Flexible Radio: A Framework for Optimized Multimodal Operation via Dynamic Signal Design

EURASIP Journal on Wireless Communications and Networking 2005:3, 284–297 c 2005 Ioannis Dagres et al.
Flexible Radio: A Framework for Optimized Multimodal Operation via Dynamic Signal Design Ioannis Dagres Institute of Accelerating Systems & Applications (IASA), National Kapodistrian University of Athens (NKUA), P.O. Box 17214, 10024 Athens, Greece Email: Andreas Zalonis Institute of Accelerating Systems & Applications (IASA), National Kapodistrian University of Athens (NKUA), P.O. Box 17214, 10024 Athens, Greece Email: Nikos Dimitriou Institute of Accelerating Systems & Applications (IASA), National Kapodistrian University of Athens (NKUA), P.O. Box 17214, 10024 Athens, Greece Email: Konstantinos Nikitopoulos Institute of Accelerating Systems & Applications (IASA), National Kapodistrian University of Athens (NKUA), P.O. Box 17214, 10024 Athens, Greece Email: Andreas Polydoros Institute of Accelerating Systems & Applications (IASA), National Kapodistrian University of Athens (NKUA), P.O. Box 17214, 10024 Athens, Greece Email: Received 16 March 2005; Revised 19 April 2005 The increasing need for multimodal terminals that adjust their configuration on the fly in order to meet the required quality of service (QoS), under various channel/system scenarios, creates the need for flexible architectures that are capable of performing such actions. The paper focuses on the concept of flexible/reconfigurable radio systems and especially on the elements of flexibility residing in the PHYsical layer (PHY). It introduces the various ways in which a reconfigurable transceiver can be used to provide multistandard capabilities, channel adaptivity, and user/service personalization. It describes specific tools developed within two IST projects aiming at such flexible transceiver architectures. Finally, a specific example of a mode-selection algorithmic architecture is presented which incorporates all the proposed tools and, therefore, illustrates a baseband flexibility mechanism. Keywords and phrases: flexible radio, reconfigurable transceivers, adaptivity, MIMO, OFDM. 1. INTRODUCTION The emergence of speech-based mobile communications in the mid 80s and their exponential growth during the 90s have paved the way for the rapid development of new wireless standards, capable of delivering much more advanced services to the customer. These services are and This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. will be based on much higher bit rates than those provided by GSM, GPRS, and UMTS. The new services (video streaming, video broadcasting, high-speed Internet, etc.) will demand much higher bit rates/bandwidths and will have strict QoS requirements, such as the received BER and the end-to-end delay. The new and emerging standards (WiFi, WiMax, DVB-T, S-DMB, IEEE 802.20) will have to compete with the ones based on wired communications and overcome the barriers posed by the wireless medium to provide seamless coverage and uninterrupted communication. Flexible Radio Framework for Optimized Multimodal Operation Another issue that is emerging pertains to the equipment that will be required to handle the plethora of the new standards. It will be highly unlikely that the user will have available a separate terminal for each of the introduced standards. There will be the case that the use of a specific standard will be dictated by factors such as the user location (inside buildings, in a busy district, or in a suburb), the user speed (pedestrian, driving, in a high-speed train), and the required quality (delay sensitivity, frame error rate, etc.). There might also be cases in which it would be preferred that a service was delivered using a number of different standards (e.g., WiFi for video, UMTS for voice), based on some criteria related to the terminal capabilities (say, power consumption) and the network capacity constraints. Therefore, the user equipment has to follow the rapid development of new wireless standards by providing enough flexibility and agility to be easily upgradeable (with perhaps the modification/addition of specific software code but no other intervention in hardware). We note that flexibility in the terminal concerns both the analog/front-end (RF/IF) as well as digital (baseband) parts. The paper will focus on the issues pertaining to the baseband flexibility and will discuss its interactions with the procedures taking place in the upper layers. 2. DEFINITIONS OF RADIO FLEXIBILITY The notion of flexibility in a radio context may be defined as an umbrella concept, encompassing a set of nonoverlapping (in a conceptual sense) postulates or properties (each of which must be defined individually and clearly for the overall definition to be complete) such as adaptivity, reconfigurability, modularity, scalability, and so on. The presence of any subset of such features would suffice to attribute the qualifying term flexible to any particular radio system [1]. These features are termed “nonoverlapping” in the sense that the occurrence of any particular one does not predicate or force the occurrence of any other. For example, an adaptive system may or may not be reconfigurable, and so on. Additional concepts can be also added, such as “ease of use” or “seamlessly operating from the user’s standpoint,” as long as these attributes can be quantified and identified in a straightforward way, adding a new and independent dimension of flexibility. Reconfigurability, for instance, which is a popular dimension of flexibility, can be defined as the ability to rearrange various modules at a structural or architectural level by means of a nonquantifiable1 change in its configuration. Adaptivity, on the other hand, can be defined as the radio system response to changes by properly altering the numerical value of a set of parameters [2, 3]. Thus, adaptive transmitted (Tx) power or adaptive bit loading in OFDM naturally fall in the latter category, whereas dynamically switching between, say, a turbo-coded and a convolutional-coded system in response to some stimulus (or information) seems to fit better the code-reconfigurability label, simply because that type of 1 “Nonquantifiable” here means that it cannot be represented by a numerical change in a parametric set. 285 change implies a circuit-design change, not just a numeric parameter change. Furthermore, the collection of adaptive and reconfigurable transmitted-signal changes in response to some channel-state-information feedback may be termed dynamic signal design (DSD). Clearly, certain potential changes may fall in a grey area between definitions.2 A primitive example of flexibility is the multiband operation of current mobile terminals, although this kind of flexibility driven by the operator is not of great research interest from the physical-layer point of view (...truncated)