Background | Objectives & Benefits | Scientific Programme | Organisation | Timetable
Economic Dimension | Dissemination Plan

C. Scientific Programme

The activitites of this Action will be conducted in 2 Working Groups (WGs):

a) WG1: Information Theoretical Description of Radio Systems
b) WG2: Communications Techniques and Systems

The Action has synergy with the activities of the COST 262 Action on Spread Spectrum Systems and Techniques in Wireless and Wired Communications. The philosophy of the Action is based on close co-operation between the scientists and engineers from the Telecommunication Industry and the Academia. In accordance with the COST spirit, the research efforts be focused and coordinated, in accordance with the objectives cited above, on the problems discussed below.

C.1 WG1: Information Theoretical Description of Radio Systems
The aim of WG1 is to provide an analytical description of mobile systems and related architectures, and to carry out detailed analyses of undiscovered areas of modern communications. The emphasis will be on mathematical studies and simulation rather than on the implementation issues. The output of WG1 can also be used as an input to WG2.

A list of research areas that the WG1 is interested includes, but not limited to:

  • Spectral efficiency (SE) and power efficiency (PE)

    The spectrum and power should be used efficiently, at every carrier frequency, geographical area, and time. The SE of currently used and possible future modulation schemes will be studied. The required Eb/N0 to achieve a specified bit error rate in a given bandwidth will be studied for modulation and coding schemes. A combined efficiency metric can be used to characterize the SE and PE jointly at a carrier frequency, geographical area and time.

  • Channel capacity (CC)

    Some of the issues to be addressed in this activity are as follows: The number of users that can be served in a given system, if their service requirements are different (QoS, delay, data rate, BER); the number of users that can be served in a completely loaded mobile system; a comparative study of candidate multiple access schemes.

  • User capacity (UC)

    The UC is computed at the air interface for a given user and the amount of information that a user can transmit through the air, in the presence of multiple users in the system. This is a rather useful concept when one restricts active transmissions in order to admit a new call in a highly loaded system.

  • Coding

    Forward error correction (FEC) coding is still an important topic for future mobile radio systems. Families of FEC codes, their properties and their applicability in modern modulation schemes should be addressed. What code properties are required for each system? Is it possible to measure these properties? What are the related properties of currently used code sets? In view of the striving needs for spectral efficiency, it is important to study the iteratively decodable error correcting codes and provide their efficient realizations. The study of turbo and low-density parity-check codes that approach the performance bound predicted by the Shannon theorem.

    To cope with randomly changing channel conditions, variable rate transmissions can also be considered. In this context, requirements for variable rate coding and variable data rate transmissions are obvious. ARQ techniques may also be utilized to keep the transmission errors below an acceptable level.

  • Modulation

    The theoretical aspects of spectrum and power efficient modulation techniques and detection algorithms will be studied in order to discover their potential for high data rate applications.

C.2 WG2: Communication Techniques and Systems
The objective of WG2 is to study the implementation of techniques and algorithms for increasing the data rates, the reliability as well as the power and spectrum efficiency of communication systems.

The research activities of the WG2 include:

  • Adaptive transmission techniques

    Adaptive transmission techniques recently emerged as a powerful tool for increasing the channel capacity of wireless networks. The motivation behind the adaptive transmission techniques is to allocate the network resources so as to take advantage of the favourable channel conditions by transmitting at high speeds and by reducing the data rate in bad channel conditions. The parameters that can be used for link adaptation depend on the channel selectivity in time, frequency, space, polarization etc.. The selectivity in time determines the update rate of link adaptation. The frequency selectivity of the channel can be exploited by multicarrier techniques such as OFDM. Similarly, space-time coding, spatial multiplexing and MIMO technology can be used to exploit the spatial selectivity.

    Adaptive techniques are based on the measurement of the parameters of the transmission channel, such as SNR, SINR, error rate and/or their combinations, and, consecutively, the selection of one or more transmission parameters, based on the optimization of a pre-assigned cost function. Thus, the parameters like the power level, the constellation size, the transmission rate, the spreading factor, the symbol rate, the code rate, and the coding scheme can be changed to match the channel conditions during a symbol period.

    Successful implementation of the link adaptation techniques over multi-user broadband frequency-selective channels requires the development of novel, practical, and high-performance signal processing algorithms for channel estimation, joint equalization/decoding, and interference suppression. All these should have negligible overhead to the throughput.

    The channel estimation is usually carried out in the receiver and the relevant parameters are fed back to the transmitter. The instantaneous feedback of the channel information to the transmitter may not always be practical due to delays in the feedback loop. Note that there is a trade-off between performance gain and amount of resources allocated to control messages. Therefore, it is important to optimise the link adaptation to yield both accuracy and robustness over a wide range of channels, adaptation rates and traffic conditions by an appropriate choice of the channel state information (CSI).

    In the absence of the CSI, one is concerned with the detection of a signal at a very low level without any a-priori knowledge about the physical channel parameters and without the need for any training sequences for channel estimation. Blind detection is a promising candidate to replace classical supervised learning-based adaptive algorithms. Similarly, the blind channel equalization will be an important tool for broadband services.

    The future communication systems will call for the adaptation of services/ applications and adaptive resource management, including the dynamic management of the allocated spectrum, the multiple access schemes and the soft channel (control the modulation and coding functionality; parametric control of the constellation size, power levels, code sizes etc).

    Link adaptation based on the sole consideration of the physical layer is not necessarily the best approach for performance improvement. System design approaches based on the joint consideration of the physical and higher layers should also be considered. Early results show significant performance improvement in this direction.

  • Software defined radio (SDR)

    The adaptive techniques will require logical functions, coordination mechanisms and systems architectures better suited for adaptation. The software-defined radio implies a radio with enough programmability, thus easy and inexpensive modification of the signal processing steps, and thereby provides different communication schemes. Thanks to the developments in digital signal processing, field programmable gate arrays (FPGAs), application specific integrated circuits (ASICs) and digital signal processors (DSPs), we now have new options for the software defined radio. The ability to work across a wide range of frequency spectrum is believed to be a requirement for next generation SDR’s. Hardware architectures, for both the baseband and RF sides, continue to improve. The SDR technology continues to evolve and is expected to be one of the key technologies of the future systems.

  • Adaptive/Reconfigurable networks

    The adaptive techniques and the SDR provide the basis for adaptive/reconfigurable networks. The adaptability and convergence of the various access technologies will require the networks to provide access to a number of radio technologies, as part of the adaptation process, while providing applications and services at QoS levels the users expect. Thus, network concepts and architectures will evolve so that they will provide dynamic adaptation to traffic and QoS needs for various multiple access technologies. This will involve changes in and across layers. Multiple access control and routing issues and the technologies for ad-hoc and self-configurable networks will have large impacts. Flexible dynamic networking is currently an area of active research.

    An ad-hoc network consists of a collection of nodes that communicate with each other, but have no fixed infrastructure and no pre-determined organization of available links. A significant feature of ad hoc networks is that rapid changes in connectivity and link characteristics are introduced due to node mobility and power control. The scalability, the energy efficiency, the QoS and security requirements are still open questions in ad hoc networks. The use of smart antennas may improve the capabilities of ad hoc networks considerably. The ad hoc networks also improve the spectrum efficiency, because they operate at a single frequency.

  • Multi-carrier systems

    With ever-increasing demand for higher data rates, the transmission bandwidth becomes wider than the coherence bandwidth of the channel, which results in frequency-selective fading. In this respect, multicarrier signalling schemes, such as OFDM, appear to be very promising. Using these schemes, the transmission channel bandwidth is divided into smaller sub-channels, which are much narrower than the original one, thus avoiding the frequency selective fading. Multicarrier signalling also provides the flexibility to adapt the transmitter to channel conditions so as to maximize the channel capacity, as described by the water-filling algorithm. Adaptive bit/power loading can be used as an effective tool to get the highest capacity from an OFDM system provided that the transmitter and receiver have the CSI.

    The susceptibility of OFDM systems to frequency and time offsets is still an active research area. Similarly, the reduction of peak to average power and the design of linear amplifiers need still to be addressed.

  • Multiple-access techniques

    Future communication systems will serve both fixed and mobile users. The signals of these users, requiring communications at different data rates, will be multiplexed to share the same transmission bandwidth. Therefore, the presence of a user should cause minimum interference to the other users. In this respect, relative merits of different multiple access schemes need to be thoroughly investigated.

    Multiuser interference mitigation is important for multicarrier-based multiple access schemes (e.g., OFDMA, MC-CDMA, etc.) in the uplink since each mobile transmitter has its own channel with its own parameters such as coherence time and coherence bandwidth. The demodulation is carried out in block-based manner for all users at a time. Furthermore, the possibility of asynchronous access may be investigated for the uplink multicarrier-based multiple-access schemes.

    The use of multicarrier based modulation schemes for multiple-access requires the channel and carrier frequency offset estimation in both uplink and downlink. The number of parameters to be estimated is directly proportional to the number of users in the uplink. The receiver complexity also constitutes an important issue for the downlink, which generally requires low-cost receivers for mobile users.

  • Multi-user detection

    Multi-user interference mitigation is a serious issue in multiple-access communication systems. Multi-user interference can be mitigated by using adaptive (smart) antennas or by applying multi-user detection algorithms in the receiver (which are very effective, but their computational complexity may render them impractical). Here, the aim would be to develop new strategies for multi-user detection and, also, to implement them using simple DSP algorithms.

  • Multiple input multiple-output (MIMO) systems

    The spatial dimension will be used in future communication systems for further improvement of the bandwidth and power efficiency. In the past, the work in this field has concentrated on smart antennas, which usually implied space-division-multiple access (SDMA) related concepts like beamforming. The MIMO systems, which employ multiple transmit and multiple receive antennas, can provide improved performance by combining modulation, coding and (space, time and frequency) diversity. Thus, signal processing in space, time and frequency has a great potential for the improvement of the system performance against interference and noise. The use of MIMO systems combined with multi-carrier modulation techniques, like MC-CDMA and OFDM together with trellis and punctured convolutional coding, is believed to provide new opportunities for spectral and power efficiency, and thus for capacity enhancement.

©2002-2007 Department of Electrical & Electronics Engineering, Hacettepe University, Ankara / Turkey