Asymmetric data access scheme

Details Asymmetric data access scheme Asymmetric data access scheme

1997-10-31

2004-09-14

Nguyen, Brian (Department: 2661)

Multiplex communications

Duplex

Communication over free space

C370S329000, C370S344000, C370S352000

Reexamination Certificate

active

06791952

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a method and apparatus for data access over a wireless link, in which a greater data rate may be provided in one direction of a link than in an opposite direction of the link.
BACKGROUND OF THE INVENTION
In a fixed wireless access (FWA) telecommunications system, subscribers are connected to a backbone telecommunications network by means of radio links in place of traditional copper wires. Each of a plurality of subscribers is provided with a subscriber radio terminal at their subscriber premises. A base station provides cellular coverage, typically in urban environments over a 5 km radius, with the plurality of subscriber radio terminals. Each base station may be connected to a backbone network, eg a Public Switched Telecommunications Network (PSTN) switch via a conventional transmission link, known as a backhaul link thereby providing the plurality of subscribers with access to the PSTN. A single base station can serve of the order of up to two thousand subscribers, making the installation and maintenance cost of a fixed wireless access system lower than that of an equivalent copper wire access network.
Referring to
FIG. 1
herein, there is illustrated schematically a radio base station and subscriber terminal of a prior art fixed wireless access system. A plurality of subscriber radio terminals
100
each comprising a transceiver
101
and an antenna
102
communicate with a radio base station
103
having a base station antenna
104
and a base station transceiver apparatus
105
. A plurality of such radio base stations
103
each communicate with a central office switch
106
to gain access to a backbone telecommunications network, eg a Public Switched Telephone Network (PSTN). In a geographical area, a plurality of base stations are distributed to provide coverage in a cellular pattern. Each base station
103
is connected to a local exchange switch
106
via a backhaul transmission line
107
which may comprise for example a terrestrial line eg fiber optic cable or coaxial cable, or a microwave transmission link. Communication between the subscriber radio terminal and the base station is via a wireless radio link
108
. Each local wireless link
108
between radio base station
103
and subscriber radio terminal
100
comprises an uplink from the subscriber antenna to the radio base station antenna, and a downlink transmitting from the radio base station antenna to the subscriber antenna and transceiver. Each radio base station operates either an omni-directional beam or a plurality of broad sectorized beams encompassing all subscribers in a cell or sector for receive and transmit, whereas each subscriber radio terminal operates a directional pencil beam directed at the base station for receive and transmit. In a prior art fixed wireless access deployment, although each nominally hexagonal cell is served by a base station located nominally at the center of the cell, current technologies permit the base station antenna to be located non-centrally within a cell area.
In the prior art fixed wireless access system a frequency spectrum allocation for the uplink is typically of a same bandwidth as a frequency spectrum allocation for the downlink. For example, the uplink may be allocated 15-17 MHz bandwidth in an available radio spectrum, and the downlink may be allocated a further 15-17 MHz bandwidth of frequency spectrum. The uplink and downlink spectrums are spaced apart by typically around 50 MHz, referred to as duplex spacing.
Typically, the uplink frequency allocation of 15 MHz is subdivided into a plurality of 300 KHz slots each occupied by a separate carrier frequency, giving 48 uplink carriers. For a 17 MHz uplink band, divided into a plurality of 300 KHz uplink frequency slots, 54 uplink carriers are available. Similarly, the allocated downlink frequency spectrum is subdivided into a plurality of 300 KHz downlink frequency slots, being symmetric with the uplink frequency allocation.
The 300 KHz frequency slots are allocated to a plurality of radio base stations over a geographical area according to a repeating frequency reuse pattern. To minimize the likelihood of interference, adjacent cells within a fixed wireless access network, or sectors within each such cell are allocated distinct groups of radio frequencies selected so as to minimize the likelihood of a transmission with any cell (or sector of a cell) causing interference in any other cells or sectors nearby. On the uplink, in a three of nine reuse pattern, every ninth frequency is reused, so although only 18 of the 54 available carrier frequencies are used per cell, the frequency pattern can be reused indefinitely, and an allocation of subscriber radio terminals to base stations giving service to around 2000 subscribers per cell can be replicated indefinitely over a geographical area.
Thus, typically in a 17 MHz uplink case, each base station may operate 18 carriers, 6 per sector, in a tri-sectored arrangement. Each carrier frequency is separated into 10 bearer time slots, providing 60 uplink bearer time slots per sector (180 bearers per cell). Of these, 2 to 6 bearer time slots per sector are reserved for an access channel, through which subscriber radio terminals request access to the radio base station leaving 54 bearer time slots per sector available for subscriber usage. Each subscriber radio terminal operates two subscriber lines, so taking account of the bearers reserved for access channels, up to a maximum of 27 radio subscriber terminals in a sector can communicate with a base station at the same time. However, as usage of subscriber terminals is statistical in nature, up to approximately 600 to 700 subscribers per sector can be accommodated since not all subscribers communicate at once.
Similarly, in the 17 MHz downlink band, the downlink frequency allocation at each base station is 18 carriers per cell, each downlink carrier corresponding to an uplink carrier in a frequency division duplex pair. In each sector, there are 6 downlink carrier frequencies, corresponding with the 6 uplink frequencies, to form 6 frequency division duplex pairs per sector. As with the uplink carrier frequencies, the downlink carrier frequencies are time division multiplexed into a plurality of bearer timeslots. Some of those bearer timeslots are used as a downlink broadcast channel which advertises available bearer timeslots to all subscribers within a sector.
For circuit switched services carried over the wireless link, where those services are characterized by having symmetric constant data rate traffic both on the uplink and downlink, eg voice traffic, the prior art symmetric allocation of frequency spectrum between the uplink and downlink beams is relatively efficient. However, for services which entail an asymmetric data rate requirement as between the uplink and the downlink, for example where the volume of traffic data on the uplink differs greatly from a volume of traffic data on the downlink, a symmetric frequency spectrum allocation for the uplink and downlink beams is inefficient. For example, taking an instance of a subscriber making Internet communications on a user terminal
109
, connected to a subscriber radio terminal
100
, a request for data sent to an Internet service provider
110
on the uplink may comprise a transmission of packets of tens or hundreds of Bytes. On the other hand, service data provided by the Internet service provider may comprise data units of the order kBytes or MBytes. Such data is downloaded from the local exchange
106
over the backhaul system
107
through the base station
103
and over the downlink. In a circuit switched application, the bandwidth is reserved and available for use for uplink and downlink communications throughout the duration of a communications session. During the download of data from the Internet, the uplink path remains reserved for use by the subscriber, although no data traffic may be actually flowing on that uplink.
In a fixed wireless access network deployment having a plurality of subscribers

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