Downlink space-frequency preequalization techniques for TDD MC-CDMA mobile radio systems

The paper considers downlink space-frequency preequalizations techniques for time division duplex (TDD) MC-CDMA. We consider the use of antenna arrays at the base station (BS) and analytically derive different preequalization schemes for two different receiver configurations at the mobile terminal: a simple despread receiver without channel equalization and an equal-gain combiner (EGC) conventional receiver. We show that the space-frequency preequalization approach proposed allows to format the transmitted signals so that the multiple access interference at mobile terminals is reduced allowing to transfer the most computational complexity from mobile terminal to the BS. Simulation results are carried out to demonstrate the effectiveness of the proposed preequalization schemes.European project IST-2001-32620 - MATRICEFCT - POSI/CPS/46701/200

Stability properties of network diversity multiple access with multiple-antenna reception and imperfect collision multiplicity estimation

In NDMA (network diversity multiple access), protocol-controlled retransmissions are used to create a virtual MIMO (multiple-input multiple-output) system, where collisions can be resolved via source separation. By using this retransmission diversity approach for collision resolution, NDMA is the family of random access protocols with the highest potential throughput. However, several issues remain open today in the modeling and design of this type of protocol, particularly in terms of dynamic stable performance and backlog delay. This paper attempts to partially fill this gap by proposing a Markov model for the study of the dynamic-stable performance of a symmetrical and non-blind NDMA protocol assisted by a multiple-antenna receiver. The model is useful in the study of stability aspects in terms of the backlog-user distribution and average backlog delay. It also allows for the investigation of the different states of the system and the transition probabilities between them. Unlike previous works, the proposed approach considers the imperfect estimation of the collision multiplicity, which is a crucial process to the performance of NDMA. The results suggest that NDMA improves not only the throughput performance over previous solutions, but also the average number of backlogged users, the average backlog delay and, in general, the stability of random access protocols. It is also shown that when multiuser detection conditions degrade, ALOHA-type backlog retransmission becomes relevant to the stable operation of NDMA

Low complexity scheduling algorithm for the downlink of distributed antenna systems

In this paper we present a low-complexity user selection algorithm for the downlink of a distributed antenna system (DAS) that achieves an optimum solution for a weighted matching problem. The user selection process is modeled as a linear sum assignment problem (LSAP). The proposed solution consists of two phases. In the first phase, a set of potential users to be scheduled is found by combining two complementary approaches: greedy and minimum-throughput-loss selection. In the second phase, the set of scheduled users is refined by selecting the users that maximize sum throughput. We provide numerical results to confirm the optimality of our user selection algorithm and to compare its performance with existing solutions

Joint design of RFID reader and tag anti-collision algorithms: a cross-layer approach

This paper investigates the potential interactions between reader and tag anti-collision algorithms of passive RFID (radio frequency identification) systems. Conventionally, reader and tag anti-collision algorithms are designed by assuming that they are independent from each other. In practice, however, readers and tags usually operate in the same frequency band. Therefore, contention between their transmissions can also potentially arise. Furthermore, reader anti-collision policies directly influence the way in which tags are activated, and thus also the way in which they collide when responding to reader’s requests. In view of this and considering the growing numbers of readers and tags, independence of both schemes can not longer be considered as a realistic assumption. This paper partially fills this gap by proposing a new cross-layer framework for the joint evaluation and optimization of reader and tag anticollision algorithms. Furthermore, the paper proposes a new approach, based on a Markov model, which allows capacity and stability analysis of asymmetrical RFID systems (i.e., when readers and tags experience different channel and queuing states). The model captures the dynamics of tag activation and tag detection processes of RFID. It also represents a first step towards a joint design of physical (PHY) and medium access control layers (MAC) of RFID. The results indicate that the proposed approach provides benefits in terms of stability and capacity over conventional solutions even when readers and tags operate in different channels. The results also provide useful guidelines towards the cross-layer design of future RFID platforms

Throughput, stability and fairness of carrier-sense multiple access with cooperative diversity

Cooperative diversity has been identified as a potential candidate for boosting the physical (PHY) layer performance of future wireless networks. However, several issues remain open today in the design of an appropriate medium access control (MAC) layer for this type of system. This paper attempts to partially fill this gap by addressing the MAC-PHY cross-layer design of a class of carrier-sense multiple access protocols where collision-free transmissions are assisted by the potential cooperative retransmission of the remaining silent terminals in the network. Unlike previous works, the analysis is focused on full asymmetrical settings, where terminals experience different channel and queuing statistics. To achieve this goal, a packet reception model is here proposed for cooperative schemes where the relaying phase is activated only when the reception of previous (re)transmissions has failed. Closed-form expressions of correct reception probability are derived for Rayleigh fading channels assuming that correct reception occurs only when the instantaneous signal-to-noise ratio (SNR) exceeds a reception threshold. This reception model allows for a MAC-layer design aware of PHY-layer information, and vice versa, PHY-layer enhancement and activation using MAC-layer information. The boundary of the throughput region (i.e., the set of all achievable throughput values) is derived in a parametric closed-form expression using a multi-objective optimization approach. A method for deriving a non-parametric form was further proposed, which allows for a geometric interpretation of the two-user case. Stability features such as backlog user distribution and backlog delay are evaluated by using a novel Markov model for asymmetrical systems. Fairness is evaluated by means of the Gini index, which is a metric commonly used in the field of economics to measure income inequality. The protocol is shown to outperform its non-cooperative counterparts under diverse network conditions that are here discussed

Set optimization for efficient interference alignment in heterogeneous networks

To increase capacity and offload traffic from the current macro-cell cellular system operators are considering the deployment of small-cells. It is expected that both the small and macro-cells will coexist in the same spectrum resulting in unsustainable levels of interference. Interference alignment is considered as an effective method to deal with such interfer- ence. By using interference alignment the small-cells align their transmission along a common direction to allow the macro-cell receiver to completely remove it. It is clear that if the two systems have no limitations on the information that may be exchanged between them to perform the signal design, then the performance may be improved in comparison to the case of no or partial cooperation. However, this full cooperation strategy requires a high-rate connection between the macro and small-cells, which may not be available. To overcome this problem we consider that the alignment direction is selected from a finite set, known to both macro and small-cell terminals. We provide sufficient conditions for this set that guarantee full-diversity, at the macro- cell, and propose an efficient method to optimize the set elements. Results show that an alignment set with a description length of 1 bit is enough to achieve the same diversity as in the case where an infinite amount of information is exchanged between both systems. The proposed set optimization method achieves better performance than random vector quantization and similar performance to Grassmannian quantization

Frequency shift based multiple access interference canceller for DS-CDMA systems

The cyclostationary properties of Direct Sequence Spread Spectrum signals are well known. These cyclostationary properties imply a redundancy between frequency components separated by multiples of the symbol rate. In this paper we present a Multiple Access Interference Canceller that explores this property and applies to UMTS-TDD. This frequency domain Canceller acts in the spreaded signal in such way that minimizes the interference and noise at its output (Minimum Mean Squared Error Criterium). The performance is evaluated in two detector configurations: one including the Frequency Shift Canceller (FSC) and the other plus a Parallel Interference Canceller (PIC). The results are benchmarked against the performance of the conventional RAKE detector and the conventional PIC detector.PRAXIS XXIFCTASILUM projectVISEF projec

Joint user scheduling and link adaptation for distributed antenna systems in multi-cell environments with imperfect CSI

This paper proposes a novel management algorithm for distributed antenna systems (DASs) that exploits the spatial diversity of the distributed architecture in order to schedule (over the same radio resource) as many transmissions as possible with the most appropriate modulation and coding schemes (MCSs). This goal is achieved by implementing a joint user scheduling and link adaptation algorithm (including power control and adaptive modulation and coding) that allows for an appropriate management of intra-cell interference. The algorithm provides the optimum set of scheduled users, the optimum serving nodes, the transmit power levels, and the MCSs that maximize the capacity of the system. In comparison with conventional approaches, where the objective is to maximize the signal-to-interferenceplus- noise ratio (SINR) of each user, in this paper the target is to satisfy a given SINR value that ensures the transmission of the chosen MCS with a particular value of block-error-rate (BLER). To achieve this goal, an iterative optimization scheme is proposed in which the set of scheduled users, the power levels, and the MCSs are modified according to channel and interference conditions. A novel method for the calculation of outer-cell interference in multi-cell configurations is also proposed. Imperfect channel state information is used throughout the system-level simulation work. Simulation results show considerable gains in terms of throughput and reduced power consumption per node when compared to conventional systems, thereby making the proposed algorithm suitable for green energy solutions

Minimum Codebook Size to achieve Maximal Diversity Order for RVQ based MIMO Systems

Multiple antenna systems provide both capacity and diversity gains relatively to single antenna based networks. These gains rely heavily on the availability of channel state information. In this manuscript we assume that the receiver has accurate channel information, which is quantized and fed back to the transmitter. Sufficient conditions for the feedback rate are provided that guarantee no loss of diversity due to channel quantization. Considering a K × M MIMO system, we show that to achieve a diversity of D = M – K + 1 which is the maximum allowable, a feedback channel with rate of log2(1 + D) bits per channel use is enough