrast to conventional cellular systems, all network transmitters are assumed to be independent of any management infrastructure and, importantly, to have the freedom to choose their own arbitrary control rules, using as input only the information on local interference and achieved signal-to-interference and noise ratio (SINR). This approach respects link's different local network conditions and preferences on quality of service. With the purpose of finding network-wide acceptable equilibria for such an individually defined power/rate allocation dynamics, the authors discuss an entirely general asynchronous and distributed algorithm, whereby stochastic channels are assumed. Moreover, optimum admission scheme for linear/linearized models is given. Numerical simulations show the efficiency of our approach to allocate comparably higher SINRs in random ad hoc networks with changing topologies and user density.

There arises a need to increase the network throughput by broadening the bandwidth. Currently, in IEEE 802.11 wireless LAN standardizations, two approaches are taken to effectively utilize multiple existing channels. One is multi-channel technique in which multiple separate frequency channels are used for communications as seen in IEEE 802.11s. The other is channel-bonding technique in which multiple frequency channels are bonded into a single broadband channel as seen in IEEE 802.11n. The former one can mitigate traffic congestion, while the latter one can increase the transmission rates. Here, in this paper, these two approaches are analytically compared and the advantages of the respective approach are clarified in terms of the network throughput and delay performances assuming the same total bandwidth and a CSMA protocol. Our numerical and simulation results indicate that under low-traffic conditions, using channel-bonding technique can achieve a low delay, while under high-traffic conditions, the performances of both throughput and delay can be improved by using multi-channel technique.

The impact of spatial correlation of shadowing on the coverage of TDMA multihop cellular systems is investigated in single-cell environments. The introduction of relaying capabilities to cellular systems may enhance the cell coverage as follows. Changing single-hop transmission to multihop transmission reduces per-hop path loss. In addition, since a multihop route is selected among a number of alternatives, the use of mobile stations suffering from severe shadowing can be decreased. In most studies on multihop cellular systems, shadowing is modeled as a location-independent log-normal random variable; however, adjacent shadowing values are spatially correlated because shadowing is caused by terrain configuration or obstacles between the transmitter and receiver. Thus, coverage enhancement due to relaying capabilities may not be achieved as expected. In this paper, first, according to the commonality between multihop transmission and symbol rate control, a similar methodology to formulate the spectral efficiency (SE) and outage probability of rate-adaptive cellular systems is used to estimate these performances of multihop cellular systems. Second, we investigate the impact of decorrelation distance, whose typical value for the urban environment is 20 meters, on the coverage of multihop cellular systems. By using a model for spatially correlated shadowing, numerical results reveal that when the coverage of single-hop cellular systems is ten times larger than the decorrelation distance, the introduction of relaying capability may enhance the coverage without significant degradation due to the spatial correlation of shadowing.

In performance evaluations of wireless ad hoc networks, both successful data reception and carrier detection probabilities are assumed to be 1 within a circle with certain radius and 0 outside the circle. In real systems, however, those probabilities are gradually changes from 1 to 0 as the distance between a terminal and access point increases. In this paper, we evaluate the throughput of wireless ad hoc networks assuming successful data reception and carrier detection probabilities to be continuous functions of the distance between a terminal and access point. We also show that new additional factors which decrease the throughput will appear under these assumptions, compared to the case where only probabilities 1 and 0 are assumed

A cooperative diversity requires accurate synchronization for relaying in multihop communications. This paper presents an iterative equalizer for multihop networks with delay diversity, which is tolerant to asynchronous relaying. We introduce frequency-domain space-time coding suited for the equalizer. In addition, we employ cooperative spatial multiplexing to improve the system throughput. We demonstrate that cooperative diversity and multiplexing can offer high spectrum-efficiency even in frequency-selective and poor-scattering channels

This paper presents a novel game-theoretic framework for distributed adaptive power control in wireless ad-hoc networks. It first shows the equivalency of two recent approaches to this noncooperative field - the QoS utility maximization approach and the best-response approach. We then choose to follow the analytically more intuitive best-response approach and analyze in an abstract way general conditions for existence of optimal outcomes (Nash equilibria) of best-response power control dynamics. Consequently, we characterize conditions for global convergence to such states without any particular technical assumption. Our work provides a mathematically more general insight to game-theoretic power control compared to recent related works. Using the herein developed framework, we discuss CIR-based utility function maximization and show conditions for applicability of linear (linearized) best-response power control in connection with illustrative simulations

This paper presents a novel framework for distributed power control for ad-hoc wireless networks. We analyze dynamic adaptive power allocation assuming that transmit power is adjusted with respect to experienced interference based on general best-response functions. For this purpose, we develop a general non-cooperative game-theoretic framework in order to characterize optimal equilibrium states and convergence of distributed power control dynamics to such states. Our work provides a more general insight to game-theoretic power control compared to most of recent works in this field. Moreover, our framework is developed in an abstract way without any technical assumption on particular modulation, coding, QoS measure definition or network architecture. To demonstrate an application of our framework, we show that stable linear best-response power control converges exponentially to a unique Nash equilibrium for any initial condition, which we confirm by numerical simulations.

Much recent interest has focused on applying multihop relaying to conventional cellular communication systems. Multihop relaying can broaden the cell coverage and enhance the capacity of cellular systems. However, as is well-known, cell coverage can also be broadened simply by lowering the transmission rate in a conventional cellular system. According to this commonality, we theoretically formulate the performance of cellular systems with multihop relaying in terms of outage probability and spectral efficiency using the same methodology to formulate the performance of cellular systems with rate adaptation. Simulation results reveal that multihop relaying and rate adaptation have very similar effects on the performance of cellular systems

A novel split soft-decision equalizer (SSE) with near-optimum performance is proposed for wireless multipath channels with large delay spread. The concept of SSE is significantly different from the traditional notions of the Viterbi algorithm and decision-feedback equalizer. Instead of dealing with received sequence as a combined sequence, it splits the received sequence into its constituent paths. Using an iterative soft-decision algorithm, reliability of the soft decisions on each decomposed element is improved iteratively. The major advantage of SSE is the independence of computation complexity on channel time dispersion. Joint design of SSE with a soft-decision decoder is also considered in this paper. Performance analysis and simulation results show that performance of the proposed algorithm comes very close to that of the logarithmic maximum a posteriori decoder.

An automatic-repeat-request (ARQ) scheme for improving the system throughput efficiency is investigated in coded multiple-input multiple-output (MIMO) transmissions. The improved ARQ scheme employs two strategies, referred to as trellis-coded modulation (TCM) reassignment and antenna permutation. TCM reassignment increases the free distance after code combining through the use of different TCM codes for retransmissions. Antenna permutation exploits spatial dimension for preventing successive frame errors by changing the connections between transmitters and transmit antennas upon retransmission. It is shown through computer simulations that this ARQ scheme achieves high throughput even in severe conditions of low signal-to-noise ratio and high Rician factor over spatially and temporally correlated Nakagami-Rice fading MIMO channels.

The capacity region of ad hoc networks with optimal routing or scheduling has been investigated. Since distributed network control algorithms can be applied without centralized information, the resulting network can be scalable. On the other hand, selfish nodes can severely degrade network performance, for example, the network capacity. We attempt to analyze the obtained network capacity region based upon distributed route selection by using game theory. Simulation results reveal that in some situations, even with optimal routing, each rational selfish node cannot determine a unique route under the assumption that nodes know not only their own end-to-end throughput, but also those of all the other nodes, as a result of their own, and all the other nodes', route selections.

This paper evaluates the effect of spatial interleaving in the Viterbi algorithm with combined intersymbol interference (ISI) equalization, trellis-coded modulation (TCM) decoding and multi-signal detection. Through computer simulations, it is confirmed that spatial interleaving provides substantial diversity gains in both frequency-flat and frequency-selective Rayleigh fading channels. In Nakagami-Rice fading channels, performance improvement is significant at low Rician factor because of overcoming the burst errors incurred by signal fading. Spatial interleaving does not require additional computational complexity, memory buffer and decoding delay, differently from time interleaving. Moreover, the optimum combined TCM and ISI maximum likelihood sequence estimation (MLSE) can employ interleaving with almost no extra complexity, and iteration between equalization and decoding is not needed through the use of spatial interleaving.

MIMO wireless systems are able to realize communication links with high spectral efficiency and high performance. By the transmission of different data streams at the same time and on the same frequency but from different transmit antennas, the spectral efficiency of MIMO systems can be increased linearly with the number of transmit antennas. However, to achieve such enormous spectral efficiencies, one of the major limitations is the complexity of the receiver that needs to separate several overlapped cochannel signals whose number increases with the number of transmit antennas. We investigate the performance of low complexity ordered successive MMSE receivers with realistic channel estimations. Generation of nulling weights by both recursive and non-recursive approaches is studied. In order to compromise performance and complexity, novel ordered successive receivers where both approaches are combined are proposed. Those new receivers are named as semi-adaptive ordered successive receivers.

This paper presents a performance comparison between space-time coding (STC) and trellis-coded cochannel interference canceller (TCC) assuming both frequency flat and frequency selective fading channels. The STC is a form of a transmit diversity scheme as it transmits multiple signals using different antennas in order to carry a single data stream. The TCC is a form of multiuser detector, which can estimate multiple signals from different transmitters. This paper describes the difference between STC and TCC, and then provides the BER performance of them over frequency flat and frequency selective fading channels. It is shown by computer simulations that TCC is as good as STC when channel impulse response is estimated, and that TCC can outperform STC over frequency selective fading channels.

This paper proposes a multi-hop radio access cellular (MRAC) scheme for achieving both high speed and high capacity and good area coverage in fourth generation mobile communications systems. In this scheme, we assume two kinds of hop stations, one is a dedicated repeater station installed at a good propagation location such as a rooftop, and the other is a user terminal that temporarily experiences good propagation conditions. For both cases, the path diversity effect can be obtained between single-hop and multi-hop paths. Four propagation models were studied representing the paths among the base station, hop stations, and mobile station. Based on the propagation models, suitable MRAC operation conditions are discussed. For mobile stations located at the cell edge area, MRAC operation is effective in reducing the transmit power. Consequently, MRAC reduces the interference in cellular systems and enhances the area coverage.

In recent years, a lot of attention has been drawn to the use of multiple antennas at both the transmitter and receiver sides. Such techniques are named here as space-time transmission and noted as STT. By their use of the available space dimension, STT techniques can achieve very high spectral efficiencies in Rayleigh fading channels with rich scattering. The use of STT techniques for indoor wireless systems with a huge number of antennas is considered. As a low complexity detection algorithm, ordered successive detection based schemes are considered. A scheme known as V-BLAST (vertical Bell Labs layered space-time) is proposed in the zero forcing (ZF) criterion (see Wolniansky, P.W. et al., Proc. ISSSE'98, 1998). In a previous work, it has been shown how this scheme can be much improved by the use of a backward iterative detection algorithm (see Benjebbour, A. et al., Proc. WPMC, p.819-24, 2000; Proc. VTC2001 Spring, 2001). An extension of the conventional V-BLAST algorithm to the minimum mean square error (MMSE) criterion is now proposed. For the proposed receiver, an ordering metric is defined. Then, the effect of ordering on both receivers is discussed. And finally, it is shown how the ordering metric in the MMSE criterion can be simplified.

A new integration scheme for an adaptive array antenna and an MAI canceller is proposed. Computer simulations show it to have a better performance with less computation load than the conventional scheme. In addition, by using part of tentative decisions as pilot symbols virtually, further considerable performance improvement is attained especially in higher E/sub b//N/sub 0/ region and in case of many users.

Cochannel interference is a major impairment that limits the capacity of cellular radio systems. A coding scheme for nonlinear cochannel interference cancellation is discussed. This canceller uses trellis-coded modulation with maximum-likelihood sequence estimation of both the desired signal and cochannel interference. This technique permits operation even when the signal-to-interference ratio is 0 dB. This performance advantage is due to the sophisticated trellis-coding designed for each user. A novel trellis coding is proposed, and its superior bit error rate performance is shown by computer simulations.

A new hybrid automatic repeat-request (ARQ) scheme that uses supplementary trellis codes for retransmission is described. In this scheme, supplementary codes are designed to compensate for the squared free distance (d/sub free//sup 2/) of the trellis code used for the previous transmission. Differently encoded codewords are combined and decoded by the same Viterbi decoder at the receiver. This scheme provides improved (d/sub free//sup 2/) upon retransmission compared with that provided by the conventional trellis coded hybrid-ARQ scheme where the same coding scheme is used for all transmissions.

Co-channel interference is a major factor limiting spectral efficiency of cellular radio systems. A nonlinear co-channel interference canceller leading to a significant increase of traffic capacity of cellular systems has been investigated. In this scheme, a maximum-likelihood sequence estimation extended to cancel co-channel interference is combined with trellis-coded modulation to enhance the co-channel interference cancelling capability. However, the bit-error rate performance of this canceller is degraded if there exists a frequency offset between the desired signal and the interference signal. In this paper, a joint frequency offset and delay profile estimation technique for a nonlinear co-channel interference canceller is proposed. In this algorithm, frequency offset is simultaneously estimated during delay profile estimation using the residual error of delay profile estimation. Computer simulations have been performed to demonstrate the performance of the proposed algorithm. It is shown that the allowable frequency offset range of the proposed canceller is over 10 times as large as that of the conventional canceller without frequency offset compensator.

Cochannel interference is a major impairment that limits the capacity of microcellular radio systems. This paper proposes a trellis-coded cochannel interference canceller, which uses trellis-coded modulation with maximum likelihood sequence estimation of both the desired signal and cochannel interference. This technique permits operation even when the signal-to-interference ratio is 0 dB. Computer simulation results show that this technique significantly reduces the bit-error rate in microcellular systems with Nakagami-Rice fading, particularly when the average signal-to-interference ratio is 0 dB. However, the technique requires: (1) synchronization of the interference and the desired signal and (2) higher computational complexity than standard techniques.

It is highly desirable to develop an efficient and flexible dynamic channel assignment algorithm in order to realize an integrated traffic TDMA mobile radio communication network. In this paper, an integrated traffic TDMA system is studied in which transmission of voice and data are assumed to occupy one and n time slots in each TDMA frame, respectively. In general, two types of channel (time slot) assignment algorithms are used: the partitioning algorithm and the sharing algorithm. However, they are not well-suited to the multimedia traffic consisting of various information sources that occupy different number of slots per frame. In this paper, assuming that voice is much more sensitive to transmission delay than data, an algorithm based on the sharing algorithm with flexible time slot management scheme is proposed as follows. To reduce the blocking and forced termination probabilities of a voice call, part of the slots of data calls are transferable to voice calls during congestion periods. At the same time, to reduce the handoff-failure of a multi-slot data call, the required number of data slots can be reduced adaptively according to the available number of slots during its handoff. These data calls whose number of slots are reduced either for voice calls or its handoff are allowed to get restored when possible. Computer simulations confirm the good performance of the proposed algorithm when compared to conventional channel assignment algorithms.

A new dynamic channel allocation algorithm with transmitter power control is proposed. The algorithm tries to allocate a channel to which subsequent power control can be applied most effectively. Namely, among the channels whose carrier-to-interference ratio (C/I) exceeds the C/I threshold by at least power control range /spl Delta/P, the channel with minimum C/I is allocated and its power is decreased by /spl Delta/P thereafter. By properly selecting /spl Delta/P, this algorithm has been confirmed to realize very low transmitter power, which guarantees a longer battery life in hand-held terminals, without sacrificing other performance compared to conventional algorithms.>

The so-called irreducible error due to frequency-selective fading is known to have a serious effect on mobile radio communication systems. Thus, the analysis of such errors is a prerequisite for making high-speed digital signal transmission over a fading channel feasible. The authors attempted to elucidate the physical mechanisms causing such errors using laboratory measurements of microscopic bit error rate. The results clarified some sources of burst errors in a multipath fading channel. In particular, large and rapid timing fluctuations of eye pattern due to severe delay distortion of the multipath channel was found to be the most dominant source of error.>

This paper describes a new modulation technique called DSK (double phase-shift keying) which shows an interesting antimultipath feature over mobile frequency-selective fading environments. The effect of this technique is shown for various multipath channels by detection waveform and theoretical bit error rate (BER) analyses. A result obtained from a laboratory test is also presented. In the latter part of the paper, a generalized form of DSK is described and extensive investigations are made so as to clarify the anti-multipath effect of DSK in view of the diversity concept and to further explore the possibility of a narrowband scheme. A major result shows that this technique can give a kind of diversity effect which is implicit in the arrival of signals via multiple paths with different delays.