Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L25/00—Baseband systems
  • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
  • H04L25/0202—Channel estimation
  • H04L25/0212—Channel estimation of impulse response
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L25/00—Baseband systems
  • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
  • H04L25/0202—Channel estimation
  • H04L25/0224—Channel estimation using sounding signals
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L5/00—Arrangements affording multiple use of the transmission path
  • H04L5/003—Arrangements for allocating sub-channels of the transmission path
  • H04L5/0058—Allocation criteria
  • H04L5/006—Quality of the received signal, e.g. BER, SNR, water filling
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04W—WIRELESS COMMUNICATION NETWORKS
  • H04W74/00—Wireless channel access

Definitions

• the invention relates to mobile communications, and especially to improved detection of invalid channel access requests in a data transmission method, a network element, a detection unit, a telecommunication system, a computer program product and a computer program distribution medium according to the appended independent claims.
• a mobile station When a mobile station is switched on, it first scans the radio interface in order to detect the presence of a network. After it receives a signal from a base station, the mobile station implements a location update, and thereby informs the network of its location and identity. After these operations, the mobile station may initiate a radio resource session by requesting channel access.
• a channel refers herein to means of unidirectional transmission of information between at least two endpoints, herein a mobile station and a base station.
• Access may be triggered in response to a communication need originating from the mobile station (a mobile originated call, location updating), or from the infrastructure side ⁇ mobile terminating call). Either way, the mobile station initiates the access procedures by sending an access request message to the base station.
• the base station decodes the received access request message, and after a successful decoding, forwards the access request to the network and thereby initiates a channel access procedure.
• the procedure comprises choosing of a free traffic channel, activating this channel in the base station and sending an assignment message to the mobile station.
• the mobile station modifies its reception and transmission configuration to adapt to the frequency and time characteristics of the new channel, and starts establishing a link layer connection for signalling messages related to the original communication need.
• the radio interface is a complex environment and provides various sources of interference, for example thermodynamic type of noise, and band noise from network or environment. Occasionally, the base station may misinterpret an invalid access request as a genuine one, and initiate unnecessarily the channel assignment procedure. Misinterpretation of bursts is also probable when mobile station at the limit of its sensitivity.
• Base transceiver stations may have internal design requirements that allow only a given number of invalid channel access requests in a certain period of time. Therefore detections made by the BTS should be as effective as possible. On the other hand, if the filtering of invalid channel access requests is too strict, genuine channel access requests may be inadvertently rejected, which degrades the receiver performance. Consequently, extreme care is required when measures for detecting and filtering out invalid channel access requests are designed.
• a prior art method of detecting invalid channel access requests known from networks implementing Global System for Mobile Communications (GSM), uses a two-staged procedure, which is based on determinations of two parameters: 1. Energy of estimated impulse response taps, where indications for channel access requests with inappropriate energies are filtered out. This stage aims to ensure that pure noise is not entered into the system.
• GSM Global System for Mobile Communications
• SNR Signal-to-noise ratio
• the procedures after the time the channel access request arrives at the base station and before the indication of the channel access request from the base station are herein referred to as receiver pre-processing.
• the procedures of receiver pre-processing may comprise, for example, timing offset correction, whitening, noise estimates, automatic frequency correction, etc.
• the detection of invalid channel access requests is integrated to the pre-processing procedure.
• the problem with the current solution is that determination of SNR is measured at a defined stage of the receiver pre-processing and therefore some of the preceding functions, like the noise whitening, may affect the levels of the measured SNR.
• a new SNR threshold are typically derived as a result of extensive simulations and analyses of measurement data collected by means of an actual product, which involves considerable amount of research and development (R&D) resources.
• R&D research and development
• the enhancements and alterations to the receiver features are unavoidable. For example, compensation of link imbalances and performance issues associated with new mobile station features typically require receiver improvements. Due to this interdependency between detection of invalid channel access requests and the pre-processing procedure, laborious adjustments are evident after any minor changes in the receiver configuration. This significantly reduces the flexibility of design and increases the R&D effort associated with releasing new products, both mobile stations and base station elements, to the market.
• An object of the present invention is thus to provide an improved solution for detection of invalid channel access requests.
• the objects of the invention are achieved by a data transmission method, a network element, a detection unit, a telecommunication system, a computer program product and a computer program distribution medium, which are characterized by what is stated in the independent claims.
• the preferred embodiments of the invention are disclosed in the dependent claims.
• An advantage of the invention is that it creates more freedom for receiver design and, by reducing the testing effort, improves the release of new products into the market. This is performed by eliminating the interdependency between detection of invalid channel access requests and receiver configuration.
• Figure 1 illustrates a simplified block diagram of a GSM mobile communication system
• Figure 2 shows a flow chart illustrating an embodiment of the invented method
• Figure 3 shows a flow chart illustrating a definition of a first threshold
• Figure 4 shows a flow chart illustrating a step of RACH ghost detection using training sequence BER
• Figure 5 illustrates a comparison of the theoretical BER limits of GMSK and BPSK
• Figure 6 comprises a functional description of a base station according to the present embodiment. Description of embodiments
• Mobile communications system 10 refers generally to any telecommunications system wherein the access point (essentially wireless access) to the system may change when users are moving within the service area of the system.
• a typical mobile communications system that comprises a Public Land Mobile Network (PLMN) and one or more mobile stations (MS).
• PLMN Public Land Mobile Network
• MS mobile stations
• the mobile communications network is an access network providing a user with a wireless access to external networks, hosts, or services offered by specific service providers.
• GSM Global System for Mobile Communications
• GSM Global System for Mobile Communications
• GSM Global System for Mobile Communications
• GSM Global System for Mobile Communications
• GSM 1800 Global System for Mobile Communications
• PCS Personal Communications System
• GSM 2+ Global System for Mobile Communications
• Services of the GSM 2+ stage include GPRS (General Packet Radio Service) and CAMEL (Customised Applications for Mobile network Enhanced Logic).
• FIG. 1 in the attached drawings illustrates a simplified block diagram of a GSM mobile communication system.
• the figure shows only the elements necessary for illustrating the invention but for a person skilled in the art it is self-evident to that a mobile communication system also comprises other prior art functions and structures that need not be described here.
• the mobile communication system comprises four parts: a network subsystem (NSS) 100, a base station system (BSS) 105, a mobile station (MS) 110, and a network management system (NMS) 115.
• NSS network management system
• VAS2g value added service platform
• Value added services generally refer to services employing standardized interfaces towards the GSM network.
• the VAS2g most often includes at least a short message centre and voice message service.
• the mobile station 110 is connected to a base transceiver station (BTS) 125 via a radio path.
• the base station system 105 consists of a base station controller (BSC) 130 and base stations 125 under it, and a transcoding and rate adaptation unit (TRAU) 135.
• the mobile services switching centre (MSC) 140 usually has a plurality of base station controllers 130 operating under it.
• the mobile services switching centre 140 communicates with other mobile services switching centres and a gateway mobile services switching centre (GMSC) 145.
• the GSM network typically accesses also other networks, such as a public switched telephone network PSTN, another mobile communication network PLMN, or an ISDN network ISDN, via the gateway mobile services switching centre (GMSC) 145.
• the network management system 115 is responsible for monitoring and controlling system resources and storing data on their use and operation in the entire system.
• the standardized interface between the base station system 105 and the network subsystem 100 is called an A interface and the one between the mobile station and the base station system is called a Um interface.
• the subscriber data of the mobile station 110 are stored permanently in the system's home location register (HLR) 150 and temporarily in the visitor location register (VLR) 155 according to the location of the mobile station 110 at a given time. Information on the visitor location register 155 that maintains the subscriber data of the mobile station 110 at a given time is transmitted to the home location register 150.
• the network subsystem typically also comprises an authentication centre (AuC) for maintaining data required to identify a subscriber and an equipment identity register (EIR) for maintaining equipment identification data.
• AuC authentication centre
• EIR equipment identity register
• a random access channel RACH is a contention access uplink channel that can be used by a mobile station to signal a number of defined messages, for example capacity request messages and access request messages.
• the RACH is mapped to the physical random access channel (PRACH).
• RACH may be used in the uplink to request allocation of a SDCCH (standalone dedicated control channel), a logical channel on the radio interface that is used for sending short messages, and during connection setup used for identification and authentication procedures and for starting the ciphering.
• Packet random access channel PRACH, CPRACH
• PRACH Packet random access channel
• the BTS base transceiver station
• the BTS base transceiver station
• the invalid RACH bursts are called as RACH ghosts, which do not lead to a useful benefit of SDCCH establishment.
• a network cannot differentiate between a true channel access request and a RACH ghost, and therefore channel allocation process will be unnecessarily started if a RACH ghost goes undetected.
• the RACH ghosts need to be identified at BTS level so that as few RACH ghosts as possible enter the network infrastructure and initiate resource allocation procedures.
• Base transceiver stations typically have internal design requirements that allow only a given number of RACH ghosts for a certain period of time; for example, a maximum of 10 RACH ghosts may be allowed within 24 hours for a BTS. To meet such requirements, detections of RACH ghosts should be very strict. However, if the RACH ghost filtering is too strict, it may end up rejecting genuine channel access requests, which in turn may degrade the original receiver performance. Thus, extreme care is to be followed in designing RACH ghost filters for the base stations.
• the flow chart of Figure 2 illustrates an embodiment of the invented method, wherein the solution is applied to the detection of RACH ghosts in a BTS of a GSM system. According to the invention, the validity of the pre- processed channel access request is evaluated on the basis of a detection rule that is not dependent on the configuration of the radio receiving unit.
• Step 210 illustrates a stage for identifying bursts caused by background noise.
• the background noise information is determined by computing a predefined number of impulse response taps, and generating a ratio of the average energy of a group of selected impulse response taps, and the average energy of a group of remaining impulse response taps. This ratio is compared to a predefined first threshold (step 215), and if the first threshold is exceeded, the channel access request is considered to be background noise, and filtered out.
• the BTS receiver initially computes from the received channel access request 73 taps and selects thereof a group of 5 taps that contribute the highest energy. The reason for using 73 taps is that this amount allows the BTS receiver up to 68 symbol periods to track the timing offset. In practise this means that the BTS is able to synchronise with a mobile within a distance of 35 km.
• ⁇ represents the selected channel tap vector, whereas h ; denotes the initial vector. These taps are complex.
• a decision criteria is formed according to
• e s represents the average energy of the selected taps and e r represents the average energy of the rejected taps, i.e.
• e t represents the average energy of the total impulse response (i.e. energy of the initial group)
• a RACH ghost is reported if the a > si .
• the flow chart of Figure 3 illustrates the definition of the first threshold s1 associated with the detection rule of step 215.
• the auto correlation coefficients (taps) tend to be flat for all lag variables.
• j£?i represents a lower threshold, which may be used to filter out pure noise.
• /J 1 is not high enough to appropriately block invalid channel access requests from entering into the system.
• use of ⁇ can provide for the Rx performance requirements, but significant amount of RACH ghosts will also enter the system.
• filtering of background noise is one of the preliminary steps, it is important that the amount of passed RACH ghosts is low so that processing burden of the next detection stages remains reasonable.
• ⁇ 2 represents a higher threshold where ghost RACH filtering is efficient, but where the degradation of bit error rate (BER) has not yet started. The degradation of BER follows from some actually valid channel access requests being rejected, instead of being forwarded for channel assignment procedure.
• S1 may be set close to ⁇ 2 experimentally.
• Eb/No where Eb corresponds to the energy per bit and No corresponds to the noise spectral density
• the value where this increase is detected is set as the first threshold, i.e. s1 ⁇ /? 2 .
• a RACH ghost flag is set to 1 (step 220), and processing of the channel access request is terminated without initiating a channel assignment procedure.
• Step 225 illustrates the general algorithms that are configured to BTS for performing preliminary operations on the channel access request before forwarding the information on the received channel access request in a CHANNEL REQUIRED -message to BSC.
• these algorithms do not comprise functions for rejecting invalid channel access requests, but are focused on operations related to timing offset correction, whitening, noise estimates, automatic frequency correction, etc.
• actual receivers may comprise further processing and/or pre-processing algorithms that are not relevant for described functions for rejecting invalid channel access requests, also in other phases of the procedure of Figure 2.
• Step 230 illustrates a step of RACH ghost detection using training sequence BER. This step is illustrated in more detail in the flow chart of Figure 4.
• the training sequence BER is determined by computing a hard decision of the training sequence soft values.
• Step 42 illustrates the detection rule of step 230, wherein the determined training sequence BER is compared against a predefined second threshold s2. If the threshold s2 is exceeded (step 43, step 235), the channel access request is considered invalid, and a RACH ghost flag is set to 1 (step 44, step 240), and processing of the channel access request is terminated without initiating a channel assignment procedure. Otherwise the processing of the channel access request in the receiver will continue normally.
• the theoretical expectation of BER for GMSK modulated signal can be given by:
• erf represents the error function.
• the error function erf(x) is twice the integral of the Gaussian distribution with 0 mean and variance of 0.5.
• Step 240 illustrates a further step of RACH ghost detection using receiver signal strength indicator (RSSI).
• RSSI receiver signal strength indicator
• the detection rule associated with this stage is based on RSSl.
• the BTS receiver calculates the received signal power for every burst.
• the signal power is computed from the received Q samples, which are passed through a low pass filter.
• the low pass filter is employed in order to remove adjacent channel interference.
• the signal power is derived from:
• the Received Signal Strength Indicator represents the difference between the received signal power p r and a reference value p s . i.e.
• the detection threshold of this stage remains independent of RX performance changes that come from modifications to the receiver configuration.
• any sensitivity improvement may be simply added to the existing threshold, i.e. no additional R&D effort is needed to tune the threshold.
• the invented solution improves the performance of RACH ghost detection in the receiver.
• a conventional method in a defined BTS test setup showed 108 RACH ghosts from 3 transceivers within a period of 111 hours, with the new solution the number of reported RACH ghosts reduced to 1 for the same test setup.
• the invented solution eliminates the interdependency between ghost RACH tuning and RX performance and therefore creates more freedom for receiver (pre-processing technology) development. This flexibility optimises RACH processing in the receiver, which has a positive impact in saving DSP cycle.
• the conventional method utilizes a predefined hard-coded SNR threshold.
• This threshold is tuned interactively with RACH ghost detection and RX Sensitivity performance, and will always be a compromise. While the new solution is independent of the receiver configuration, the RX performance may actually be improved in certain areas (for example in typical urban environment, TU). By means of the optimised succession of stages, the new solution enables reduction of the time for call set-up processing (channel request). Early filtering of RACH ghosts also eliminates unnecessary allocation of valuable spectrum.
• the base station comprises processing means 610, an element that comprises an arithmetic logic unit, a number of special registers and control circuits.
• memory means 615 Connected to the processing means are memory means 615, a data medium where computer-readable data or programs or user data can be stored.
• the memory means typically comprise memory units that allow both reading and writing (RAM), and a memory whose contents can only be read (ROM).
• the base station also comprises an interface block 620 with input means 625 for inputting data from the network infrastructure (basically the BSC), for internal processing in the base station, and output means 630 for outputting data from the internal processes of the base station to the network infrastructure.
• the base station also comprises a transceiver block 635 configured with receiving means 640 for receiving information from the air interface and for inputting the received information to the processing means 610, as well as with transmitting means 645 for receiving information from the processing means 610, and processing it for sending via the air interface.
• the implementation of such a transceiver unit is generally known to a person skilled in the art.
• the processing means 610, memory means 615, interface block 620, the interface block 620, and the transceiver unit 635 are electrically interconnected for performing systematic execution of operations on the received and/or stored data according to predefined, essentially programmed processes of the unit.
• the operations comprise the functionality of the receiver unit of the base station BTS as described above.

Landscapes

• Engineering & Computer Science (AREA)
• Signal Processing (AREA)
• Computer Networks & Wireless Communication (AREA)
• Power Engineering (AREA)
• Quality & Reliability (AREA)
• Mobile Radio Communication Systems (AREA)

Applications Claiming Priority (3)

Publications (2)

Family

ID=37604117

Family Applications (1)

Country Status (2)

Families Citing this family (1)

Family Cites Families (5)

• 2006
  • 2006-07-04 WO PCT/FI2006/050312 patent/WO2007003718A1/en not_active Ceased
  • 2006-07-04 EP EP06778515.4A patent/EP1908322A4/de not_active Withdrawn

Also Published As

Similar Documents

Legal Events