Signal processing in mobile communications toward 2020

Signal processing for mobile communications had a major impact as an essential tool of economic grow in the last decade. This can be concluded by the features obtained in cellular networks (GSM, UMTS, LET, etc.) and more recently by the performance of wireless computer networks (Wifi, Wimax). Next decade will bring dramatic developments in the area by different reasons, among them

- Increasing demand of broadband services like HDTV and data file acces of increasing size.

- Increasing rates available in fixed networks (DSL, 1000-base-T, FTTH and FTTB), whose users will expect to access in a wireless context.

- Gain in efficiency available through coordinated networks of devices and autonomous sensors, with application to security and vigilance.

These applications introduce a series of specific challenges in the area of signal processing, as for example:

- To increase the system bandwidth efficiency in an order of magnitude.

- To improve the requirements of quality of service.

- To widening the possible network architectures to more heterogeneous forms, like mesh, multi-hop and point-to-point.

- To coordinate a multiplicity of autonomous devices using different standards.

In particular, the area of signal processing it is able to provide solutions to these challenges abording the following concrete problems

- Interference reduction and control. Unlincensed bands of the spectrum are increasingly been used by a variety of services. That leads to increasing congestion and interferences. As a consequence, new (adaptive) techniques for medium access will be required for the spectrum use. These new techniques will improve the performance in the case of non coordinated use of the unlicensed bands.

- Adaptability in multiple access systems. Adaptability to improve the performance can be used at different levels (physical, link, etc.). This area should consider the development of alternatives to coordinate the adaptability of specific systems and its impact in other systems that use the same allocation of spectrum.

- Interaction between adaptive techinques. The objective would be to study the impact of the previous adaptive techinques, in which there is an interaction of several levels of the OSI model. The interaction can lead to a degradation of the general performance, and as a consequence, it will be required to put main emphasys in efficient techniques.

- Power control criteria. Power control schemes can be summarized as a very demanding algorithm that is looking to provide the required performance in a single link, taking into account that for the other users power received is minimized maintaining the quality of service. This model is suitable when user have differents sources. When the sources are shared, these power control schems are not longer optimal and new alternatives will be required to guarantee fairness.

- Efficient RF fron-end design. Baseband processing design is often performed independently of the RF implementation issues. High spectral efficiency modulations (to obtain the required savings in bandwidth) require high degree of linearity and high peak-to-average power ratio. This leads to transmitters with low power efficiency and complex RF solutions. New concepts in design will be required for the case in which the RF and baseband signal processing is performed jointly observing low complexity in addition to higher spectral and power efficiencies.

- MIMO operation limited by interferences. MIMO technologies provide high benefits in spectral efficiency when the channel conditions are apropiated and known. The possibility to separate multiple bit sequences send on the same channel requires knowing how they are relationated. However, since interference is unknown in general, methods to mitigate or to reduce their influence will be required. These methods could include an antenna processing more elaborated, such as a combination of MIMO detection and beamforming, or just new techniques of signal separation.

- Exploration of the radio channel for new applications. Developments in MIMO technologies put in evidence that channel can not be considered as a linear tube but as tridimentional complex process. Channel structure must be explored to obtain the best performance. This will require also the combined design of antennas and detection strategies.

- Aspects of optimal architectures. An important part of the available technological advances considered hardware and software design separately. Complementation in the design with optimization purposes must be one of the main methodologies.

- Trade-off between coordinated and uncoordinated use of the spectrum. To many actual systems the design decision to use coordinated or not coordinated access is fundamental. This happens generally at the MAC level through a regulated process of licensed on unlicensed channel allocation. Different trade-offs between both types of systems must be considered and their interactions at different levels of the OSI model.

- Energy efficient system design. Design of energy efficient wireless systems is an important problem and is highly complex. Is important since mobiles operate with limited life bateries. Is also a complex problem since the performance depends on (in an interrelated form) differents subsystems: antenna, amplifier, modulation, channel coding, network protocols, etc. To optimize the performance, the relationship among the different subsystems must be taking into account and solve the trade-off considering the energy constraints in all of them as a whole.

- Multifunctionality and reconfigurability. In terms of channel coding, it will be required to develop practical techniques of low complexity, as an alternative to those obtained based on theoretical bounds. This includes, in the long term, techniques that use multifunctionality and reconfigurability, i.e., coding that allows an approximation to theoretical bounds, using adaptive rates and low complexity. The evident manner to achieve this is to integrate the functions of the physical level (equalization, synchronization, detection, channel estimation, etc.) with efficient coding techniques and iterative decoding techniques.

- Combined source and channel coding. The increasing importance of internet and 3G mobile systems has also increased the interest in robust multimedia communication systems on no reliable channels. Instead of considering source and channel separately, significative improvement could be otained by combining source and channel coding through an interactive design among levels of the OSI model.

- Space – time processing. Diversity techniques (space – time coding or space – frequency coding) become rapidly a new frontier of wireless commnunications due to huge advantages in terms of capacity and performance. This includes space – time coding for single carrier modulation, optimal receivers for impulsive noise channels, combined designs based on diversity and space – time coding and OFDM (orthogonal frequency division multiplexing), etc. New solutions will be required that solve the trade-off between available degrees of freedom of diversity and maximum rate transmission.