We propose a new radio resource management algorithm which aims at minimizing the base station supply power consumption for multi-user MIMO-OFDM. Given a base station power model that establishes a relation between the RF transmit power and the supply power consumption, the algorithm optimizes the trade-off between three basic power-saving mechanisms: antenna adaptation, power control and discontinuous transmission. The algorithm comprises two steps: a) the first step estimates sleep mode duration, resource shares and antenna configuration based on average channel conditions and b) the second step exploits instantaneous channel knowledge at the transmitter for frequency selective time-variant channels. The proposed algorithm finds the number of transmit antennas, the RF transmission power per resource unit and spatial channel, the number of discontinuous transmission time slots, and the multi-user resource allocation, such that supply power consumption is minimized. Simulation results indicate that the proposed algorithm is capable of reducing the supply power consumption by between 25% and 40%, dependend on the system load.

Due to the data traffic explosion, cellular networks tend to have larger cell density and smaller cell size, which will give rise to significant intercell interference. In this work, we focus on receiver side signal processing techniques to perform intercell interference suppression and successive interference cancellation, where adaptive antenna arrays are used to develop a receiver beamforming approach within an iterative receiver. Specifically, the matrix inversion for calculating the beamforming weights is avoided, which drastically reduce the computational complexity.

In this paper, a novel scheduling mechanism that enhances both network spectral and energy efficiency is presented. In Pareto Optimal Scheduling (POS), mobile stations (MSs) are scheduled based on path gains such that the sufficient conditions for Pareto optimal power control (POPC) are fulfilled. This is performed in such a manner to maximise the number of concurrently transmitting MSs. Furthermore, a Stepwise Removal (SR) algorithm is introduced for the situation where links do not meet the sufficient conditions for power control. In this case, links are removed in order for other MSs to achieve their signal-to-interference-plus-noise ratio (SINR) targets. The targets of these remaining MSs are updated to prevent losses in system spectral efficiency caused by the link removals. Large network simulation results show that significant gains in spectral efficiency can be achieved over standard power control techniques, while additionally providing substantially improved energy efficiency.

In this paper, Pareto Femto-Cell Scheduling (PFCS), a novel scheduling mechanism for randomly deployed femto-cells, is presented. Here, the signal-to-interference-plus-noise ratio (SINR) targets of femto-users are adapted such that the sufficient conditions for Pareto optimal power control (POPC) are fulfilled. Furthermore, interference from full bandwidth users is managed such that as many mobile stations (MSs) as possible can transmit. Due to the random nature of femto-base station (FBS) deployment, interference graphs are used to group femto-cells (and hence, users) such that target spectral efficiencies can be achieved at Pareto optimum power. Simulation results show that PFCS achieves significant system capacity gains over other SINR-target-based power allocation techniques, while maximising coverage in dense mobile environments. Furthermore, substantial power savings can be achieved.

The principles and benefits of soft decisions are well known and widely applied. The advantage of knowing that a decision is reliable or not is obvious. Belief propagation within a factor graph enables a unified use of soft information for both channel estimation and data detection. However, if soft information does not reflect the true reliability of a decision, the achievable performance may degrade. In this paper, the calculation of reliability information is refined to consider the event of unreliable soft decisions. The proposed solution is based on the mean bit error probability calculated after each iteration and integrates nicely within the existing factor graph. Simulation results are provided to illustrate the performance gains.

With the explosion of wireless communications in number of users and data rates, the reduction of network power consumption becomes more and more critical. This is especially true for base stations which represent a dominant share of the total power in cellular networks. In order to study power reduction techniques, a convenient power model is required, providing estimates of the power consumption in different scenarios. This paper proposes such a model, accurate but simple to use. It evaluates the base station power consumption for different types of cells supporting the 3GPP LTE standard. It is flexible enough to enable comparisons between state-of-the-art and advanced configurations, and an easy adaptation to various scenarios. The model is based on a combination of base station components and sub-components as well as power scaling rules as functions of the main system parameters.

We consider in this paper downlink scheduling at the Medium Access Layer (MAC) layer for delay sensitive traffic in wireless systems based on Orthogonal Frequency Division Multiple Access (OFDMA). We refer in particular to next generation wireless systems such as 3GPP-LTE. We propose an Opportunistic Packet Loss Fair (OPLF) scheduling algorithm based on calculating a simple dynamic priority function which depends on the Head of Line (HOL) packet delay, the packet loss rate (PLR) and the achievable instantaneous downlink rate of each user. This algorithm overcomes the main limitations of existing algorithms, achieving better performance than state-of-the art algorithms, such as Modified Largest Weighted Delay First (M-LWDF), Proportional Fair (PF), and Packet Loss Fair (PLF), in terms of throughput, PLR and fairness among users.

In this paper bandwidth efficient pilot design for a MIMO-OFDM downlink is addressed. By exploiting the spatial correlation of signals originating from neighboring transmit antennas, the principle of pilot aided channel estimation (PACE) by two-dimensional (2D) interpolation in time and frequency is extended to the spatial domain; giving rise to three-dimensional (3D) PACE. By invoking the multi-dimensional sampling theorem conditions for regular pilot patterns to attain minimum overhead are derived. Provided sufficient spatial correlation 3D-PACE reduces pilot overhead by 50% compared to conventional 2D-PACE. Unfortunately, spatial correlation may not be universally available: while for outdoor macro-cells, where transmit antennas are typically mounted above rooftop, the assumption of spatially correlated channels is justified, in indoor deployments where spatial correlation tends to be low, spatial interpolation may not be feasible. On the other hand, in indoor environments cell sizes together with mobile velocities are substantially smaller, giving rise to lower Doppler and channel delay spreads. We show that a sophisticated pilot design is able to retain the low pilot overhead of 3D-PACE by exploiting these heterogeneous correlation properties, in the way that high spatial correlation compensates for low correlation in time and/or frequency, and vice versa.

A graph-based receiver is presented that iteratively performs soft channel estimation and soft data detection. Reliability information of data symbols is utilized to improve channel estimation, and in turn, soft channel estimates refine data symbol estimates. The proposed multi-dimensional factor graph introduces transfer nodes that exploit correlation of adjacent channel coefficients in an arbitrary number of dimensions (e.g. time, frequency, and spatial domain). This establishes a simple and flexible receiver structure that facilitate soft channel estimation and data detection in multi-dimensional dispersive channels, and supports arbitrary modulation and channel coding schemes. Simulation results demonstrate that the proposed multi-dimensional graph-based receiver outperforms iterative and non-iterative state-of-the-art receivers.

The sum rate of spectrum-sharing in decentralized and self-organizing wireless networks is investigated in this paper. Such networks pose the following two fundamental challenges: 1) cochannel interference and 2) the hidden node problem. For a slotted shared wireless medium, where resources are partitioned into time-frequency slots, time-multiplexed receiver initiated busy burst (BB) transmissions solve these problems by establishing an exclusion region around an active receiver by means of receiver feedback. The size of this exclusion region is controlled by an interference threshold that determines whether a user is allowed to transmit on a specific time-frequency resource unit. We propose a novel approach for setting the interference thresholds based on a heuristic derived for a two-link network. First, for two-links, the optimum threshold value is derived that maximizes the sum rate. Second, for multiple links, the new heuristic threshold that only relies on locally available information is derived. It is demonstrated via simulations that heuristic thresholding achieves superior sum rate compared to a fixed system-wide threshold. To complement simulation results, an analytical approach is developed to approximate the probability density function (pdf) of the sum of the interferers in BB setting with fixed threshold with a cumulant-based shifted log normal fitting method.

For cellular networks, the increased cell density and decreased cell size will give rise to significant inter-cell interference. In this work, an adaptive antenna array and an iterative receiver are combined to facilitate inter-cell interference suppression and cancellation at the receiver end. Iterative receivers exchange soft information between different receiver modules, e.g., symbol detector and channel decoder. An adaptive antenna array is integrated within the iterative receiver, which performs interference suppression and successive interference cancellation. The beamforming weight vectors are updated in each iteration using interference rejection combing, which takes into account the soft information from the channel decoder and previously estimated interfering signals. The proposed algorithm can effectively suppress and cancel the interference signal with various signal-to-interference ratios under different channel models.

We consider single cell multi-user OFDMA downlink resource allocation on a flat-fading channel such that average supply power is minimized while fulfilling a set of target rates. Available degrees of freedom are transmission power and duration. This paper extends our previous work on power optimal resource allocation in the mobile downlink by detailing the optimal power control strategy investigation and extracting fundamental characteristics of power optimal operation in cellular downlink. We find that only a system wide allocation of transmit powers is optimal rather than on link level. The allocation strategy that minimizes overall power consumption requires the transmission power on all links to be increased if only one link degrades. Furthermore, we show that for mobile stations with equal channels but different rate requirements, it is power optimal to assign equal transmit powers with proportional transmit durations. To relate the effectiveness of power control to live operation, we take the power model into consideration which maps transmit power to supply power. We show that due to the affine mapping, the solution is independent of the power model. However, the effectiveness of power control measures is completely dependent on the underlying hardware and the load dependence factor of a base station (instead of absolute consumption values). Finally, we conclude that power control measures in base stations are most relevant in macro stations which have load dependence factor of more than 50%.

In order to quantify the energy savings in wireless networks, the power consumption of the entire system needs to be captured and an appropriate energy efficiency evaluation framework must be defined. In this paper, the necessary enhancements over existing performance evaluation frameworks are discussed, such that the energy efficiency of the entire network comprising component, node and network level contributions can be quantified. The most important addendums over existing frameworks include a sophisticated power model for various base station (BS) types, which maps the RF output power radiated at the antenna elements to the total supply power of a BS site. We also consider an approach to quantify the energy efficiency of large geographical areas by using the existing small scale deployment models along with long term traffic models. Finally, the proposed evaluation framework is applied to quantify the energy efficiency of the downlink of a 3GPP LTE radio access network.

With considerable interest being garnered in recent years, femto-cells are seen as a major contender to significantly increase capacity and fill coverage holes in 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) cellular networks. User- deployed femto-cells, each exclusively serving a set of registered users and sharing the same frequency spectrum as the overlay macro-cells are already defined in 3GPP specifications. Such a co- channel and random deployment of femto-cells can cause heavy downlink (DL) interference to a user equipment (UE) in the vicinity of one or more femto-cells and not belonging to their closed subscriber groups (CSGs). In this paperwe focus on protection of the most important LTE DL control channel, known as the physical control format indicator channel (PCFICH). Failure to decode this channel correctly results in the loss of the subsequent subframe. A technique which mitigates femto-to-macro PCFICH interference by carefully manipulating the physical cell identity (PCI) of the aggressor femto-cell is presented. It is shown that employing this technique results in a significant improvement of PCFICH effective signal- to- interference-plus-noise ratio (SINR) without the need for any additional network signaling.

This paper elaborates on self-organized and distributed interference management for femtocells that share the available radio resources with macrocells. A multi-agent learning approach is examined, based on distributed Q-learning, where femtocell base stations control their transmit power, such that the femtocell capacity is maximized, while the aggregated downlink interference generated at macro users' receivers is maintained within acceptable limits. The distributed Q-learning algorithm is carried out at the femto nodes, in the way that the interference is controlled at each resource block. The contribution of this work is to integrate multi-user scheduling in the operation of the macrocell network, so that instantaneous changes, with 1 ms granularity, are encountered in the perception that the femtocell agents get of the environment under observation. We demonstrate that, by relying on 3GPP Long Term Evolution (LTE) compliant signalling from the macro network on the intended macrocell scheduling policies, the proposed learning approach allows each femto node to react on these instantaneous changes in the environment, such that the femto-to-macro interference is appropriately controlled.

In this paper, a novel decentralised coordinated beamforming technique for interference mitigation in multi-user multiple-input-multiple-output (MIMO) networks is proposed. When the available spectrum is fully reused in all cells, the base station (BS) can potentially activate a beam which causes detrimental co-channel interference (CCI) towards mobile stations (MSs) served by neighbouring cells. This problem is solved by requiring each active MS to transmit a busy burst (BB) in a time-multiplexed feedback channel using the spatial precoder utilised for receiving data. Likewise, the BS must scan the feedback channel using the same precoder which it intends to use for transmitting data in the next slot. Provided that channel reciprocity holds, the feedback channel and the data channel become reciprocal. This enables the BS to listen to the feedback channel and ascertain the amount of CCI it potentially causes towards the active MSs if it were to apply the particular spatial precoder. The BS is permitted to utilise only such precoders that cause CCI lower than a predetermined threshold. The impact of the threshold parameter for selecting spatial precoder in cellular networks is investigated and a novel heuristic for setting the thresholds in the downlink (DL) mode is proposed. It is demonstrated that the proposed heuristic achieves a 14% increase in spectral efficiency and a 6.3-fold increase in cell-edge user throughput compared to a transmit beamforming approach without interference awareness.

We consider resource allocation over a wireless downlink where Base Station (BS) power consumption is minimized while upholding a set of required link rates. A Power and Resource Allocation Including Sleep (PRAIS) method is proposed that combines resource sharing, Power Control (PC), and Discontinuous Transmission (DTX), such that downlink power consumption is minimized, which can be formed into a convex optimization problem. Unlike conventional approaches that aim at minimizing transmit power, in this work the BS mains supply power is chosen as the relevant metric. Based on a linear power model, which maps a certain transmit power to the necessary mains supply power, we quantify the fundamental limits of PRAIS in terms of achievable BS power savings. The fundamental limits are numerically evaluated on link level for four sets of BS power model parameters representative of envisaged future hardware developments. We establish an expected lower limit for PRAIS of 27W to 68W depending on load per link for BSs installed in 2014, which provides a 61% to 34% gain over conventional resource allocation schemes.

Iterative receiver structures that jointly perform channel estimation and decoding promise substantial performance gains. However, these gains only materialize with sufficiently accurate initial channel estimates. In this paper, initialization by multi-objective particle swarm optimization (MOPSO) is investigated. MOPSO supports low-complexity initial channel estimation with superimposed training symbols. Furthermore, it is shown that MOPSO works well in rank-deficient scenarios with arbitrary training sequences. Numerical results validate the performance enhancement of MOPSO initialization integrated within a graph-based iterative receiver.

We address interference avoidance by resource partitioning in densely deployed femtocell networks. The main objective is to protect user equipments (UEs) that are located near the cell boundary of two or more femtocells from detrimental downlink interference. The available frequency bands are divided into subbands that are distributed among femtocells in a way that directly adjacent cells do not occupy the same subbands. For this purpose, a novel centrally controlled resource partitioning method is developed based on graph coloring that assigns subbands in terms of resource efficiency. The proposed algorithm strikes a balance between interference protection and spatial frequency reuse of subbands, and is well suited for randomly deployed wireless networks. System-level simulations reveal that cell edge capacities are significantly boosted without causing a degradation in average system throughput.

Femto-cells promise substantial gains in system spectral efficiency due to an enhanced reuse of radio resources. Femto-cells may be employed in a co-channel fashion such that the bandwidth used by the macro-cell is reused by the femto-cell. Femto-cells maintain an exclusive list of user equipments (UEs) that they serve. In this case, a foreign macro UE trapped inside a femto-cell experiences heavy interference from this femto-cell. As per the current 3rdGeneration Partnership Project (3GPP) discussions for Rel-10, this paper investigates the issue of femto-to-macro interference on the control channel. The control channel is very crucial because if it is incorrectly decoded, the following data region is completely lost. In line with the 3GPP discussions on the inter-cell interference coordination (ICIC) topic, interference-reducing techniques that are backward compatible with Rel-8/9 UEs are introduced in this paper. Employing these techniques results in a significant improvement of control channel performance for trapped macro UEs.

A novel decentralized coordinated multipoint (CoMP) technique facilitated by receiver feedback is proposed. If multiple base stations (BSs) access the same radio resource; downlink transmissions from one BS cause co-channel interference (CCI) towards towards users served by neighboring BSs. To mitigate CCI in a multiple-input-multiple-output (MIMO) network, the mobile station (MS) scheduled in a tagged cell (i.e. the active user) emits a busy burst (BB) on a time-multiplexed feedback slot, applying the precoder used earlier for receiving data. Prior to transmission a neighboring BS must scan the feedback slot using the spatial precoder(s) intended for data transmission. Consequently, in a time division duplex (TDD) system the channels of the data and the feedback slots become reciprocal. Therefore, by listening to the feedback slot, a neighboring BS can predetermine the CCI potentially imposed towards the active user for each of its available precoders. This information enables the BS to select spatial precoders autonomously so that CCI is limited to a predetermined threshold. Simulation results demonstrate that the proposed technique achieves 20% increase in spectral efficiency, 6-fold increase in cell-edge user throughput, and 40% reduction in transmitted energy, compared to transmit beamforming without interference coordination.

In shared wireless medium access, receiver initiated busy burst (BB) transmission mitigates the problem of co-channel interference. Receiver feedback allows potential transmitters to ascertain the amount of interference their transmission would cause to victim receivers in adjacent communication links. The BB concept mitigates excessive co-channel interference, by maintaining an exclusion region around any victim receiver: interferers within the region must refrain from accessing the shared wireless medium. In this paper, an analytical framework for interference modelling of the BB protocol is developed, via cumulant-based shifted log normal approximation of its probability density function (pdf). The interference pdf is then applied to derive the pdf of the signal-to-interference ratio (SIR).

We address joint channel estimation and data detection based on factor graphs. The considered graph-based approach utilizes reliability information of channel estimates to facilitate soft-output data detection, and in-turn reliability information about the data symbols is taken into account for channel estimation. In this paper graph-based soft channel estimation and detection is extended to an OFDM based air interface, where the channel response varies in two dimensions; time and frequency. Initial channel estimates obtained by training symbols are conveyed by a two dimensional (2D) factor-graph in time and frequency with only a linear increase in complexity. The required training overhead for the proposed 2D graph-based soft channel estimation scheme may be substantially reduced by taking the redundancy introduced by the channel coding into account.

Decentralized synchronization requires cooperation among network participants so that all nodes agree on a common reference timing. What happens if one node does not follow local synchronization rules and randomly transmits? This paper studies the resilience of two classes of decentralized slot synchronization against random disturbances, and quantifies the impact of the random behavior. It is shown that the coupling strength is a key factor for resilience, and that the synchronization approach based on the theory of coupled oscillators generally behaves better and is more robust than the approach that updates clocks based on the average neighboring timings.

Mobile communications are increasingly contributing to global energy consumption. The EARTH (Energy Aware Radio and neTworking tecHnologies) project tackles the important issue of reducing CO2emissions by enhancing the energy efficiency of cellular mobile networks. EARTH is a holistic approach to develop a new generation of energy efficient products, components, deployment strategies and energy-aware network management solutions. In this paper the holistic EARTH approach to energy efficient mobile communication systems is introduced. Performance metrics are studied so to assess the theoretical bounds of energy efficiency and the practical achievable limits. Moreover, various deployment strategies focusing on their potential to reduce energy consumption are studied, whilst providing uncompromised coverage and user experience. This includes heterogeneous networks with a sophisticated mix of different cell sizes, which may be further enhanced by energy efficient relaying and base station cooperation technologies. Finally, scenarios leveraging the capability of advanced terminals to operate on multiple radio access technologies (RAT) are discussed with respect to their energy savings potential.

This paper presents a biologically inspired approach for distributed slot synchronization in wireless networks. This is facilitated by modifying and extending a synchronization model based on the theory of pulse-coupled oscillators. The proposed Meshed Emergent Firefly Synchronization (MEMFIS) multiplexes synchronization words with data packets and adapts local clocks upon the reception of synchronization words from neighboring nodes. In this way, a dedicated synchronization phase is mitigated, as a network-wide slot structure emerges seamlessly over time as nodes exchange data packets. Simulation results demonstrate that synchronization is accomplished regardless of the arbitrary initial situation. There is no need for the selection of master nodes, as all nodes cooperate in a completely self-organized manner to achieve slot synchrony. Moreover, the algorithm is shown to scale with the number of nodes, works in meshed networks, and is robust against interference and collisions in dense networks.

In this paper, challenges regarding the provision of channel state information (CSI) on a multi-user (MU) MIMO-OFDM downlink are addressed. Reference symbols (pilots) support coherent detection at the receiver and enable link adaptation at the transmitter, but add overhead and consume transmit power. As spatial transmit processing combined with multiple access essentially requires the provision of dedicated and common pilots, i.e. pilots that include respectively exclude user-specific transmit processing, the potential gains of MU-MIMO may partly be canceled by the incurred pilot overhead. Fortunately, spatial correlation at the transmitter has the potential to substantially reduce overheads, by introducing spatial reuse of dedicated pilots, and by utilizing spatial interpolation of common pilots. We demonstrate that a reduction in overhead is traded with compromised channel estimation accuracy, which particularly limits the attainable spectral efficiency in the high SNR regime. This implies that a bandwidth efficient pilot design is to be complemented by advanced channel estimation schemes, which enhance accuracy by utilizing data symbols together with pilots.

Reference symbols (pilots) support coherent detection, but add overhead and consume transmit power. A sophisticated pilot design needs to trade-off the attainable channel estimation accuracy with the induced losses in bandwidth & power efficiency. MIMO transmission, transmit beamforming, and spatial multiplexing further complicate pilot design. In this paper a unified analytical framework is developed to evaluate the attainable spectral efficiency of different pilot types for MIMO- OFDM employing fixed transmit beamforming.

Interference aware user scheduling in fixed grid-of-beam (GoB) transmission is envisaged to significantly benefit from the receiver initiated busy burst (BB) protocol. Fixed GoB scheduling relies on the knowledge of the location of the intended users as well as the vulnerable users which effectively is provided by the BB protocol via exploitation of channel reciprocity. This paper studies the hybrid BB and GoB (BB + GoB) approach in a Manhattan environment. The new proposed hybrid interference avoidance scheme with an underlying score based scheduler is evaluated by means of system level simulations, and is compared against a pure GoB approach with the same scheduler as well as the BB based interference avoidance techniques applied to omnidirectional antennas. The results show an improvement in both system throughput and fairness (defined as cell edge user throughput). In particular, system throughput of up to 238.5 Mbps/cell or user throughput of up to 8.88 Mbps/user for the lower 10%-ile of users are shown to be feasible. In particular, the hybrid BB + GoB scheme exhibits a 16-fold improvement at the lower 10%-ile compared to pure GoB technique.

This paper presents a novel approach for setting the thresholds for busy burst (BB) enabled interference avoidance in TDD (time division duplex) systems. One of the key issues in the BB concept is the appropriate choice of the threshold, which determines whether a user is allowed to transmit on a specific time-frequency resource unit. For a two-link network we derive the optimum threshold value that maximises the sum rate. Furthermore, for multiple links, a new heuristic threshold that only relies on locally available information is derived. It is demonstrated via simulations that the heuristic threshold provides superior sum rate than a fixed system-wide threshold.

This paper presents the integration of power control and the busy burst (BB) technique for a TDD OFDMA (time division duplex orthogonal frequency division multiple access) system in an urban Manhattan grid type of environment. The objective is to combine power control with the BB protocol to develop a power efficient interference-avoidance technique. Previous work has demonstrated that the time multiplexed BB can be used to achieve interference aware medium access. It has also been demonstrated in other work that the implicit feedback mechanism of BB can be exploited to achieve Pareto optimal SINR-based (signal-to-interference-plus-noise ratio) power control. However, these two applications are not compatible the former depends on fixed signal powers across users, while the latter makes use of variable signal power to achieve optimum power control. This paper proposes an amendment to the BB technique that allows the two approaches to co-exist by exploiting the frequency granularity of OFDMA to frequency multiplex two BBs one with variable and one with fixed power. The resulting control protocol exhibits an eight-fold reduction at the 50th percentile of used energy with no additional overhead as compared to the single BB protocol.

We address pilot aided channel estimation (PACE) for MIMO-OFDM operating in spatially correlated channels. Pilot symbols are only inserted on selected transmit antennas, and the channel response on the remaining transmit antennas is obtained by means of interpolation. By extending the principle of channel estimation by interpolation to the spatial domain, three dimensional (3D) PACE operates in time, frequency and space. Provided an appropriately dimensioned pilot grid, 3D-PACE is shown to substantially reduce the required pilot overhead without sacrificing channel estimation accuracy.

In this paper the capacity of decentralized wireless networks is addressed. An exclusion region is introduced that protects active receivers from destructive interference of nearby transmitters. An exclusion range imposes an upper bound on the interference that a transmitter may cause to receivers of competing links. While an exclusion region avoids excessive interference and thus improves capacity per link, the spatial reuse in terms of concurrently served links is compromised. The resulting trade-off is elaborated by computer simulations, so to optimize the exclusion range as a function of the user density and maximum transmit power. We demonstrate that by an appropriately specified exclusion range, the network capacity is substantially enhanced. In this context, it has been found that the exclusion range that maximizes the system capacity does not vary greatly when changing the a priori user density. In addition, the trade-off between maximizing the system capacity and maintaining fairness is investigated.

This paper analyses busy burst (BB) enabled interference avoidance for multi-user orthogonal frequency division multiple access (OFDMA) in a Manhattan grid deployment scenario. Upon successful reception of data, the receiver transmits a BB in a time-multiplexed mini-slot. Exploiting the channel reciprocity of time division duplex (TDD), an exclusion region around a victim receiver is established, where potential transmitters are denied access to reserved resources. The size of the exclusion region is determined by a threshold parameter, known to the entire network. System level simulations compare the system and user throughput of BB enabled interference avoidance with greedy resource allocation that does not attempt to avoid interference. It is shown that by adjusting the BB threshold parameter, system throughput can be traded with fairness in terms of cell-edge user throughput. By tuning the BB threshold, either up to 45% increase in system throughput or up to 7-fold increase in cell-edge user throughput are feasible compared to greedy resource allocation. Moreover, by an appropriate adjustment of the BB power further gains in system throughput without compromising fairness are achieved.

In this paper decentralized power control in ad hoc networks is addressed, so to maintain a certain carrier-to- interference ratio (C/I) at the receiver. The objective is to extend the Pareto optimal C/I power control algorithm to time division duplex (TDD) links, where each node in the network transmits and receives data to/from other nodes. By exploiting the channel reciprocity of TDD, we demonstrate that Pareto optimal C/I power control can be implemented without any explicit feedback of the achieved link quality.

Decentralized slot synchronization protocols inspired from nature have recently gained considerable interest. By adjusting the internal time reference of a node in response to the detected timing of a received synchronization word, and by following simple rules, synchronization emerges from an initially completely uncoordinated situation. However these protocols typically assume that synchronization words are detected error free. In this paper the performance of biologically inspired slot synchronization is investigated when a realistic timing synchronization scheme is employed. We consider an ad hoc network where nodes communicate over an orthogonal frequency-division multiplexing (OFDM) air interface. The transmission is organized in frames, and each frame is preceded by a synchronization preamble with known repetitive parts. Specifically one common synchronization preamble is employed for all transmitting terminals, as, e.g., for the wireless LAN standard 802.11. Computer simulations verify that provided a sufficiently long synchronization word, reliable slot synchronization is maintained using a practical synchronization unit.

Under the horizontal spectrum sharing scenario where two cognitive radios are able to find an unutilised resource and access to the resource is coordinated between them, it is shown in our earlier work that either single or simultaneous transmission at maximum power maximises system spectral efficiency. While in general suboptimal, sequential transmission, an average of two single transmissions, is also investigated as a more fair alternative to single transmission. In this paper, the system spectral efficiencies of simultaneous, single and sequential transmission schemes are compared for the 2-link scenario where the two communication pairs are chosen from uniformly distributed nodes on a disk. The simultaneous transmission scheme is shown to provide smaller expected system spectral efficiency for higher transmit power than sequential or single transmission schemes. Introducing additional interferers, however, significantly reduces the advantage of single and sequential schemes over simultaneous transmission. However, under the scenario of transmission to the closest neighbour, simultaneous transmission is shown to result in higher average system spectral efficiency than the other two schemes over the wide range of transmit powers.

The present article develops a decentralized inter-base station slot synchronization algorithm suitable for cellular mobile communication systems. The proposed algorithm is derived from the theory of pulse-coupled oscillators, commonly used to describe synchronization phenomena in biological systems, e.g. spontaneous synchronization of fireflies. In order to maintain synchronized base stations (BSs), even when there is no direct link between them, some selected user terminals (UTs) participate in the network synchronization process. Synchronization emerges thanks to two synchronization words, one transmitted by BSs and the other by UTs, and each group helping the other synchronize. It is demonstrated through simulations that in a typical indoor environment, synchronization is always achieved within 10 super-frames when coupling parameters are appropriately chosen, without any a priori assumption on the initial timing misalignments of BSs and UTs.

This paper analyzes the system spectral efficiency of a 2-link ad hoc network. The analysis shows that there are exactly three operating points that maximize system capacity: either both links transmit with maximum power simultaneously; or one single link transmits with maximum power while the other is silent. The impact of the scheduling policy on the system spectral efficiency is also studied: simultaneous transmission or sequential access where the two links share the medium by dedicated time/frequency slots without causing interference. An exhaustive numerical search over a wide range of pathloss parameters shows that sequential transmission is superior to simultaneous transmission in about two third of the cases. Even when simultaneous transmission outperforms sequential transmission, the gains are typically not substantial. Furthermore, we show that in case of simultaneous transmission power control causes only marginal degradation in terms of system spectral efficiency, while transmitting with maximum power pays off for sequential transmission. This highlights the importance of the joint optimization of power and rate adaptation together with scheduling for power constraint ad hoc networks.

In this paper we analyze the capacity of MIMO-OFDM systems that utilize pilot symbols to acquire estimation of the radio channel at the receiver. An analytical framework is developed to derive the optimal pilot boost and optimal pilot overhead over time-variant frequency selective channels. In addition, to illustrate the achievable capacity, several numerical examples are given assuming different pilot boosts, pilot overheads, and number of transmit antennas.

By the generalized multi-carrier (GMC) principle a unified framework to describe various multi-carrier as well as single carrier approaches is established. In this paper the GMC principle is extended to pilot design and channel estimation, so that a unified description of pilot aided channel estimation (PACE) by interpolation in time and frequency is established. This applies to frequency domain pilots which are embedded in the GMC signal, as well as pilots sequences time multiplexed with data-bearing GMC blocks. A comparative performance evaluation of the two different GMC variants OFDM and single carrier with various pilot allocation schemes is carried out, also taking into account practical constraints such as the peak to average power ratio (PAPR) and required power amplifier back-off. It is found that a serial modem with time or frequency multiplexed pilots could be designed with a power amplifier with about 2 dB lower maximum power rating than that of a corresponding OFDM modem.

Cross-layer optimization (CLO) promises significant gains in comparison to a conventional system design, which does not allow for information exchange across layers. One of the key challenges in CLO is the exchange of parameters between optimizer and layers. In this paper a model-based approach is presented that drastically reduces the amount of parameters that are to be exchanged. The optimizer employs models of the respective layers that emulate the communication system within the optimizer. The layers then only need to pass a small number of model parameters to the optimizer. This general concept is applied to CLO between application (APP) layer and medium access control (MAC) layer of a radio communications system. Our proposed model for the MAC layer is suitable for a transmitter without instantaneous channel state information (CSI). Simulation results demonstrate that the proposed model-based CLO is able to exploit the available diversity to enhance the system capacity. Dependent on the application characteristics, the same perceived quality in terms of mean opinion score (MOS) is maintained, while increasing the number of served users by up to 25%. Compared to known CLO approaches, much fewer parameters need to be exchanged.

This paper presents a new cross-layer approach to achieve active link adaptation and QoS support in ad hoc networks. Upon data reception in a transmission slot, receivers broadcast a feedback signal in a time-multiplexed mini-slot. The level of this feedback or busy signal is made dependent on the received signal from the intended transmitter and the required SINR (signal-to-interference-plus-noise ratio). Potential new interfering transmitters adjust their data transmit power based on the received busy signal power. It is demonstrated that due to the channel reciprocity in TDD (time division duplex) the received busy signal power is sufficient information to be able to maintain the SINR at the already existing links. Since multiple links are coupled via the busy signal, convergence and stability issues are analyzed.

This paper analyses the busy burst (BB) enabled interference avoidance mechanism applied to anadhocindoor scenario with time division duplex (TDD) mode of WINNER parameters. The BB algorithm exploits the channel reciprocity of the TDD mode. Upon successful data reception, a receiver broadcasts a busy signal. A potential interfering transmitter can directly infer the level of interference it would cause to the vulnerable receiver from the received BB signal power. The new transmitter can, thus, autonomously decide whether to transmit, or to refrain from transmission so that interference at the vulnerable receiver is below a given threshold. As a consequence, significant co-channel interference is avoided and the system performance is ameliorated. The results show that using the BB protocol, approximately a three fold increase in throughput and a significant reduction in packet delay and packet expiration rate is achieved compared to uncoordinated random medium access in suchadhocnetworks.

This paper presents a fully decentralized link adaptation algorithm to manage interference in an ad hoc network operating in TDD mode. The algorithm, called busy tone interference tolerance signaling, is of low complexity and easy to implement. Two functions are explored to set the busy tone signal power dependent on the level of tolerable interference, and their performance is compared with a system with fixed transmit powers. Results show that an appropriately chosen function for setting the busy-tone exhibits significantly enhanced power efficiency, implying that battery life can be extended, while providing a similar capacity than a fixed power system.

In this paper iterative channel estimation (ICE) for coded OFDM systems over dispersive channels is addressed. ICE utilizes data symbol estimates fed back from the channel decoder to refine initial channel estimates obtained by pilot symbols. In the presence of decision feedback errors, the performance of ICE may degrade severely. The proposed threshold controlled ICE avoids decision feedback effects by choosing between iterative and non-iterative channel estimates. The threshold is conditioned on the statistical information of the decoded data at the decoder output, as well as the filter coefficients and the channel statistics.

In this paper a reference timing is imposed onto a network that synchronizes in a completely distributed manner. Synchronization is not always obtained because normal nodes are deaf while transmitting and cannot always receive synchronization messages from reference nodes. To counter this unwanted effect the transmission scheme of reference nodes is modified so that synchrony can be ensured and all network members agree on the imposed reference timing

In this paper, an analytical framework is established to dimension the pilot grid for OFDM in terms of overhead and power allocation. With the constraint that the receiver solely relies on pilot symbols for channel estimation, the capacity for OFDM over time-variant frequency selective channels is maximized. The authors show that the maximum capacity is achieved by placing pilots with maximum equidistant spacing given by the sampling theorem, if perfect interpolation is assumed and pilots are appropriately boosted. Allowing for realizable and possibly sub-optimum estimators where interpolation is not perfect, a semi-analytical best pilot allocation strategy is presented.

Amplify-and-forward (AF) is a simple but effective relaying concept for multihop networks that combines transparency regarding modulation format and coding scheme with ease of implementation. Recently, we introduced an OFDM-AF schemes that achieves significant SNR gains by reordering groups of received subcarriers (chunks) at the relay station to optimally couple them into the 2n hop channel. This reordering process, however, destroys the correlation between chunks, so channel estimation techniques based on interpolation no longer work. To mitigate this problem, we propose to insert a new pilot grid at the relay station in addition to the existing pilot grid. At the mobile, channel estimation is performed in two steps, where the 2n and 1 hop channels are estimated separately. The proposed pilot and channel estimation scheme allows to fully exploit the gains through chunk reordering at the relay station with only a small increase in complexity.

This paper presents a new analytical framework for the analysis of inter-cellular timeslot allocation over TDMA (time division multiple access) based air-interfaces using TDD (time division duplexing). The analysis is used to evaluate the delay- throughput performance of an adaptive decentralized inter- cell interference mitigation technique that exploits the inherent channel reciprocity of TDD. The main principle is that receivers upon successful transmission of a packet transmit a busy burst on a succeeding minislot. Potential transmitters in neighboring cells sense the minislot prior to signal transmission. With this mechanism, inter-cell interference to the existing link is avoided. The performance of this MAC (medium access control) protocol is compared against blind timeslot allocation. The new MAC protocol is shown to facilitate network self-organization, offering superior delay and throughput compared to the state of the art, with modest overhead and complexity requirements.

In this paper a novel decentralized and interference aware medium access control (MAC) protocol combined with a dynamic subchannel selection algorithm for OFDM (orthogonal frequency division multiplexing) is presented. The protocol resolves several drawbacks of the existing radio resource allocation techniques for OFDM systems, such as the hidden and exposed node problem, and the requirement for a time-invariant channel. The proposed scheme enables the transmitter to determine the level of interference it would cause to already active links prior to any transmission. This is achieved through a busyslot signaling that exploits the channel reciprocity offered by the TDD mode. Compared to existing radio resource allocation schemes the required signaling overhead is significantly reduced. The interference awareness allows the system to avoid significant co-channel interference (CCI) and to operate with full frequency reuse. When applied to a broadband OFDM air interface, this principle can also exploit the frequency selective fading channel on the intended as well as the interference links. This results in an autonomous subcarrier allocation algorithm which can dynamically adapt to time-varying channels. Due to its decentralized nature, the algorithm can be utilized in both cellular and ad hoc networks. Simulation results for a cellular system show that the new decentralized medium access and channel assignment algorithm is capable of achieving spectral efficiencies of up to 2.25 bits/Hz/cell using 16 QAM (quadrature amplitude modulation).

This paper presents an extension for adaptive transmission of a multi-user scheduling algorithm to a cellular environment. In an effort to manage inter-cell interference, scheduling between neighboring base stations is coordinated. To this end, the amount of buffered data at each base station is exchanged within a small group of base stations. More precisely, the sectors that illuminate each cell form inter-sector scheduling groups. We divide the scheduling into two separate operations, inter and intrasector scheduling, that are performed separately. From a comparison against a scheme with a static frequency reuse of three we observe a 20-50% reduction in average packet delays, depending on the traffic model used.

This paper presents a new analytical framework for the analysis of intercellular timeslot allocation over TDMA (time division multiple access) based air-interfaces using TDD (time division duplexing). The analysis is used to evaluate the performance of an adaptive decentralized intercellular interference mitigation technique that exploits the inherent channel reciprocity of TDD. The main principle is that receivers upon successful transmission of a packet transmit a busy burst on a succeeding minislot. Potential transmitters in neighboring cells sense the minislot prior to signal transmission. With this mechanism, intercellular interference to the existing link is avoided. The performance of this MAC (medium access control) protocol is compared against blind timeslot allocation. The new MAC protocol is shown to facilitate network self-organization, offering superior delay and throughput compared to the state of the art, with modest overhead and complexity requirements

In this paper1, we investigate iterative channel estimation for coded OFDM over frequency-selective, time-varying channels. Provided that an initial channel estimate is available, iterative channel estimation (ICE) utilizes data symbol estimates fed back by the channel decoder. The efficiency of these iterative approaches, however depends on the reliability of the data estimates. In the presence of decision feedback errors, the channel estimation performance is severely degraded. We propose an optimized ICE based on the minimum mean squared error (MMSE) criterion that incorporates statistical information of decision feedback errors, readily available in the form of soft information. Moreover, an efficient implementation of the channel estimation unit is described. Simulation results indicate that the new scheme outperforms existing techniques in terms of channel mean-squared error (MSE) and bit error rate (BER).

In this paper we address channel estimation for uplink OFDM over frequency-selective, time-varying channels. In the considered multi-user uplink, data flows of several users are assigned to time-frequency slots, termed chunks. In this case, only dedicated pilots located in chunks associated to a specific user can be used for channel estimation. We investigate pilot-aided and iterative channel estimation techniques based on Wiener filtering and study the impact of the chunk size on the channel estimation performance. We show by means of numerical simulations that iterative approaches are particularly beneficial for channel estimation using dedicated pilots per chunk

In this paper pilot aided channel estimation (PACE) for OFDM is addressed. For PACE equidistantly spaced pilot symbols allow to reconstruct the channel response by means of interpolation. The optimum minimum mean squared error (MMSE) estimator performs smoothing and interpolation jointly. To reduce the complexity of the optimum MMSE estimator, we propose to separate the smoothing and interpolation tasks. The separated smoothing and interpolation estimator (SINE) consists of a MMSE based smoother which only operates at the received pilot symbols, and an interpolator which is independent of the channel statistics. We show that the separated approach gets close to the optimum MMSE, while the complexity is grossly reduced. However, at high SNR an error floor is observed, which is caused by edge effects, i.e. subcarriers near the beginning and end of the band suffer from an increased interpolation error.

In this paper we investigate the performance of a downlink multi-carrier code division multiple access (MC-CDMA) system employing a receiver with iterative channel estimation (ICE). The MC-CDMA system transmits M data symbols per user and per orthogonal frequency division multiplexing symbol in parallel, with each data symbol being spread with a Walsh-Hadamard code of length L. Due to the superposition of spread data signals (chips), zero-valued subcarriers can occur, which cannot be used to compute least-squares (LS) channel estimates in ICE. Therefore, we propose the modified LS channel estimation method to overcome this problem and to avoid noise enhancement in ICE. We further investigate the robustness of ICE with the modified LS method if the channel statistics are not known perfectly. Simulation results demonstrate that a signal-to-noise ratio gain of 1 dB can be achieved when employing robust ICE with the modified LS method.

In this paper1an efficient implementation of robust pilot-symbol aided channel estimation (PACE) for OFDM is presented. We propose to cyclically delay the received signal after removal of the cyclic prefix. The purpose of the cyclic delay is to match the received signal after OFDM demodulation to a channel estimation filter with real valued coefficients. Compared to the conventional approach which requires complex valued coefficients, the computational cost is cut by half, without any degradation in performance. Moreover, the impact of a timing synchronization unit on the considered channel estimation filter is examined.

In this paper, we investigate a gradient based joint detection algorithm for MC-CDMA. The algorithm is based on the improvement of a suboptimum initial estimation, which itself can be obtained from an already known low complexity detection algorithm. For the improvement strategy, the algorithm calculates the gradient of the Euclidean distance function between the rebuilt initially estimated transmission signal and the received signal and goes into the direction of the steepest descent in order to find an improved estimation, which is closer to the received signal. Simulation results show, that for 4-QAM modulation and a spreading length of 8, an error performance close to maximum likelihood detection can be achieved with a complexity which is about 26 times lower.

This paper addresses pilot-symbol aided channel estimation (PACE) for OFDM based systems employing multiple transmit antennas. Two approaches are investigated: first, all N/sub T/ transmit antennas use all available pilot symbols simultaneously, resulting in N/sub T/ superimposed signals; second, a subset of available pilots is allocated for each transmit antenna exclusively. The Wiener filter and the corresponding mean square error (MSE) are derived. By applying the sampling theorem, the minimum number of pilots required to estimate the channel of N/sub T/ superimposed waveforms is derived.

For OFDM based systems using cyclic delay diversity (CDD), a multiple input channel is transformed to a single input channel, with increased frequency selectivity. While CDD provides additional frequency diversity which can be utilized by the outer channel decoder, a conventional SISO channel estimator may have difficulties to track the increased dynamics of the received signal. In this paper, an enhanced scheme for pilot-symbol aided channel estimation (PACE) is presented, which utilizes the properties of CDD.

The concept of a multi-cell environment for an MC-CDMA downlink scenario is presented; it is based on a propagation model taking into account path loss and shadowing. The interference of adjacent cells is studied and the Gaussian approximation (GA) is investigated for simplifying the interference simulation. The major influence of the interfering cells is restricted to the peripheral area of the desired cell. In particular, it is shown, by simulations, that the GA for the impinged interference signals is an appropriate assumption. This drastically decreases the simulation complexity for a high number of interfering cells, e.g., one or two tiers around the desired cell.

We address pilot-symbol aided channel estimation (PACE) for orthogonal frequency division multiplexing (OFDM) based systems, employing multiple transmit antennas. A novel approach is presented which estimates and separates N/sub T/ superimposed signals, corresponding to N/sub T/ transmit antennas, by exploiting the correlation in two dimensions of the received signal. More specifically, we apply the concept of using two one-dimensional (1D) estimators instead of a truly two dimensional (2D) estimator, by dividing the estimation as well as the separation task into two stages. The first stage separates a subset of the N/sub T/ superimposed signals, together with estimating the channel response in the first dimension. The second stage further separates the signals of each subset in the second dimension, yielding an estimate for all transmit antennas. For a space-time block coded OFDM system we demonstrate that the proposed estimator can track the channel variations even at high Doppler frequencies.

In this paper the trade-off between time diversity and the inter-carrier interference (ICI) in an OFDM based MC-CDMA system is discussed. Our approach is to partially shift each spectrum of an OFDM symbol at the transmitter by using different phase shifts in the time domain at different antennas after the OFDM modulation. The summary signal at the receiver has a wider Doppler spectrum as the result of different uncorrelated Rayleigh fading channels. This leads to an artificially increased selective time channel transfer function, which is more favorable for the channel decoder at the receiver. The trade-off between Doppler diversity and ICI is found at about 9% maximum Doppler spread of the subcarrier distance for flat fading channels and frequency selective fading channel models. The distance to the independent Rayleigh channel bound by applying an MMSE detector is less than 2 dB for convolutional coding and about 2 dB for turbo coding at a BER of 10/sup -3/ for flat fading channels. For frequency selective fading channels this approaches to less than 1 dB to the independent Rayleigh channel bound.

We addresses channel estimation based on the discrete Fourier transform (DFT) applied to OFDM-based MIMO systems. By exploiting the properties of the DFT, channel estimation schemes for MIMO-OFDM system can be simplified. The Fourier transform translates phase shifts in the frequency domain to delays in the time domain. In order to exploit this feature, phase shifted pilot sequences are a perfect match to the Fourier transform in terms of separating the N/sub T/ superimposed signals, corresponding to N/sub T/ transmit antennas, without any further processing. The proposed estimator is shown to be optimum for the sample spaced channel, i.e. the channel tap delays are multiples of the sampling duration. A sub-optimum approximation for the non-sample spaced channel is also suggested.