Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
2000-04-04
2003-04-01
Le, Thanh Cong (Department: 2749)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C370S465000, C370S468000
Reexamination Certificate
active
06542736
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to data communications in a radio communications system, and more specifically, to adaptation of a radio link to a mobile terminal based on current radio communication conditions.
BACKGROUND AND SUMMARY OF THE INVENTION
There is an ever increasing convergence of the media industry (including television, video, three dimensional graphics, electronic publishing, and entertainment), the computer industry (including desktop computing, personal computers connected by local area networks, electronic mail, web sites etc.), and the telecommunications industry (both fixed and wireless communications networks). All of these converging industries rely on high-speed data communication capabilities.
High-speed data communication is particularly important for Internet communications. The Internet offers access to an extraordinary variety of information resources across the world. Typically, users make that access from a fixed location, such as their home, business, or school. However, cellular telephones, coupled with an increasing variety of other wireless devices, such as wireless laptops and personal digital assistance (PDAs), are changing otherwise fixed points of access to the Internet to include mobile access by these types of mobile terminals. For the sake of simplicity, the term “mobile terminal” is used to encompass all types of wireless devices.
Mobile radio packet data communications employ a different model than the circuit-switched model used, e.g., for traditional mobile radio voice communications. In circuit-switched communications, each communication link is allocated a dedicated radio channel, i.e., a frequency in an FDMA system, a time slot in a TDMA system, or a code in a CDMA system, for the duration of the communication with a mobile radio. Data to other users is not delivered over that dedicated channel, even if there are periods of silence in the communication when no data is being transmitted. Thus, although circuit-switched channels ensure minimal delay and a guaranteed bit rate, which is important for certain applications like voice communications, they are typically underutilized and also are usually limited in bandwidth. That limited bandwidth, while acceptable for certain applications like voice communications, is not well suited for many high-speed data applications that require considerably more bandwidth.
Packet-based data communications are better suited for high-speed data communications. Data packets are delivered individually using a “best effort,” packet-switched network like the Internet. Individual packet routing means that the bandwidth may be used efficiently and that higher bandwidth applications may be accommodated. While wireline data terminals, e.g., personal computers, are capable of utilizing higher, packet-switched network bandwidth, wireless data terminals are at a considerable disadvantage. The bandwidth of the radio interface separating the wireless data terminals from wireline, packet-switched networks like the Internet is limited.
Accordingly, considerable efforts are being made to increase the bandwidth for wireless data communication. That increased bandwidth is particularly important in the radio “down link” direction from the radio network to the mobile terminal. For example, a mobile terminal user might send in the radio “up link” direction, a low bandwidth request, e.g., a command, to download a web page from a site on the Internet. Downloading the web page and other information (especially graphics) from that web site requires considerably more bandwidth.
Another problem confronting data communications over the radio interface is the variable quality of the radio channel or link from base station to mobile terminal (downlink). The radio channel or link quality depends on a number of factors including the distance between the mobile terminal and a transmitting base station in the radio network, interference from other transmitting base stations and mobile terminals, path loss, shadowing, short term multi-path fading, etc. If the channel quality is good, the base station may modify the signal transmission parameters to increase the data transmission rate from the base station to the mobile terminal. On the other hand, if the channel quality is bad, the signal transmission parameters may need to be adjusted to lower the data transmission rate to ensure that the signal is reliably received.
The process of modifying one or more signal transmission parameters to compensate for channel quality variations is sometimes referred to as “link adaptation,” where “link” refers to the radio link between a base station and a mobile terminal. Link adaptation may be accomplished by changing the transmit power of the base station, e.g., increasing the transmit power level for data transmitted to mobile terminals with a bad channel quality. Link adaptation may also be accomplished by changing the type of modulation and amount of channel coding applied to the data to be transmitted by the base station. Moreover, link adaptation may also be performed in the uplink by the mobile terminal.
The present invention is concerned with the latter type of link adaptation because in wide band code division multiple access (WCDMA) communication system, increasing the base station transmit power for an individual subscriber communication adversely impacts other mobile subscriber communications and ultimately limits the overall system capacity. In contrast, changing the modulation and/or coding format to match the current channel quality, without increasing the transmit power, does not adversely impact other mobile subscriber communications or the overall system capacity. For example, mobile terminals close to the base station (presumably experiencing a high channel quality) may be assigned a higher order modulation that provides higher bit rates. A lower order modulation offering a lower bit rate may be employed for communications with mobile terminals farther away from the base station (presumably experiencing lower channel quality). Similarly, when the channel quality is relatively good, higher code rates (i.e., less redundancy) may be employed. Lower code rates (more redundancy) are employed for lower channel quality.
The underlying assumption is that the maximum data rate reasonably supported with each mobile terminal, (i.e., the maximum data rate under the current radio channel conditions to meet a certain performance standard such as a maximum bit error rate), depends on the channel quality experienced by the mobile terminal.
Each base station may be divided into multiple sectors, where each sector serves a particular portion of the geographical area surrounding the base station. For example, each sector of a three sector base station serves approximately one third of the total geographical area surrounding that base station. The mobile terminal estimates the channel quality by measuring the signal quality of pilot signals or other broadcast signals transmitted by nearby “candidate” base station sectors, where some of the sectors may be associated with different base stations. Based on the estimated channel qualities, the mobile terminal determines a maximum data rate at which the mobile terminal can receive data for each base station sector and selects the sector with the highest data rate. The mobile terminal sends a rate/sector request message to one or more base stations in the radio network including information about the current estimated maximum supportable transmission rate as well as the currently requested sector to make the downlink transmission to the mobile terminal. That message also identifies a currently requested base station.
In order to track the rapidly changing conditions of the radio channel, the channel quality measurements are performed at a high rate and the corresponding rate/sector request messages are sent at a high rate in order to track those rapid changes. When a base station receives these rate/sector request messages from several mobile terminals, the base station chooses wh
Dahlman Erik
Parkvall Stefan
Cong Le Thanh
Nguyen Huy D
Nixon & Vanderhye PC
Telefonaktiebolaget LM Ericsson (publ)
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