Multiplex communications – Communication over free space – Combining or distributing information via code word channels...
Reexamination Certificate
1999-06-11
2003-04-01
Le, Thanh Cong (Department: 2684)
Multiplex communications
Communication over free space
Combining or distributing information via code word channels...
C370S320000, C375S152000
Reexamination Certificate
active
06542493
ABSTRACT:
BACKGROUND OF THE INVENTION
I. Field of the Invention
This invention relates generally to communication systems. More specifically, the invention relates to random access communication systems.
II. Description of the Related Art
The use of wireless communication systems for the transmission of digital data is becoming more and more pervasive. In a wireless system, the most precious resource in terms of cost and availability is typically the wireless link itself. Therefore, one major design goal in designing a communication system comprising a wireless link is to efficiently use the available capacity of the wireless link. In addition, it is also important to minimize the delay associated with data transmissions.
In a system in which multiple units compete for finite system resources, a means must be developed to regulate access to such resources. In a digital system, remote units tend to generate bursty data. The bursty data is characterized in that it has a high peak-to-average traffic ratio, meaning that large blocks of data are transferred during short periods of time interposed between significantly longer periods of idleness. Dedication of an individual communication channel to each active unit does not result in efficient use of system capacity in a system in which units generate bursty data because, during those times when the remote unit is not utilizing the system, the allocated channel remains idle. The use of dedicated channels also may impose a hard limit on the number of remote units which may simultaneously use the system regardless of the usage patterns of the remote units. In addition, the use of dedicated channels may cause unacceptable delay if the slice of capacity allocated to each remote unit is so small that data transfer rates are greatly compromised.
The characteristics of the inbound and outbound traffic tend to differ significantly in a digital data system. For example, in a system which provides wireless Internet services, a typical inbound transmission from a remote unit is relatively short, such as a request for a web page. However, a typical outbound data transfer to a remote unit tends to be rather large. For example, in response to a request for a web page, the system may transfer a significant amount of data. Because the characteristics of the inbound and outbound channel are very different, system efficiency may be increased by developing two distinct protocols for the inbound and outbound links.
A random access ALOHA protocol was developed for use in the inbound link from a remote unit in a digital data system. The basic idea behind ALOHA is quite simple: the remote units transmit whenever they have data to send. If the remote units are using a communication resource which can only be accessed by one remote unit at a time, the information from each remote unit is destroyed if two units transmit at the same time causing a collision. In a system where the remote unit can monitor the random access channel, the remote unit may listen to the channel in order to determine whether its transmission is the victim of a collision. In a system in which the remote unit does not or cannot monitor the random access channel, the remote unit may detect a collision based upon the absence of an acknowledgment received from a hub station in response to a transmission. According to standard ALOHA operation, whenever a collision occurs, the remote unit waits a random amount of time and retransmits the data. The waiting time must be random or the colliding remote units generate collisions in lockstep over and over again.
FIG. 1
is a timing diagram showing the operation of a pure ALOHA random multiple access system. As shown in shown in
FIG. 1
, five remote units designated A, B, C, D and E are transmitting packets of data within a common communication channel. Whenever two remote units transmit at the same time, a collision occurs and both transmissions are lost. In a pure ALOHA system, if the first bit of a new transmission overlaps just the last bit of a transmission already in progress, both transmissions are totally destroyed and both have to be retransmitted at some other time. For example, in the frequency modulated (FM) channel shown in
FIG. 1
where no two packets may contemporaneously be transmitted, a packet
12
transmitted by remote unit B collides with a packet
10
transmitted by the remote unit A and a packet
14
transmitted by remote unit C. The remote unit A must retransmit the information in the packet
10
, the remote unit B must retransmit the information in the packet
12
and the remote unit C must retransmit the information in the packet
14
.
FIG. 1
shows the remote unit C retransmitting the packet
14
as packet
14
R.
In a pure ALOHA system, if the average packet transfer rate is low, most packets are transferred without a collision. As the average packet transfer rate begins to increase, the number of collisions increases and, hence, the number of retransmissions also increases. As the average packet transfer rate increases linearly, the probability of retransmissions and multiple retransmissions increases exponentially. As some point as the average packet transfer rate increases, the probability of successful transmission falls below a reasonable number and the system becomes practically inoperable. In a pure ALOHA system, the best channel utilization which can be achieved is approximately 18%. Below 18%, the system is underutilized. Above 18%, the number of collisions increases such that the throughput of the system begins to fall.
The introduction of a satellite link within a digital communication system complicates the multiple access dilemma. The use of a geosynchronous satellite typically introduces a 270 millisecond (msec) delay between transmission of a signal from a remote unit and reception of that same signal at a hub station. Due to the delay introduced by a satellite link, scheduled access schemes which require the remote unit to request system resources before beginning any transmission are impractical for many applications. Therefore, a satellite link which serves a great number of remote units which transmit bursty data is a likely environment in which to implement an ALOHA system.
If an ALOHA system is implemented in a satellite system in which the remote units can't or don't monitor the random access channel, in the event of a collision, the remote unit does not know of the collision for at least 540 msec. In addition to the notification delay, the remote unit typically must wait some random amount of time before retransmitting the data (to avoid lockstep retransmissions). The retransmitted signal is once again subjected to the 270 msec time delay. The cumulative delay of such a transmission can easily exceed one second. In a fully loaded system, the delay can be significantly longer due to the increased probability of repeated collisions. Therefore, when using a satellite link, it is advantageous to limit the number of retransmissions attributable to collisions as well as other causes. The number of retransmissions due to collisions can be decreased by simply reducing the allowable system load.
The satellite link also introduces challenges concerning the successful transmission of data over the link. Due to the high level of interference and high path loss which characterizes a satellite link, typically a relatively robust physical interface must be used. One physical interface commonly used over satellite links is direct sequence spread spectrum (DSSS). In a prior art DSSS system, the communication channel is defined by a maximal length binary spreading sequence. Each discrete binary value which makes up a spreading sequence is referred to as a “chip.” The spreading sequence is selected such that the autocorrelation of the sequence with itself is nearly zero for all chip-aligned offsets other than zero. A maximal length pseudo noise (PN) sequence of length n has the property that its circular correlation with itself (autocorrelation) is either 1 or 1
for any chip-aligned offset. The correlation of th
Cong Le Thanh
Gantt Alan T.
Knobbe Martens Olson & Bear LLP
Tachyon, Inc.
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