Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1997-07-03
2001-04-24
Kuntz, Curtis (Department: 2643)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S063300, C455S424000, C455S562100, C455S514000
Reexamination Certificate
active
06223031
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a process and a device for observing the quality of frequencies or channels in a radio communication system. In particular it relates to a process for obtaining in a mobile radio communication system a measure of the quality of frequencies or channels which are intended to be used in the uplink and of frequencies or channels intended to be used in the downlink by generating, from the base station, values of a quality parameter for the frequencies or channels.
PRIOR ART
In a radio communication system, radio contact occurs between a primary radio station and at least one secondary radio station. The primary radio station can be a fixed base station and the secondary radio station can be a mobile station which is hand-carried or placed in a vehicle. A base station has responsibility for the radio coverage of a certain geographical area, a so-called cell.
Between a transmitter and a receiver in the radio communication system, a radio link can be established. The link is two-way, and thus one speaks of a downlink establishing the link in the direction from a base station in the radio communication system to a mobile station, and an uplink forming the link in the opposite direction from the mobile station to the base station. Transmitting and receiving of radio traffic for different links occurs on physical channels, which can be defined by a certain frequency in a Frequency Division Multiple Access system (FDMA) or by a combination of a certain frequency and a certain time slot in a Time Division Multiple Access system (TDMA). In a Code Division Multiple Access system (CDMA), a channel can be defined by a code. In general, the frequencies and the channels available in a radio communication system can be disturbed by other radio traffic to varying degrees, even by radio signals on the same channel which is used for other links, each channel in the system having a certain interference level.
In certain radio communication systems, frequency hopping is permitted, which means that a link changes between various frequencies according to a certain sequence. This can occur apparently randomized or sequentially. A frequency which proves to be of poor quality is then shared between a number of links and thus the interference for an individual link will not be as noticeable.
The spacing between an uplink frequency and the corresponding downlink frequency is called the duplex spacing. This spacing is in most radio communication systems fixed. Due to the limited number of available frequencies in a radio communication system, the same frequency must be re-used in different cells. In order to avoid interference between cells using the same channel simultaneously, so-called co-channel interference, the same frequency should not be used at the same time in adjacent cells. There should also be a certain re-used distance between two cells using channels on the same frequency.
In the GSM-system, each frequency is divided into eight time slots constituting a TDMA-frame. There are two different main types of so-called logical channels: speech channels and control channels. In each cell there is a duplex frequency pair, c
0
, of which at least one time slot, time slot zero (T
0
), is only used for control channels. This frequency pair is called a BCCH-carrier, where BCCH stands for Broadcast Control Channel. Other time slots on the BCCH-carrier are used for speech channels, and other frequency pairs can be carriers of solely speech channels. On the downlink of the BCCH-carrier, the mobile station receives continuous information concerning, among other things, cell identity. Furthermore the mobile station must be able to carry out measurements of the signal strength on the BCCH-carrier of the neighbouring cells and report the measuring results on the uplink of the BCCH-carrier. This means that, regardless of whether any information is transmitted for the moment on the BCCH-carrier, the same power must be transmitted continuously. This is done by sending out so-called “dummy bursts” when a time slot is not used.
Frequency hopping is usually not used on the BCCH-carrier. Each cell has its own BCCH-carrier. It is of particular importance that this frequency pair be of good quality as regards interference, since the information sent on the control channels will, inter alia, be used by mobile stations and the base station when connecting calls, handover, signal strength measurements on the neighbouring stations, etc.
A number of closely lying cells using together all frequency pairs which are allocated to the radio communication system without re-using any frequency pair, is called a cell cluster. Extensive planning work is required to obtain optimum re-use of frequencies. Field studies are usually formed to estimate the interference situation in various parts of the radio communication system. This type of cell planning where a pattern, according to which the various frequencies or channels of the system are to be allocated to the cells to obtain the lowest disturbance level, is called fixed frequency or channel allocation.
A radio communication with a fixed frequency allocation is incapable of adapting itself to geographical changes or changes in traffic in the system. Today each base station is manually allocated a number of frequencies, and one of these is the BCCH-carrier. Various frequency pairs are used as BCCH-carriers in different cells and the BCCH-carrier is today changed manually. The network must be regularly re-adjusted and re-planned due to the building of new base stations and transceivers and due to changes in the environment, for example, the erection of new buildings or due to increased traffic within a certain area. New measurements and predictions must be made to find a suitable frequency planning. When the measurements and the allocation of frequencies are often done manually, frequency planning in a radio communication system with fixed frequency allocation, if it must be re-done, is expensive for the operators.
The cell size varies within the radio communication system depending on the user density within various geographical areas. In sparsely populated areas, a base station can cover a radius of several kilometers. In order to be able to handle a high concentration of mobile stations in densely populated areas, the cell size should be reduced. In order to achieve high capacity in a radio communication system within a densely populated geographical area, various cell layers can be superimposed.
A so-called micro cell can have a diameter of several hundred meters and cover e.g. a sports arena, thoroughfares such as crossings and streets or a portion of a highway. Characteristic of a micro cell is that its base station antenna/antennae is/are placed outdoors at below roof level.
Still smaller indoor cells, so-called pico cells, can cover e.g. a floor of a building or an entire building. There are also so-called “pico cells with distributed antenna system” which means that there are several radio antennae placed within one pico cell.
A layer with micro cells is covered by at least one macro cell, and a micro cell can in turn cover several pico cells. A covering cell is called an umbrella cell. The various layers are organized relative to each other in certain cell structures in a known manner and with certain repeating distances for the frequency of the channels included in the radio communication system.
In micro and pico cells, the base station antennae are placed below roof level, and therefore the range of an antenna is usually less than the range for a base station in a macro cell. Compared with a base station in a macrocell, a base station in a micro or pico cell is less affected by interference from surrounding base stations since fewer disturbance signals will reach the relatively lonely placed base station. This means that the same frequency can be re-used more often in a micro or pico cell than in a macro cell.
In micro and pico cells, it will be particularly ineffective to use fixed channel allocation. The layout of micro and
Burns Doane Swecker & Mathis L.L.P.
Kuntz Curtis
Telefonaktiebolaget LM Ericsson (publ)
Tieu Binh K.
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