Multiplex communications – Communication over free space – Signaling for performing battery saving
Reexamination Certificate
1995-06-06
2001-12-18
Patel, Ajit (Department: 2662)
Multiplex communications
Communication over free space
Signaling for performing battery saving
C455S574000, C340S007320, C340S007100
Reexamination Certificate
active
06331971
ABSTRACT:
BACKGROUND
Applicant's invention relates generally to radio communication systems and more particularly to radio communication systems in which evaluations of remote unit servers for potential control channel reselection are performed.
The growth of commercial radio communications and, in particular, the explosive growth of cellular radiotelephone systems have compelled system designers to search for ways to increase system capacity without reducing communication quality beyond consumer tolerance thresholds. One way to increase capacity is to use digital communication and multiple access techniques such as TDMA, in which several users are assigned respective time slots on a single radio carrier frequency.
In North America, these features are currently provided by a digital cellular radiotelephone system called the digital advanced mobile phone service (D-AMPS), some of the characteristics of which are specified in the interim standard IS-54B, “Dual-Mode Mobile Station-Base Station Compatibility Standard”, published by the Electronic Industries Association and Telecommunications Industry Association (EIA/TIA). Because of a large existing consumer base of equipment operating only in the analog domain with frequency-division multiple access (FDMA), IS-54B is a dual-mode (analog and digital) standard, providing for analog compatibility in tandem with digital communication capability. For example, the IS-54B standard provides for both FDMA analog voice channels (AVC) and TDMA digital traffic channels (DTC), and the system operator can dynamically replace one type with the other to accommodate fluctuating traffic patterns among analog and digital users. The AVCs and DTCs are implemented by frequency modulating radio carrier signals, which have frequencies near 800 megahertz (MHz) such that each radio channel has a spectral width of 30 kilohertz (KHz).
In a TDMA cellular radiotelephone system, each radio channel is divided into a series of time slots, each of which contains a burst of information from a data source, e.g., a digitally encoded portion of a voice conversation. The time slots are grouped into successive TDMA frames having a predetermined duration. The number of time slots in each TDMA frame is related to the number of different users that can simultaneously share the radio channel. If each slot in a TDMA frame is assigned to a different user, the duration of a TDMA frame is the minimum amount of time between successive time slots assigned to the same user.
The successive time slots assigned to the same user, which are usually not consecutive time slots on the radio carrier, constitute the user's digital traffic channel, which may be considered a logical channel assigned to the user. As described in more detail below, digital control channels (DCCs) can also be provided for communicating control signals, and such a DCC is a logical channel formed by a succession of usually non-consecutive time slots on the radio carrier.
According to IS-54B, each TDMA frame consists of six consecutive time slots and has a duration of 40 milliseconds (msec). Thus, each radio channel can carry from three to six DTCs (e.g., three to six telephone conversations), depending on the source rates of the speech coder/decoders (codes) used to digitally encode the conversations. Such speech codes can operate at either full-rate or half-rate, with full-rate codes being expected to be used until half-rate codes that produce acceptable speech quality are developed. A full-rate DTC requires twice as many time slots in a given time period as a half-rate DTC, and in IS-54B, each radio channel can carry up to three full-rate DTCs or up to six half-rate DTCs. Each full-rate DTC uses two slots of each TDMA frame, i.e., the first and fourth, second and fifth, or third and sixth of a TDMA frame's six slots. Each half-rate DTC uses one time slot of each TDMA frame. During each DTC time slot, 324 bits are transmitted, of which the major portion, 260 bits, is due to the speech output of the codec, including bits due to error correction coding of the speech output, and the remaining bits are used for guard times and overhead signaling for purposes such as synchronization.
It can be seen that the TDMA cellular system operates in a buffer-and-burst, or discontinuous-transmission, mode: each mobile station transmits (and receives) only during its assigned time slots. At full rate, for example, a mobile station might transmit during slot
1
, receive during slot
2
, idle during slot
3
, transmit during slot
4
, receive during slot
5
, and idle during slot
6
, and then repeat the cycle during succeeding TDMA frames. Therefore, the mobile station, which may be battery-powered, can be switched off, or sleep, to save power during the time slots when it is neither transmitting nor receiving. In the IS-54B system in which the mobile does not transmit and receive simultaneously, a mobile can sleep for periods of at most about 27 msec (four slots) for a half-rate DTC and about 7 msec (one slot) for a full-rate DTC.
In addition to voice or traffic channels, cellular radio communication systems also provide paging/access, or control, channels for carrying call-setup messages between base stations and mobile stations. According to IS-54B, for example, there are twenty-one dedicated analog control channels (ACCs), which have predetermined fixed frequencies located near 800 MHz. Two frequency bands, each about 25-MHz wide (the A- and B-bands), are provided for transmission and reception. The AVCs and DTCs are also located within the A- and B-bands. Since these ACCs are always found at the same frequencies, they can be readily located and monitored by the mobile stations.
It will be understood that a communication system that uses ACCs has a number of deficiencies. For example, the format of the forward analog control channel specified in IS-54B is largely inflexible and not conducive to the objectives of modern cellular telephony, including the extension of mobile station battery life. In particular, the time interval between transmission of certain broadcast messages is fixed and the order in which messages are handled is also rigid. Also, mobile stations are required to re-read messages that may not have changed, wasting battery power. These deficiencies can be remedied by providing a DCC, one example of which is described in U.S. patent application Ser. No. 07/956,640 entitled “Digital Control Channel”, which was filed on Oct. 5, 1992, and which is incorporated in this application by reference. Using such DCCs, each IS-54B radio channel can carry DTCs only, DCCs only, or a mixture of both DTCs and DCCs. Within the IS-54B framework, each radio carrier frequency can have up to three full-rate DTCs/DCCs, or six half-rate DTCs/DCCs, or any combination in-between, for example, one full-rate and four half-rate DTCs/DCCs. As described in this application, a DCC in accordance with Applicant's invention provides a further increase in functionality.
In general, however, the transmission rate of the DCC need not coincide with the half-rate and full-rate specified in IS-54B, and the length of the DCC slots may not be uniform and may not coincide with the length of the DTC slots. The DCC may be defined on an IS-54B radio channel and may consist, for example, of every n-th slot in the stream of consecutive TDMA slots. In this case, the length of each DCC slot may or may not be equal to 6.67 msec, which is the length of a DTC slot according to IS-54B. Alternatively (and without limitation on other possible alternatives), these DCC slots may be defined in other ways known to one skilled in the art.
Also, U.S. patent application Ser. No. 07/956,640 also shows how a DCC may be defined alongside the DTCs specified in IS-54B. For example, a half-rate DCC could occupy one slot and a full-rate DCC could occupy two slots out of the six slots in each TDMA frame. For additional DCC capacity, additional half-rate or full-rate DCCs could replace DTCs. In general, the transmission rate of a DCC need not coincide with the halfrate
Burns Doane Swecker & Mathis L.L.P.
Patel Ajit
Telefonaktiebolaget LM Ericsson (publ)
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