Multiplex communications – Pathfinding or routing – Combined circuit switching and packet switching
Reexamination Certificate
1995-06-07
2002-04-09
Marcelo, Melvin (Department: 2663)
Multiplex communications
Pathfinding or routing
Combined circuit switching and packet switching
C370S354000
Reexamination Certificate
active
06370135
ABSTRACT:
TECHNICAL FIELD
This invention relates generally to wireless data communication systems. More particularly, the invention relates to mobile data base stations supporting transmission and reception of data in multiple modes.
BACKGROUND ART
The modern analog cellular system for mobile wireless duplex voice transmission is called “Advanced Mobile Phone Service” (AMPS). The AMPS cellular network uses the FCC assigned carrier frequency range of 800 to 900 MHz. Automobile-mounted cellular units transmit voice signals to a cellular base station within a given cell using up to one watt of power. Hand-held cellular units using battery power supplies transmit voice signals to a cellular base station within a given cell using up to one quarter watt of transmission power.
The analog human voice was the signal that the AMPS system was first designed to communicate. The AMPS system has been optimized for carrying as many analog voice signals within a given bandwidth of a channel as possible. Mobility of the cellular telephone using low power mobile units, FM modulation, and the higher carrier frequency range (800 MHz-900 MHz) is achieved through a cellular arrangement of base stations whereby a user's signal is handed off to the next cell site as he or she moves into a different cell area. This cellular hand-off can cause a temporary loss in transmission or reception. Temporarily losing a voice signal is not critical because a user knows when there is a signal loss and the voice information can be retransmitted. However, signal loss, even though temporary, poses special problems for transmission of digital data. Some other AMPS cellular problems causing loss in voice signals are drops in signal strength, reflections, Rayleigh fading, and cellular dead spots.
The availability of portable computers naturally results in the desire to conduct wireless transmission of digital data from a remote location. Presently, the AMPS voice cellular system is being used to transmit digital data in the form of circuit-switched cellular data across AMPS carrier channels. Raw (baseband) digital data must be converted so that it can be transmitted and received across the analog AMPS system. One disadvantage to using the AMPS system for data transmission is that a narrow channel bandwidth and errors in transmission limit the baud rate for transmitting and receiving the digital data. As previously stated, loss of raw digital data may be caused by other problems in the AMPS mobile cellular system.
In the circuit-switched mode of data communication, a single channel is set aside for specific communication, and is dedicated thereto until that communication is complete. When sending data in a conventional cellular circuit-switched mode, a modem creates a waveform from the data in a manner similar to that of a wireline modem. The modem combines amplitude and phase modulation to create modulation based upon multiple bits per symbol. The resulting modulated signal is then transmitted over the cellular voice channel in the same way that it would be sent over a wireline connection, i.e., as an audio signal. The modulation constellations used for cellular modems are, although similar, generally simpler than that used for their wireline counterparts. However, they are somewhat similar. Consequently, the modulated circuit-switched cellular signals are particularly vulnerable to the sort of degradations that occur on a normal cellular voice channel which is far noisier than a typical wireline voice channel.
In general, performance of cellular circuit-switched modems is quite poor as soon as the signal strength becomes relatively low or the interference level becomes high. For example, while the sensitivity of the cellular voice receiver is about −103 dBm in a fading channel, the circuit-switched connection begins to degrade at about −80 dBm. Normally, such degradation can be overcome by shifting the bit rate downwards. However, even at 4.8 kbps, it is difficult to maintain a reliable connection when the signal level dips below −90 dBm. Even at high signal levels in the cellular circuit-switched arrangement, it is rare to be able to achieve more than 14.4 kbps data rate.
In addition, the performance of the circuit-switched modem is impacted by the nature of the backhaul as well as the nature of the airlink. The backhaul can cause severe echo problems that necessitate good echo cancellation techniques requiring complex circuitry resulting in increased costs and additional station space. Thus, conventional circuit-switched AMPS data transmission is expensive as well as risky.
Another problem of data transmission with a mobile subscriber station occurs when a subscriber station is moving at the edge of a cell or coverage area. Such circumstances can result in a substantial loss of data, or substantial delays due to the necessity of re-transmitting the data. These problems have been addressed in part by the Cellular Digital Packet Data (CDPD) system described in the CDPD specification, Version 1.1, incorporated herein as background information. The CDPD communication system shares the same carrier frequencies assigned to the AMPS channels as indicated in the CDPD specification.
The base unit or mobile data base station (MDBS
1
, as illustrated in FIG.
1
), of an exemplary CDPD system utilizes a channel within an AMPS cell to establish a link and communicate to a user's mobile end system. The MDBS may use other frequencies outside of AMPS that are made available to it by service providers. The mobile end system or subscriber station (M-ES
2
) is a portable computer, hand-set or other portable electronic device containing a subscriber communication unit. The MDBS serves as a communication link between the user of the M-ES
2
and a service provider's network of wire lines, microwave links, satellite links, AMPS cellular links, or other CDPD links (such as mobile data intermediate system MD-IS
3
, intermediate systems
4
,
5
,
6
) to convey data to another mobile end system, computer network, or non-mobile or fixed end-user system (F-ES
7
,
8
).
The CDPD network is designed to operate as an extension of existing communication networks, such as AMPS networks and the Internet network. From the mobile subscriber's perspective, the CDPD network is simply a wireless mobile extension of traditional networks. The CDPD network shares the transmission facilities of existing AMPS networks and provides a non-intrusive, packet-switched data service that does not impact AMPS service. In effect, the CDPD network is entirely transparent to the AMPS network, which is “unaware” of the CDPD function.
The CDPD system employs connectionless network services (CLNS) in which the network routes each data packet individually based on the destination address carried in the packet and knowledge of current network topology. The packetized nature of the data transmissions from an M-ES
2
allows many CDPD users to share a common channel, accessing the channel only when they have data to send and otherwise leaving it available to other CDPD users. The multiple access nature of the system makes it possible to provide substantial CDPD coverage to many users simultaneously with the installation of only one CDPD transmitter/receiver in a particular sector (transmitting range and area of a standard AMPS base station transceiver).
The airlink interface portion of the CDPD network consists of a set of cells. A cell is defined by the geographical boundaries within which the RF transmission between an M-ES and an MDBS are received at acceptable levels of signal strength. It is not sufficient that the subscriber receive an adequate signal level. The base station must also receive a good signal from the subscriber station (M-ES) for the subscriber to be considered within the cell. The transmitter supporting the cell may be located centrally within the cell, with transmission being carried out via an omni-directional antenna, or the transmitter located at the edge of a cell and transmission carried out via a directional antenn
Cirrus Logic Inc.
Marcelo Melvin
Skjerven Morrill & MacPherson LLP
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