Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Reexamination Certificate
1999-07-14
2003-06-03
Vo, Nguyen T. (Department: 2682)
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
C370S337000
Reexamination Certificate
active
06574202
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to the transmission and reception of information in a wireless communication system and more particularly to a method of interleaving information conveyed over a wireless communication system.
2. Description of the Related Art
Wireless telecommunication systems use various schemes to allow multiple users to use (i.e., share simultaneously) a particular bandwidth. The bandwidth is part of or all of the band of frequencies allocated to the telecommunication system for conveying (i.e., transmitting and receiving) information. The wireless telecommunication systems typically use techniques known as multiple access schemes to allow a plurality of users to share a given bandwidth. One multiple access scheme that is widely used in many wireless telecommunication systems is called Time Division Multiple Access (TDMA).
A TDMA wireless telecommunication system allows multiple users to use the same bandwidth by creating time frames (hereinafter “frames”) comprising time slots (hereinafter “slots”) within which each user is allowed to convey their information. Each slot or a group of slots are defined and are assigned to a particular user. Thus, each user is allowed to transmit and/or receive information in that user's assigned slot. In this manner, many users are able to use the same bandwidth thus increasing the user and/or information capacity of the system.
FIG. 1
depicts a typical scenario in a TDMA system where user
1
is conversing with user
2
. For the sake of clarity, only two users are shown. In an actual system, there is usually more than one base station and many more users throughout the system some of whom obtain access to base station
106
via a wireline telephony network such as the Public Switched Telephone Network (PSTN) (not shown). The users have mobile equipment
100
and
112
(e.g., cellular phone) which they use to communicate with each other. Base station
106
represents part of the TDMA system's equipment which relays the user information in accordance with the protocol used by the system. The protocol is a particular set of rules by which the system equipment and the user equipment initiate communication, convey information and terminate communication. Typically, protocols are established communication standards that are well defined and which are followed by many wireless telecommunication systems. Wireless communication channels
102
and
110
are commonly referred to as the uplink, which is the channel used by the users to transmit information to base station
106
. Wireless communication channels
104
and
108
are commonly referred to as the downlink, which is the channel used by the users to receive information from base station
106
. The information is conveyed over the uplink and downlink in the form of Radio Frequency (RF) signals. Base station equipment
106
comprises radio equipment (e.g., transmitters and receivers) and other system equipment owned by a system or network provider.
FIG. 2
depicts a particular frame format used by a protocol referred to as the IS-136 standard. The IS-136 standard is a protocol used by many TDMA wireless communication systems in North America and South America. The frame format is the particular arrangement of the slots within frame
200
. In particular, frame
200
has a time duration of 40 milliseconds and is divided equally into six slots. Thus, each slot has a time duration of 6.67 milliseconds. For IS-136, the frame format (i.e., six slots per frame) is the same for the uplink and the downlink.
In IS-136, either three (3) users or six (6) users are allowed to convey information within the frame format discussed above. The number of users conveying information per frame depends on the mode at which the TDMA system is operating. In IS-136 compliant systems there are two modes of operation. One mode of operation is referred to as the Full Rate mode in which each user is assigned two slots per frame. The other mode of operation is called the Half Rate mode in which each user is assigned one slot per frame. Thus, there are six users per frame in the Half Rate mode and three users per frame in the Full Rate mode. A particular slot allocation scheme in IS-136 Full Rate mode for the uplink and the downlink has a first user assigned to slots
1
and
4
, a second user assigned to slots
2
and
5
and a third user assigned to slots
3
and
6
.
TDMA signals, especially uplink signals, are by nature bursty signals; that is the signals for any particular user are transmitted and received as short intermittent bursts each of which carries a relatively great amount of information. The RF signals which carry the user information are exposed to various well known distorting effects such as phase jitter, frequency offset, amplitude and phase fading and multipath distortions. When a transmitted or a received burst of information is adversely affected by channel distortions, a substantial amount of the conveyed information is lost. Therefore, to reduce the likelihood of losing significant amounts of information, a certain amount of processing of the information is done prior to transmission. The intent of the processing is twofold: first it is to introduce time diversity to the information with the use of well known channel coding techniques and second, to spread out in time the burst of information with the well known technique of interleaving. The processing of information to be transmitted to mobile stations and information received from mobile stations is performed by system equipment typically located at base station
106
.
Time diversity adds repetitions to the user information to reduce the likelihood of losing information due to channel distortions. The repetitions add redundancy in the information rendering said information more robust (better able to withstand noisy conditions) with respect to a noisy communication channel. The technique of interleaving spreads out a burst of information over a period of time further adding time diversity to the information. Thus an entire slot of information for a user is less likely to be lost since the information is spread across two or more slots. Channel coding introduces time diversity to information within a slot by adding information (e.g., bit padding—adding repetitive blocks of bits) to the information. Because channel coding actually adds information to the user information to be conveyed, it is costly since it reduces a system's overall capacity. Further, the time diversity introduced by channel coding is limited because such processing is restricted to within a slot. Interleaving reduces the likelihood that a user's entire slot of information would be lost.
Referring to
FIG. 3
, there is shown the traditional 2-slot interleaving process for an IS-136 compliant TDMA system in the Full Rate mode; this interleaving scheme is referred to as the IS-136 1-4 interleaving scheme and is currently used in IS-136 TDMA communication systems. There are three users per frame, User A, user B, user C. Information to be transmitted by each of the users are stored in their respective buffers (i.e., temporary storage devices). That is, user A information is stored in buffer
301
A,
302
A,
303
A etc . . . , user B information is stored in buffer
301
B,
302
B,
303
B, etc . . . and user C information is stored in buffer
301
C,
302
C,
303
C, etc . . . The capacity of each of the buffers is expressed in terms of time. For example, all of the buffers in
FIG. 3
are 20 millisecond buffers meaning that all buffers transfer information at a certain rate and thus the time duration for transferring information determines the amount of information transferred. Similarly, each slot has an associated capacity that depends on the rate of transmission of information and the time duration of the slot. Note, the fractions shown (i.e., ½) next to the lines going from a buffer to a time slot indicate a specific portion of the information stored in the buffer that is being transmit
Himayat Nageen
Seymour James Paul
Tonello Andrea M.
Lucent Technologies - Inc.
Ly Nghi H.
Vo Nguyen T.
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