Ubiquitous and seamless connectivity can be achieved in a heterogeneous network environment, under the condition that both terminals and network enable feature the necessary reconfiguration capabilities to support horizontal (between access point adhering to one standard) and vertical (between access points operating different standards) roaming.
Recently, the need for reconfiguration support is receiving attention in specific standardization initiatives, as illustrated below;
This confirms that technological answers are due to answer the user’s need for seamless connectivity. These technological answers need to include:
Multi-mode radios: user terminals should encompass radios that can generate the different waveforms as specified in the various standards. One solution to generate these various waveforms is to conceive flexible radios.
Control solutions for reconfiguration: Control solutions for flexible radios should assure reliable connectivity, allocating the available resources in the most efficient way.
On top of seamless connectivity to one service, users want to enjoy a multitude of services on one terminal: Streaming interactive connectivity (voice and video), high-speed data access, broadcasting reception, and short range connectivity to devices in the close proximity.
In conclusion from a functionality point of view, flexibility is desired! A closer inspection of the communication schemes (see the table below for a summary of the major specifications for key wireless standards) reveals that this concept is quite challenging.
Fortunately, we see some common trends in broadband access schemes, which enable to optimize flexibility in the radios. For example, modulation schemes applying frequency domain processing are recurrently used for achieving high rates in fading environments. Also, the use of multiple antennas processing, in its most advanced flavor ‘Multiple Input Multiple Output’ (MIMO), is becoming commonplace.
Flexibility is a property of radio systems and networks, and thus it does not get easily categorized within the various layers of the familiar OSI communication model (or the related IP network model). Rather, it manifests itself across the layers, and its scientific inquiry calls for a multi-layered approach, one that explores synergies rather than separations. One can distinguish between platform (or equipment)-centric flexibility, network-centric flexibility and service/application centric flexibility.
Traditionally, radios have featured little or no flexibility, in the sense that they have been designed to be compatible to a specific standard. We call this class dedicated radios. More recently, a certain degree of flexibility in the waveform has been introduced within one standard. A clear example is the recent IEEE802.11n, where not only different constellation sizes and code rates should be supported, but also several operating frequencies and bandwidths have been defined.
