Multiplex communications – Communication over free space – Combining or distributing information via time channels
Reexamination Certificate
1999-08-11
2003-10-14
Trost, William (Department: 2683)
Multiplex communications
Communication over free space
Combining or distributing information via time channels
C370S335000, C370S347000, C370S337000, C370S252000, C370S902000, C370S503000, C455S067150, C455S561000, C455S502000, C455S091000, C455S515000, C455S449000
Reexamination Certificate
active
06633559
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to wireless communications systems and methods, and more particularly, to apparatus and methods for increasing range in wireless communications systems.
BACKGROUND OF THE INVENTION
Wireless communications systems are commonly employed to provide voice and data communications to subscribers. For example, analog cellular radiotelephone systems, such as those designated AMPS, ETACS, NMT-450, and NMT-900, have long been deployed successfully throughout the world. Digital cellular radiotelephone systems such as those conforming to the North American standard IS-54 (superseded by IS-136) and the European standard GSM (Global System for Mobile Communications) have been in service since the early 1990's. More recently, a wide variety of wireless digital services broadly labeled as PCS (Personal Communications Services) have been introduced, including advanced digital cellular systems conforming to standards such as IS-136 and IS-95, lower-power systems such as DECT (Digital Enhanced Cordless Telephone) and data communications services such as CDPD (Cellular Digital Packet Data). These and other systems are described in
The Mobile Communications Handbook
, edited by Gibson and published by CRC Press (1996).
FIG. 1
illustrates a typical terrestrial cellular communication system
20
. The cellular system
20
may include one or more terminals
22
, communicating with a plurality of cells
24
served by base stations
26
and a mobile telephone switching office (MTSO)
28
. Although only three cells
24
are shown in
FIG. 1
, a typical cellular network may include hundreds of cells, may include more than one MTSO, and may serve thousands of terminals.
The cells
24
generally serve as nodes in the communication system
20
, from which links are established between terminals
22
and the MISO
28
, by way of the base stations
26
serving the cells
24
. Each cell
24
will have allocated to it one or more dedicated control channels and one or more traffic channels. A control channel is a dedicated channel used for transmitting cell identification and paging information. The traffic channels carry the voice and data information. Through the cellular network
20
, a duplex radio communication link may be effected between two terminals
22
or between a terminal
22
and a landline telephone user
32
through a public switched telephone network (PSTN)
34
. The function of a base station
26
is to handle radio communication between a cell
24
and terminals
22
. In this capacity, a base station
26
functions as a relay station for data and voice signals.
Those skilled in the art will appreciate that “cells” may have configurations other than the omnidirectional cells
24
illustrated in FIG.
1
. For example, the coverage areas conceptually illustrated as a hexagonally-shaped area served by a base station
26
may actually be subdivided into three sectors using separate directional antennas mounted at the base station
26
, with the sector antenna having patterns extending in three different directions. Each of these sectors may itself be considered a “cell” As will be appreciated by those skilled in the art, other cell configurations are also possible, including, for example, overlaid cells, microcells, picocells and the like.
Cell size in time-division multiplexed communications systems is typically limited by the effect of propagation delays on synchronizing the arrival of transmissions from variously located terminals to the slotted frame structures used by base station transceivers. In order to synchronize transmissions from terminals located in a cell, the base station terminal typically transmits a respective timing advance value (TA) to a respective terminal. The terminal advances its transmissions to the base station according to the timing advance value to compensate for the propagation delay between the terminal and the base station. Typically, the timing advance values instruct the terminals to advance their uplink transmissions such that the transmissions from all the terminals served by a base station arrive at the base station in synchronism with a common receive frame structure.
When a terminal attempts to access a system, however, such propagation delay information typically is unavailable. Accordingly, conventional time-division-multiplexed systems commonly utilize a random access channel (RACH) to receive an access request burst from such an unsynchronized terminal and use propagation delay gained from the received RACH burst to determine an appropriate timing advance for the terminal. Upon powering up or handoff to a new base station, an unsynchronized terminal searches for and receives a control channel from the base station that provides an initial timing reference. To initiate use of the base station, the terminal then transmits a RACH burst at a predetermined time in relation to the control channel timing reference. Upon receipt of the RACH burst, the base station can determine round-trip time delay based on the delay between the transmission of the control channel timing reference and the receipt of the RACH burst. The base station uses this round-trip time delay to determine an appropriate timing advance for the terminal.
The RACH typically is a slotted channel that is designed to tolerate significant variation in RACH burst timing. Each RACH slot typically includes a significant amount of “guard time” so that RACH bursts in adjacent slots are less likely to overlap. The amount of guard time provided typically limits maximum cell size, as the amount of guard time determines the maximum delay variation in RACH bursts that can be received by a base station.
For example, in systems conforming to the GSM recommendations, cell size is typically limited by: (1) the number of guard bits (68.25) provided in slots assigned to a RACH logical channel for random access bursts; (2) the number of bits (6) allotted to the timing advance message field in slow associated control channel (SACCH); and (3) synthesizer switching time required between receipt and transmit bursts at terminals operating in half-duplex mode. RACH bursts are used by terminals to achieve access, e.g., at handoff or initial access, and typically have relatively long guard periods (68.25 bits or 252 &mgr;secs). Using an 8.25 bit guard time, the remaining 60.0 bit period (221.5 &mgr;sec) of a slot is available for roundtrip time estimation. The roundtrip delay between a terminals and a base station should be within 221.5 &mgr;sec; otherwise, a RACH bust may overlap and/or collide with the next time slot. The maximum of 221.5 &mgr;sec roundtrip delay thus generally provides for a maximum cell radius of 33.2 km. For cells larger than 33.2 km distance, a RACH burst may collide with the next slot burst, and thus may not allow the base station to estimate the correct roundtrip delay and decode the RACH burst.
Once a connection has been established between a terminal and a base station in a GSM system, the base station continues to measure the time offset between its own burst schedule and bursts received from the terminal. Based on these measurements, the base station periodically provides the terminal with timing advance information in the form of a 6-bit timing advance value (TA) transmitted on the slow associated control channel (SACCH) at a rate of twice per second. The base station estimates round-trip delay on the random-access channel (RACH) on the common control channel (CCCH), and uses this estimated round-trip delay to determine the appropriate timing advance value to send to the terminal. Typically the timing advance value sent by the base station corresponds to the absolute delay between the base station and the terminal in terms of the number of bit periods, such that the 6-bit timing advance value provides a range of from 0 bit periods to 63 bit periods of advance, with a resolution of 1 bit period.
Referring to
FIGS. 2A and 2B
, the uplink frame in GSM is typically delayed by 3 slot periods with respect to the downlink frame (GSM slots h
Asokan Ramanathan
Raith Alex Krister
Ericsson Inc.
Le Danh C
Myers Bigel & Sibley & Sajovec
Trost William
LandOfFree
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