Multiplex communications – Communication techniques for information carried in plural... – Adaptive
Reexamination Certificate
1998-10-19
2004-03-02
Sam, Phirin (Department: 2661)
Multiplex communications
Communication techniques for information carried in plural...
Adaptive
C370S329000, C370S437000
Reexamination Certificate
active
06700902
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to communications, and more particularly, to a method for improving data package delivery of wireless communications through dynamic packet sizing.
BACKGROUND OF THE INVENTION
With the acceptance of wireless communications as a safe and reliable means by which to transfer data, more and more applications are being developed to support this need. These applications can be broadly divided up into two categories, mobile and telemetry.
Mobile devices are those devices where the end device is mobile, such as cellular phones, pagers, and Personal Digital Assistants (PDAs). Telemetry devices are those devices that are in a fixed point, such as an alarm sensor, or a remote meter reading device. Typically, mobile devices are usually operated by a human and are being moved around (such as a device mounted to a vehicle), while telemetry devices are usually unmanned and are fixed.
Wireless DataPacket (WDP) networks, such as ARDIS (Advanced Radio Data Information Systems) and BellSouth Wireless Data's Mobitex (formerly known as RAM Mobile Data), transfer data between a first device (hereinafter also referred to as tower, base station, or carrier device) and a second device (hereinafter also referred to as remote, modem, or end user device) using data packets. These traditional wireless systems use a fixed packet size to transfer data.
Packets are typically sent using a digital modulation scheme with built in error correction. This error correction is necessary due to the nature of radio frequency (RF) and the effects that the atmosphere has on the signals. When a device receives a data packet, the device will decode the message, using the inverse of the modulation scheme and then calculate a Bit Error Rate (BER), which is a measure of the number of bits that are in error inside the packet. If the BER is low enough, the device can fix the errors in the packet, thus making the packet useable. If the BER is too high, the method of error correction cannot guarantee correct data, thus rendering the packet unusable, forcing it to be discarded. Several negative impacts result if correct data cannot be guaranteed, including negative impacts on the network carrier only, on the application only, and on both the network carrier and the application. Some carriers only bill the customer on “delivered” packets, meaning correct packets. So, if a packet must be discarded by either the end user device or the tower device, the network carrier cannot bill the customer for that data, thus resulting in a loss of revenue for the carrier.
A negative impact on the end user device is that the information that it was trying to send was not correctly delivered to the tower device, thus requiring it to re-try the transmission of the packet. This is time consuming and unproductive for the end user. If none of the re-tries are successful, then the end user device is not able to get the information to the tower, rendering the application useless. Also, some carriers bill customers for all packets, whether the packets are delivered or discarded, thereby resulting in a higher bill to the customer. A negative impact on both the carrier and the application comes from areas that have highly congested packet traffic. As the traffic increases, the probability of the packet correctly reaching its destination is reduced. It can be shown that the throughput, T, of a pure ALOHA scheme, is T=Re
−2R
where R=&lgr;&tgr;, &lgr; is the mean arrival time in packets per second (assuming a Poisson distribution) and T is the average duration of the packet, as described in “Wireless Communications, Principles and Practice,” Theodore S. Rappaport, published by Prentice Hall PTR, © 1996, pg. 412. This shows that as the average number of packets is increased or the average duration of the packets is increased, the throughput of the system is decreased. This results in an increased number of packet collisions, which causes the packets to be undeliverable, leading to un-billable revenue activity by the network carrier and a failure of the application for the end user.
A wireless communications system or radio system generates a radio frequency signal that contains information. The system propagates or transmits that signal through the atmosphere with enough strength to be received at the appropriate location. The system preferably performs this function with a high degree of reliability under many different conditions.
Radio signal reception may best be described in terms of probabilities. It is difficult to calculate the actual signal level and effective receiver sensitivity with absolute certainty due to the fluctuations of both signal and noise levels caused by signal reflections. However, it is possible to predict a radio coverage area with a relatively high degree of accuracy.
As shown in
FIG. 1
, when radio signals operate in an environment free of obstructions (free space), their behavior may be predicted by subtracting radio signal losses from gains. Gains enhance or increase signal strength while losses attenuate or reduce that strength. The signal's gains minus its losses will determine whether the signal is strong enough for a receiver to recognize the signal.
To determine radio propagation performance in free space, the following gain and loss factors may be considered. Gains that are of most importance are transmitter output power/receiver sensitivity and the transmitter antenna. The RF transmitter is the source of power used by the system. A typical RF network fixed transmitters may operate with about 45 watts of power. A typical subscriber unit device may operate with about ¾ to one watt of power. This discrepancy is offset by the performance of the receivers and the topology of the area of operation. Base station receivers are typically more sensitive and selective (i.e., they “hear better”) than the receivers in subscriber devices.
With regards to the transmitter antenna, a base station antenna converts an electrical signal to an electromagnetic wave that radiates through the atmosphere. Subscriber units detect these waves and translate them back to the original message sent by the host computer. Most antennas radiate electromagnetic waves evenly in all directions. Accordingly, for a typical ground-based system, waves radiate up into the sky and are wasted. For this reason, most networks use antennas that concentrate the radiated signal in the desired directions by reducing signals radiated above the horizon.
Some of the losses that are important in determining radio propagation performance in free space are the transmission line, the free space attenuation, and the subscriber unit antenna. With respect to the transmission line, a typical network's transmitter/receivers are connected to base station antennas by a transmission line. This line has a loss associated with it that is proportional to its length. Losses associated with free space attenuation result when an electromagnetic wave travels unobstructed through the atmosphere, it loses its power in proportion to the distance that it travels. Several factors cause this attenuation. First, the atmosphere offers resistance to the signal and lowers its strength. Second, as the wave radiates outward, the area it covers increases. Subsequently, the wave's radiated energy must cover a larger area causing the signal strength to decrease at any particular point. Another loss factor to consider is losses associated with the subscriber unit antenna. Subscriber units typically have internal antennas. For an antenna to provide a gain, or at least have no loss, it must have a certain characteristic length and remain unobstructed by any metallic objects. These features are much easier to design into an exterior antenna. However, an exterior antenna may reduce user convenience by limiting mobility.
Accordingly, in free space, a received signal level can be easily calculated by factoring together gains and losses. If the received signal level is greater than the minimum sensitivity of the
Elster Electricity LLC
Sam Phirin
Woodcock & Washburn LLP
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