Pulse or digital communications – Spread spectrum
Reexamination Certificate
2000-03-06
2004-02-24
Tse, Young T. (Department: 2634)
Pulse or digital communications
Spread spectrum
C375S219000, C455S088000
Reexamination Certificate
active
06697415
ABSTRACT:
BACKGROUND
1. Technical Field
The present invention relates generally to communication networks utilizing spread spectrum radio transceivers, and, more specifically, to multi-hop RF networks wherein participating devices utilize spread spectrum transceivers that are capable of operating in any of a variety of spread spectrum modes. The spread spectrum modes include, for example, direct sequence transmission across a spreading bandwidth or channelized across the spreading bandwidth, frequency hopping transmission across all or a part of the spreading bandwidth, a hybrid combination of direct sequence transmissions and frequency hopping transmissions, and transmissions on a portion of the spreading bandwidth. The selection of a spread spectrum mode of operation depends upon signal conditions and characteristics of members capable of communication within the RF communication network.
2. Description of Related Art
Communication devices within a wireless local area network employ wireless communication links to transfer data and commands within the local area network. Typical units within a wireless local area network include stationary wireless access devices, mobile radio units, mobile image capture units, printing units, and other units operative with the data and commands. These units often link to a wired local area network through a wireless access device to transfer data and commands to devices located on the wired network. The wireless local area networks typically employ cellular communication techniques to provide the wireless communication links within the local are network.
One common installation of a wireless local area network serves factory automation functions. Because hard-wiring a local area network within a large, dynamic facility is both expensive and difficult, the wireless local are network provides traditional network functions as well as additional functions germane to the wireless attributes of the network. However, due to difficult transmission and interference conditions within a factory, establishing and maintaining sufficient wireless communication ties oftentimes proves difficult. Attenuation of transmitted signals, multi-path fading, ambient noise, and interference by adjacent cells often disrupts communication within the wireless local area network.
Spread spectrum transmissions are often used in attempts to overcome communication problems. With spread spectrum transmissions, the bandwidth over which information is broadcast is deliberately made wide relative to the information bandwidth of the source information. Spread spectrum transmission techniques include direct sequence transmission, frequency hopping transmission, a combination of direct sequence transmission and frequency hopping transmission, and may include other techniques that deliberately transmit over a wide spectrum.
Direct sequence spread spectrum transmitters typically spread by first modulating a data signal with a pseudo random chipping sequence at a multiple of the source data clocking rate. Once constructed, the composite modulation is coupled to a carrier via modulation techniques and then transmitted. Phase modulation is typically employed, but frequency modulation or other types of modulation may also be used. Circuitry in a receiving units receives the signal, decodes the signal at the multiple of the source data clocking rate using a particular chipping sequence, and produces received data. In a typical direct sequence system, the pseudo random chipping sequence applied by the receiving unit corresponds to all, or respective portions, of the transmitted signal. In this fashion, the receiving unit receives only intended data and avoids receiving data from adjacent cells operating on the same frequency. Direct sequence spread spectrum modes also provide significant noise rejection characteristics since each component of the source data is essentially transmitted multiple times. The received signal is therefore a composite that may be averaged or weighted to avoid receiving improper data or falsing based upon noise.
A frequency hopping system commonly uses conventional narrowband modulation but varies the modulation frequency over time in accordance with a known pattern or algorithm, effectively moving the modulated signal over the intended spreading bandwidth. The spread spectrum signal is only discernible to a receiver that has prior knowledge of the spreading function employed and which has obtained synchronization with the spreading operation at the transmitter. By spreading transmissions over the spreading bandwidth, particular portions of the spreading bandwidth within which transmission is difficult may be substantially avoided.
In the United States and many other countries, spread spectrum communications is used commercially within designated Industrial, Scientific and Medical (ISM) bands. These bands are structured as multi-use bands containing non-communications equipment such as industrial and commercial microwave ovens as well as low power consumer grade transmitters, vehicle location and telemetry systems and other spread spectrum devices of differing characteristics. Operation in ISM bands is unlicensed and uncoordinated, so equipment operating in these bands must be designed to operate successfully without knowledge of the types of devices that may be used in close proximity. The spread spectrum system design must also take into consideration the occupants of the spectrum adjacent to the ISM bands which may be both potential sources of interference to, and susceptible to interference from, various types of spread spectrum products.
Various forms of modulation across the spreading band may be utilized in commercial spread spectrum packet data communication systems. Full band direct sequence systems occupy the entire width of an ISM band. The spreading ratio, the ratio of the bandwidth of the spread spectrum modulated signal to the information bandwidth of the source modulation, determines the process gain of the system. Regulations within the United States mandate a minimum process gain of 10 dB, which is determined from ten times the logarithm of the spreading ratio. Process gain is a measure of the ability of a spread spectrum system to resist interference. The larger the spreading ratio, the more resistant the system is to interference within the receiver bandwidth. Wide bandwidth modulation is reasonably resistant to low or moderate levels of interference, but even systems with relatively high process gains experience difficulties when subject to strong interference.
When system throughout requirements dictate high data rates, the minimum process gain requirements in the regulations necessitate using wide bandwidth transmissions. For example, a well-known system NCR Wavelan uses Quadrature PSK modulation at 1 million symbols per second to achieve 2 megabits per second (MBPS) data rates with a source information bandwidth available in the US ISM band at 902 MHz band. In practice, implementation constraints dictate that this system uses the full 26 MHz band. Systems operating at other data rates, including the original Norand system, utilize the full bandwidth at lower source data rates, e.g., 200 kilobits per second (KBPS). Utilization of a wider spreading bandwidth in this case provides greater rejection of multipath fading typical of the indoor RF signal propagation environment.
When it is anticipated the direct sequence systems may be used in environments with strong in-band interference, a design choice is to employ channelization to reject interference. In the case of channelized direct sequence (DS) modulation, the spreading bandwidth is reduced to a fraction of the total available bandwidth, and a frequency-agile frequency generation systems is employed. By selecting the carrier frequency of operation, communications can be established in a portion of the band where interference is not present. This technique requires the use of selective filters in the receiver intermediate frequency (IF) section to provide the necessary interference rejection. These
Broadcom Corporation
McAndrews Held & Malloy Ltd.
Tse Young T.
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