There is a need for personal communication systems that allow portable devices, such as pagers, telephones, computer terminals, and so forth, carried on the person to communicate with each other and with fixed location devices. For example, a user may want to store in a personal computer a message received over the air by a paging terminal. Known prior personal communication systems typically require that these two devices be interconnected by wire, which makes it cumbersome to affix them to users and/or interconnect them to each other, and thus, inconvenient to use.
In medical environments, systems for gathering information such as blood pressure, EKG readings, and so forth typically require that instruments taking the readings from a patient be connected, by wire, to a patient-carried system component that monitors or stores the information. These system are also cumbersome to affix to a user.
In other applications, wireless systems are currently used to transmit information between system components by, for example, radio waves, microwaves, infra-red signals and so forth. These systems may not be suitable for sending information between the user-mounted system components discussed above because of problems with interference in the immediate environment or between the signals transmitted from the various devices.
For example, devices in systems that use infra-red signals should optimally communicate with line-of-sight transmissions, which are not always possible between devices carried by a user. Further, the infra-red systems suffer from interference with ambient light, which can not always be controlled by the user. And, for systems transmitting signals at high frequencies, the users' bodies absorb the radiating energy, and thus, degrade the signals.
Further, such systems are subject to government regulation, since their signals radiate significantly. Also, these systems allow others to eavesdrop on the transmissions.
Wireless transmission systems have also been used to determine relative position. Such systems determine the position of a transmitter based on the timing or strength of signals received by various receivers. These systems are not well suited for and can be unreliable for determining position and orientation at close distances.