Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1999-02-16
2001-02-27
Tsang, Fan (Department: 2746)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S423000, C455S452200, C455S513000
Reexamination Certificate
active
06195554
ABSTRACT:
TECHNICAL FIELD
This invention relates generally to wireless communication systems, and more particularly to a method and device for reassigning a transmission channel based on interference level and channel quality measurements.
BACKGROUND OF THE INVENTION
Wireless communication technology has experienced unprecedented growth, fueled by advances in radio frequency, satellite, and microelectronic technologies and by the convenience of access to telephony and portable wireless devices. Several technologies have emerged for providing wireless communications, including analog systems such as Advanced Mobile Phone System (AMPS), and digital systems such as Global System for Mobile Communications (GSM) and Digital AMPS (D-AMPS). Cellular radio systems operate in an interference-limited environment and typically rely on frequency reuse plans and Frequency Division Multiple Access (FDMA), in analog systems, and Time Division Multiple Access (TDMA), in digital systems, for example, to maximize capacity.
Because bandwidth and radio spectrum are limited resources shared by all users in a wireless network, schemes to divide the available network bandwidth among as many subscribers as possible are often employed. One such scheme involves a combination Time and Frequency Division Multiple Access (TDMA/FDMA) method. For example, 25 MHZ bandwidth can be divided into 124 carrier frequencies spaced 200 kHz apart, with one or more carrier frequencies assigned to each Base Station in the network. Each of the carrier frequencies, in turn, can be divided in time using a TDMA scheme to define logical channels. Channels, in turn, may be divided into dedicated subchannels which are allocated to a mobile station with common channels used by mobile stations in the idle mode.
A Traffic Channel (TCH) is used to carry speech and data signals. Traffic channels are defined in GSM using a 26 multi-frame, or a group of 26 TDMA frames. With 26 frames, 24 can be used for traffic, one for a control channel, and one unused. In other systems, other methods are used, such as CDMA or FDMA. An Idle Channel Measurement (ICM) algorithm that uses dynamic channel assignment is usually utilized by the Base Station Controller (BSC) to select channels for transmission of speech and data signals.
In addition to dividing bandwidth among subscribers, transmission channels are assigned or allocated to incoming calls to the network. Two methods of channel assignment are typically used in the industry: fixed channel assignment and dynamic channel assignment. Fixed channel assignment involves assigning a predetermined set of channels or frequencies to the base station or BSC. If all the channels within the cell of the base station or BSC are occupied, either the call is blocked, meaning the mobile handset does not receive service, or borrowing strategies are used to borrow channels from neighboring cell. With dynamic channel assignment, channels are not allocated to a base station or BSC permanently. The Mobile Switching Center (MSC) assigns the BSC some frequencies, which remain fixed for a period of time. Periodically, some measurements are made regarding channel utilization, and adjustments are made to the channel assignments as needed. The frequencies or channels are assigned by the BSC each time a call request is made.
A problem with prior art channel assignment algorithms and devices is that channel assignments are made based on the best interference level available. This results in an unbalanced use of traffic channels, with all voice and data calls being assigned to the best quality transmission channels even when a lower quality channel is available to service a call. Continuous monitoring of transmission channel interference levels and improved assignment algorithms would provide a more balanced network which could be utilized by a greater number of subscribers.
SUMMARY OF THE INVENTION
The present invention achieves technical advantages by reassigning transmission channels in a wireless communications network based on interference levels and channel quality using forward and backward reassignment. Interference measurements and channel quality measurement are used to reassign a call to a higher or lower quality channel, freeing up the network for more calls. The use of medium level quality channels rather than high quality channels results in a better balanced network and improved transmission channels in the network.
According to the present invention, disclosed is a method of assigning transmission channels based on channel quality and interference level. The frequencies of the network are discriminated to determine which transmission channels have interference thresholds less than a predetermined blocking threshold of interference, and a transmission channel is preassigned with an interference level less than the predetermined blocking threshold to an incoming call. The channel quality and interference level of the preassigned transmission channel are continually monitored, and a transmission channel is reassigned to the incoming call when the monitored levels of channel quality and interference levels change beyond a set of predetermined thresholds.
Also disclosed is a method of assigning transmission channels in a wireless communications network based upon interference and channel quality measurements. The method includes the steps of discriminating the frequencies of the network to determine which transmission channels have a interference level less than a predetermined blocking threshold of interference. Next, the channel with the highest available interference level less than the blocking threshold is preassigned to an incoming call. The remaining transmission channels are categorized based on channel quality and interference levels in the network using a rule table that creates bands of interference and channel quality. Next, a decision is made whether to continue the call on the first channel, drop the call, reassign the call to another channel with more interference than the current channel, or reassign the call to a channel having interference less than the current channel, based upon the measured channel quality and the interference levels. A tradeoff is made between good quality on some channels and bad quality on other channels to achieve a more uniform quality distribution across the network.
Also disclosed is a device for assigning transmission channels in a wireless communications network. The device includes an algorithm comprising a discrimination function adapted to discriminate the frequencies of the network to determine which transmission channels are less than a predetermined blocking threshold of interference, and a preassignment function adapted to preassign a first transmission channel with an interference level within a band less than the predetermined blocking threshold of interference to an incoming call. The algorithm also includes a monitoring function adapted to continually monitor the channel quality and interference level of the preassigned transmission channel, and a reassignment function for reassigning a second transmission channel to the incoming call when the monitored levels of channel quality and interference levels change beyond a set of predetermined thresholds. A rule table is stored in the device and utilized to reassign transmission channels in a wireless network on channel quality and interference level measurements.
By providing for both forward and backward reassignment, the invention is particularly advantageous to fully utilized networks with heavy traffic conditions. A further advantage is improved balance in the use of transmission channels of the wireless network. When the overall channel quality becomes more uniform over the entire network, a higher capacity can be achieved. Another advantage includes the free-up of low interference channels.
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H'Mimy Hossam H.
Shah Ali R.
Ericsson Inc.
Jackson Blane J.
Navarro Arthur I.
Tsang Fan
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