Convergence of different wireless communication systems and networks is becoming more prevalent, as well as seamless connections between wireless and backbone wired networks.  Adaptive technologies in mobile transceiver design, network and application services can provide an important role in supporting such diverse mobile multimedia services.  These trends in wireless communications bring several fundamental challenges for wireless system and network designs. 
The nature of mobile multimedia communication is dynamic, due partly to the fast variation of wireless channels, and partly to the wide range of user applications and requirements.  The user mobility and the short wavelength of a broadband wireless signal mean that the system throughput can vary substantially within a few microseconds or a few feet in distance.  Similarly, the traffic of wireless communications also changes from the constant low rate voice communications, to high sporadic internet browsing and broadband video communications. 
The traditional design methodology for mobile multimedia communication is to devise the wireless system for the maximum data request under the “worst case” wireless channel condition.  Such a design could result in a scenario that all the system resources are committed to one user and no one else could be accommodated.  In contrast to the “worst-case” design methodology, considerable bandwidth, battery power, latency, and other communication resources can be conserved by adapting the transmission parameters to current channel conditions and application requirements. 
There is a need to develop flexible transmission technologies which can adapt to current mobile multimedia communication conditions and requirements in the most efficient and reliable way. 
The fast evolution of wireless communications also brings challenge of efficient spectrum utilization.  Today's wireless communication systems are characterized by a fixed spectrum allocation policy, i.e. the spectrum is regulated by governmental agencies and is assigned to license holders on a long term basis for large geographical regions.  With the existing radio spectrum regulatory framework, access to radio spectrum is frustratingly difficult.  According to Federal Communications Commission (FCC), temporal and geographical utilization rate of the assigned spectrum can be as low as 15% [1, 2] at any location and at any given time.  Although the fixed spectrum assignment policy generally served well in the past, the dramatic increase in wireless communications in recent years poses a looming challenge due to spectrum overcrowding.  Improving the spectrum utilization efficiency is required to support the wireless communications that will continue to fuel the economic growth.  The limited availability of spectrum and the inefficiency of its usage necessitate a new communication paradigm termed cognitive radio to exploit the existing wireless spectrum opportunistically. 
US Patent Application, Publication Number US 2008/0014880 A1, invented by Hyon et al.,  discloses a signaling method between a cognitive radio (CR) base station and a CR terminal in a CR environment, in which a channel division method is used for the signaling method, the method including: detecting a channel usage of an incumbent system, which communicates with a CR base station; sensing an outband channel to communicate with the CR base station; receiving an EOS, which is broadcasted from the CR base station via the outband channel according to a pre-determined period; and transmitting a sensing report signal with respect to the channel to the CR base station.  This technique is designed for point to multipoint communications were a base station and mobile CR users have pre-arranged signal form a to exchange information.  This is achieved through signaling transmission using outband channel which would require extra bandwidth. 
US Patent Application, Publication Number US 2008/0080604 A1, inventor Hur et al.,  discloses spectrum-sensing algorithms and methods for use in cognitive radios and other applications.  The spectrum-sensing algorithms and methods may include receiving an input spectrum having a plurality of channels, performing a coarse scan of the plurality of channels of the input spectrum to determine one or more occupied candidate channels and vacant candidate channels, where the coarse scan is associated with a first resolution bandwidth and a first frequency sweep increment, performing a fine scan of the occupied candidate channels and the vacant candidate channels to determine actually occupied channels and actually vacant channels, where the fine scan is associated with a second resolution bandwidth and a second frequency sweep increment, and storing an indication of the actually occupied channels and the actually vacant channels.  The signal detection method disclosed is power/energy detection.  The sensing decision is based on the existence of signal power and may not be able to distinguish signal from interference. 
US Patent Application publication number US 2008/0089389 A1, inventor Hu, relates to cognitive radio based wireless communications of dynamic spectrum access networks, and in particular to a method of addressing zero-delay frequency switching for cognitive dynamic frequency hopping.  The method combines regular (periodic) channel maintenance with dynamic frequency hopping over a cluster of vacated channels that are initially setup such that the switching delays for channel setup and channel availability check are eliminated.  The method disclosed does not manipulate the physical layer. 
Cognitive radio is a revolutionary technology that provides improvements in efficiency of spectrum usage.  Ever since Joseph Mitola III [3, 4] established the phrase “cognitive radio” in his thesis, many definitions of what a true cognitive radio can look like have been discussed in literature.  The cognitive radio is normally defined as an intelligent wireless communication system that is aware of its environment and uses the methodology of “understanding-by-building” to learn from the environment and adapt to statistical variations in the input stimuli, with the efficient utilization of the radio spectrum as the primary objective [5].  The Federal Communications Commission (FCC) defines cognitive radios as radio systems that continuously perform spectrum sensing, dynamically identify unused spectrum, and then operate in this spectrum at times when it is not used by incumbent radio systems [1].  Modern wireless LAN IEEE 802.11 devices operate with a listen-before-talk spectrum access and with dynamically changing frequencies and transmission power [6, 7].  However, such existing standards provide only a subset of the required techniques for cognitive radio, and do not cover the full range of objectives for efficiently using the spectrum.  On the other hand, the terrestrial TV broadcast band is currently in the process of being reorganized for the roll-out of digital video broadcast [8, 9].  This change is pursued in parallel in many regulatory domains worldwide.  With the introduction of the single frequency transmission network and advanced equalization technique, the total number of the Digital TV channels would be significantly reduced to maintain the current terrestrial TV coverage [10].  It is therefore envisioned to allow such unlicensed reuse of the some of the TV broadcast band for cognitive radios that scan all TV channels throughout the band and operate only upon identification of spectrum opportunities. 
In the drawings, embodiments of the invention are illustrated by way of example.  It is to be expressly understood that the description and drawings are only for the purpose of illustration and as an aid to understanding, and are not intended as a definition of the limits of the invention.