MXPA00001670A - A method for selecting a combination of modulation and channel coding schemes in a digital communication system - Google Patents

Abstract

A communication system that supports multiple modulation and channel coding schemes selects an optimum RF link by measuring link quality parameters, such as C/I ratio. All of the available RF links are characterized based on the measured link quality parameters by calculating mean values and variances of the parameters. Based on the characterization of the RF link, user quality values, such as user data throughput and speech quality values, are estimated. The communication system selects the RF link that provides the best user quality value.

Description

METHOD FOR SELECTING A COMBINATION OF CHANNEL ENCODING AND MODULATION SCHEMES IN A SYSTEM OF DIGITAL COMMUNICATION BACKGROUND OF THE INVENTION This invention relates generally to the field of communication systems and, more particularly to digital communication systems that support multiple channel coding and modulation schemes. In wireless digital communication systems, standardized air interfaces specify most of the system parameters, including modulation scheme, channel coding scheme, burst format, communication protocol, symbol rate, etc. For example, the European Telecommunications Standard Institute (ETSI) (European Telecommunications Standards Institute) has specified a Global System for Mobile Communication (GSM) standard that uses time division multiple access (TDMA). ) to communicate control, voice and data information on physical radio frequency (RF) channels or links using a minimum Gaussian shift manipulation (GMSK) scheme at a symbol rate of 271 ksps. In the United States of America, the Telecommunication Industry Association (TIA) (Telecommunications Industry Association) has published several Provisional Standards, such as IS-54 and IS-136, which define several versions of advanced digital mobile telephony service ( D-AMPS), a TDMA system that uses a differential QPSK modulation scheme (DQPSK) to communicate data on RF links. Digital communication systems employ various linear and non-linear modulation schemes to communicate voice or data information in bursts. These modulation schemes include GMSK, quadrature phase shift manipulation (QPSK), quadrature amplitude modulation (QAM), etc. The GMSK modulation scheme is a non-linear low level modulation (LLM) scheme with a symbol rate that supports a specified user bit rate. As the object of increasing the user's bit rate, high-level modulation (HLM) schemes can be employed. Linear modulation schemes, such as a QAM scheme, can have different levels of modulation. For example, a 16 QAM scheme is used to represent the 16 variations of 4 data bits. On the other hand, a QPSK modulation scheme is used to represent the 4 variations of the data bits. In addition to various modulation schemes, digital communication systems can support various channel coding schemes that are used to increase the reliability of communication. For example, a General Packet Radio Service (GPRS), which a GSM extension to provide packet data service, supports 4 channel coding schemes. A convolutional half-speed coding scheme, the coding scheme CSl, which is the "mother" channel coding scheme of GPRS. The CSl scheme is interrupted to obtain speed coding schemes of about 2/3 and approximately, the coding schemes CS2 and CS3. GPRS also supports an uncoded scheme, known as a CS4 coding scheme. Generally speaking, channel coding schemes encode and intersperse data bits of a burst or a burst sequence to prevent their loss under degraded RF conditions, for example, when RF links are exposed to fading. The number of coding bits used for the data bit channel coding corresponds to the error detection accuracy, with a higher number of coding bits offering greater error detection accuracy in the bits. For a given raw bit rate, a high number of encoding bits, however, reduces the user bit rate since the encoding bits reduce the number of user data bits that can be transmitted in a burst. The communication channel typically introduces errors in the sequence. In order to improve coding efficiency, the encoded bits are interleaved, before transmission. The purpose of the interleaving is to distribute the errors in several code words. The perfect interleaved term is used when the sequence of received data bit errors is not correlated. The less correlated the data bits received in the receiver, the easier it is to recover lost data bits. On the other hand, if interleaving is not effective, large portions or blocks of transmitted data bits may be lost under degraded RF link conditions. Therefore, error correction algorithms may not be able to recover lost data. TDMA systems subdivide the band of available frequencies into one or more RF channels. The RF channels are divided into 'several physical channels corresponding to time segments in TDMA frames. The logical channels are represented in one or several physical channels, where modulation and coding schemes of channels are specified. An RF link includes one or more physical channels that support the logical channels. In these systems, the mobile stations communicate with a plurality of dispersed base stations by transmitting and receiving bursts of digital information in uplink and downlink RF channels. An increasing number of mobile stations in use today has generated the need for a greater number of voice and data channels within cellular telecommunication systems. As a result, the base stations are closer together, with an increase in interference between mobile stations operating on the same frequency in neighboring or nearby cells. Even when digital techniques gain more useful channels from a given spectrum of frequencies, there remains a need to reduce the interference, or more specifically to increase the ratio between the strength of the carrier signal and the interference (ie, carrier-to-carrier reversal). and interference (C / I)). In RF loops that can handle lower C / I ratios, they are considered more robust than those that can only handle higher C / I ratios. According to the modulation and channel coding schemes, the degree of service deteriorates more rapidly as the quality of the link decreases. In other words, the performance of data or the degree of service of more robust RF links deteriorates less quickly than those of less robust RF links. Higher level modulation schemes are more susceptible to link quality degradation than lower level modulation schemes. If an HLMN scheme is used, the data throughput drops very rapidly with a drop in link quality. On the other hand, if an LLM scheme is used, the data production and the degree of service does not deteriorate so rapidly under the same interference conditions. Accordingly, link adaptation methods, which provide the ability to dynamically change the modulation scheme, channel coding, and / or the number of time segments employed, based on the channel conditions, are employed to balance the speed of user bits against link quality. In general terms, these methods dynamically adapt a combination of channel coding system, modulation, and the number of assignable time segments to achieve optimal performance over a wide range of C / I conditions. One evolutionary pathway for the next generation of cellular systems is to employ a high-level modulation (HLM), for example, a 16 QAM modulation scheme, to provide increased user bit rates compared to existing standards. These cellular systems include increased GSM systems with GPRS extension, enhanced D-AMPS systems, International Mobile Telecommunication 2000 (IMT-2000) (International Mobile Telecommunication 2000), etc. A high level linear modulation, for example a 16 QAM modulation scheme, has the potential to be more spectrum efficient than, for example, GMSK, which is a low level modulation scheme (LLM). Because higher-level modulation schemes require a higher minimum C / I ratio for acceptable performance, their availability in the system becomes limited to certain areas of system coverage or certain parts of the cells where they are stored. They can maintain more robust links. In order to provide various communication services, a corresponding minimum user bit rate is required. In voice and / or data services, the user bit rate corresponds to voice quality and / or data production, with a higher user bit rate producing better voice quality and / or higher output of data. The total user bit rate is determined by a selected combination of techniques for voice coding, channel coding, modulation scheme, and for a TDMA system, the number of time segments assignable per call. Data services include transparent services and non-transparent services. Transparent services, which have a minimum link quality requirement, provide white user bit rates. A system that provides transparent communication services varies the raw bit rate for g maintain a constant user bit rate with the required quality. Conversely, in non-transparent services, for example GPRS, the user bit rate may vary, due to the fact that erroneously received data bits are retransmitted. Unlike non-transparent services, transparent services do not retransmit data bits received erroneously. Accordingly, transparent services have a constant point-to-point transmission delay, and non-transparent services have a non-constant point-to-point transmission delay. A communication system can provide a data service through a number of RF links that support different channel combinations, voice coding, and / or modulation schemes. For example, the system can offer a multimedia service by employing two or more separate RF links that separately provide audio and video signals. In this scenario, one of the RF links can use an HLM scheme and the other link can use an LLM scheme. In order to provide a constant user bit rate in a TDMA system, lower level modulation schemes may employ a higher number of time segments than higher level modulation schemes. In addition, digital communication systems must also select an appropriate combination of channel coding and modulation schemes based on the quality of the link. For example, in the case of a high quality link, a higher level modulation or a lower channel coding results in a higher user bit rate, which can be used profitably by different services of communication. For example, in a non-transparent data service, the production of user data is increased. In the case of a voice service, the increased user bit rate can be used to display an alternative voice coder with a higher quality. Therefore, systems that support multiple channel and modulation coding schemes offer sufficient flexibility to select an optimal combination of modulation and channel coding schemes. A conventional method for selecting an optimal combination of channel coding and modulation schemes considers that link quality parameters are well known at a given time. Usually, these methods determine link quality parameters by measuring, in predefined cases, one or more of the received signal strengths (RSS) or frequency of errors in the bits (BER), etc. Using these instantaneous measurements, these methods also consider that the quality of the user in the function of the link quality parameters is perfectly known for all combinations of channel coding schemes and modulation. Since these parameters vary continuously, the average measurement of link quality parameters does not provide an accurate indication of user quality, especially after the selection of a link with a different combination of modulation and channel coding schemes. One method dynamically encompasses the user bit rate in a TDMA system to achieve optimum voice quality over a wide range of channel conditions. This system continuously monitors the link quality by making instantaneous measurements of an RF link C / I ratio. The system dynamically adapts its combination of channel coding and modulation schemes and the number of assignable time segments to optimize voice quality for the measured conditions. In addition, the system determines cost functions to derive a cost of using RF links with different coding and modulation schemes to improve voice quality. The user quality, however, varies considerably with variations in link quality parameters. Figure 1 shows a link performance of two modulation schemes, that is, the QPSK and 16 QAM schemes, which are exposed to 3 conditions channel dots an additive White Gaussian Noise (AWGN) channel condition, a channel condition of fast Rayleigh fading, and a slow Rayleigh fading channel condition. In figure 1, the link performance is expressed in terms of BER. For a given C / I ratio, the AWGN channel offers the best performance, due to the lack of fading depressions. In a fast Rayleigh fading channel, where the fading is constantly changing rapidly to make effective use of interleaving, the link performance is degraded compared to the AWGN channel. In a slow Rayleigh fading channel, where the fading varies slowly in such a way that the interleaving is not effective, the worst link performance is obtained. Conventional methods employ a mean C / I ratio to determine the channel condition. As shown in Figure 1, however, an average C / I ratio for different channel conditions can be the same, when the link performance can be quite different. Therefore, more information is required to accurately estimate link performance, if different combinations of channel coding and modulation are employed. An additional factor that affects user quality is the time dispersion the receiver equalizers can not effectively handle large time dispersions.

As a result, link performance is degraded, even though the C / I ratio distribution remains the same. Therefore, average measurements of C / I, BER or single time dispersion are not enough to estimate the performance of a selected link. Accordingly, there is a need for an effective link selection method in systems that support various channel coding and modulation schemes. SUMMARY OF THE INVENTION The present invention addresses this need and is exemplified in a selection method that statistically characterizes combinations of channel coding and modulation schemes available through the use of measured link quality parameters to determine which combination offers the best user quality The method of the present invention measures at least one link quality parameter of at least one RF link, for example, a C / I, BER, the strength of the received signal, or time dispersion. Then, at least one measurement of channel characteristic is calculated based on the measured link quality parameter by calculating both its average value and its variance. By introducing the variance of for example the C / I ratio, it is possible to estimate the type of channel conditions to which a transmission is supplied. Accordingly, it is possible to estimate how a modulation change and / or channel coding scheme can affect the quality of the link. In an exemplary embodiment, the measurement of each channel principle can be calculated for each of the available combinations of channel coding and modulation schemes of an RF link. Then, a user quality estimator estimates the user quality values, for example, production of user data or voice quality values, based on the measurement of calculated channel characteristics. Finally, the present invention selects a combination of channel coding and modulation schemes in an RF link that provides the best user quality. In accordance with some of its more detailed features, the present invention presents a map of the calculated channel characteristic measurement with estimated user quality values of the supported combinations of channel coding and modulation schemes. The map creation function can use simulation results, laboratory results or results derived from the normal operation of a coding system. In accordance with another aspect of the invention, the selection method determines an optimum transmission power for each combination of channel coding and modulation schemes based on the measured link quality parameter. Then, link quality values are estimated based on the optimal transmission power. Likewise, data bursts are transmitted on the selected RF link at the optimum transmission power. Other features and advantages of the present invention will be apparent from the following description of the preferred embodiment, in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a diagram of the performance of two variable modulation RF links under 3 different channel conditions. Figure 2 is a block diagram of a communication system that usefully employs the present invention. Figure 3 is a diagram of a subdivided RF channel that is employed in the communication system of Figure 2. Figure 4 is a diagram of a normal transmission burst transmitted in the RF channel of Figure 3. The figure 5 is a block diagram of a mobile unit that is employed in the communication system of FIG. 2. FIG. 6 is a block diagram of a radio base station that is used in the communication system of the Figure 2. Figure 7 is a block diagram of a radio transceiver that is used in the base station of Figure 6. Figure 8 is a flow diagram of a link selection method in accordance with an exemplary embodiment of the present invention. Figure 9 is a block diagram of the selection method of Figure 8. Figure 10 is a flow chart of a power selection scheme in accordance with another aspect of the invention. Figure 11 is a graph of link performance of two combinations of channel coding and modulation schemes. DETAILED DESCRIPTION With reference to Figure 2, a communication system 20 is presented, in accordance with an exemplary embodiment of the present invention that supports multiple modulation schemes. In an exemplary embodiment of the present invention, the system 10 supports 3 modulation schemes: a first LLM scheme (LLM1), a second LLM system (LLM2), and an HLM scheme. The LLMl scheme is a non-linear modulation scheme, such as a GMSK modulation scheme used in GSM systems. An LLM2 scheme is a linear modulation scheme, such as QPSK. Finally, an HLM scheme is a higher level linear modulation scheme, for example, a 16 QAM scheme, which could be supported by the second generation of improved GSM systems, which are still to be standardized. The communication system 10 also supports the channel coding schemes of the GSM GPRS extension. The system 10, therefore, supports the channel coding schemes CSl, CS2, CS3, and CS4. The system 10 supports various combinations of channel coding schemes and modulation in several RF links. Even though the system 10 is described with reference to the exemplary channel and modulation coding schemes specified above, it will be appreciated that a wide range of modulation and coding schemes can be employed to implement the present invention. The mode of operation of GSM communication systems is described in documents ETS 300 573, ETS 300 574 and ETS 300 578 of the European Telecommunication Standard Institute (ETSI) (European Institute of Telecommunications Standards), which are incorporated herein by reference. Accordingly, the operation of the GSM system is described to the extent necessary to understand the present invention. Although the present invention is described in accordance with what is incorporated in a GSM system, those skilled in the art will note that the present invention could be employed in a wide variety of other digital communication systems, such as those based on PDC standards or D-AMPS and improvements thereof. The present invention can also be used in CDMA or a hybrid of the CDMA and TDMA communication systems. The communication system 10 covers a geographical area that is subdivided into communication cells, which together provide communication coverage to a service area, for example, an entire city. Preferably, the communication cells are conformed in accordance with a cell pattern that allows some of the spaced cells to employ the same uplink and downlink RF channels. In this way, the cell pattern of the system 10 reduces the number of RF channels required to cover the service area. The system 10 can also employ frequency hopping techniques, for example, to avoid "dead spots". The initial selection of modulation schemes depends preferably on the measured or predicted link quality parameters of a new RF link. Alternatively, the initial selection may be based on a predefined cell parameter. Due to a possible difference in the link robustness for schemes LLM1, LLM2 and HLM, a mobile station 12 continues to use an LLM1 scheme until the channel characteristic allows the use of other schemes, in which case a procedure is initiated. of link adaptation to switch modulation scheme from the LLMl as LLM2 scheme, or to the HLM scheme. When no information is transferred from a mobile station 12 or to a mobile station 12, for example during idle states or GPRS wait states, the mobile station 12 preferably measures whether the link quality parameters of the mobile station 12 are in the same state. different RF links. For example, the mobile station 12 measures the interference on the candidate RF links for future use as well as the strength of the received signal from its current link. The measurement results are used to determine a distribution of channel characteristics measurements. These measurements serve as the basis for deciding which combination of modulation and channel coding schemes should be used subsequently. In accordance with the present invention, during an ongoing communication, user quality values are estimated based on channel characteristics, which are expressed in terms of variations and average values of link quality parameters. The channel characteristics are derived based on link quality parameter measurements in a predefined period. In this way, the system 10 estimates user quality values provided by combinations of channel coding schemes and modulation of one or more RF links. By comparing the estimated user quality values of these combinations, the present invention selects a combination of channel coding and modulation in an RF link that provides the best user quality value. For example, to provide a non-transparent service, the system 10 estimates the user quality values of available combinations of channel coding and modulation schemes in one or several RF links in terms of data production S. For a period of predefined time, the system 10 continuously measures the link quality parameters and calculates their average values and variances. The present invention is based on statistical measurements to characterize an RF link. Even when the exemplary modality employs mean values and variances, other statistical measurements can also be used, for example, median standard deviation, etc. The system 10 calculates the average values of such link quality parameters as a C / I ratio or BER values that are obtained in the predefined period of time. Based on measured link quality parameters in the predefined period of time, the system 10 also determines the variances of one or more of the link quality parameters. Based on the variances, the system 10 estimates the data outputs S for all combinations of channel coding schemes and modification in one or several RF links. The system then selects a new combination of channel coding and modulation schemes in an RF link, if the switching to the new combination in this RF link offers a higher data throughput S than what is obtained by the current combination. For a voice service, the system 10 may employ a user quality value measurement different from the data production S used for a non-transparent data service. Preferably, the user quality value in a voice service is expressed in terms of speech quality value Q which can be based on estimated frame erasure frequency (FER) and / or frequency of errors in user bits residual (RBER) that originates from the use of several voice coding schemes. In this arrangement, the present invention estimates Q-quality voice values for different combinations of channel coding and modulation schemes. Then, system 10 selects a combination that offers the best estimated voice quality value. System 10 is designed as a hierarchical network with multiple levels to handle calls. Employing an assigned set of uplink and downlink RF links, several mobile stations 12 operating within the system 10 participate in the calls using the allocated time segments. At a high hierarchical level, a group of Mobile Service Switching Centers (MSCs) 14 are responsible for routing calls from an origin to a destination. In particular, they are responsible for establishing, controlling and terminating calls. One of the MSCs 14 known by gate MSC, handles communication with a public network of switched telephones (PSTN) 18, or other public and private networks. Different operators support different communication standards with different modulation schemes and applied coding. The same operator can also support different modulation and channel coding systems in different cells. For example, one operator can support one LLMl modulation scheme, and one channel coding scheme CS4 only, while another operator can support all channel coding and modification systems. The communication system 10 employs the present invention to select a combination of modulation and channel coding schemes that offer the best user quality value. At a lower hierarchical level, each of the MSCs 14 is connected to a group of base station controllers (BSCs) 16. The primary function of a BSC 16 is the management of radio resources. For example, based on the strength of the received signal reported in the mobile stations 12, the BSC 16 determines whether a transfer is initiated. According to the GSM standard the BSC 16 communicates with an MSC 14 through a standard interface known as the interface A, which is based on the mobile application part of the CCITT signaling system number 7. At a still lower hierarchical level, each one of the BSCs 16 controls a group of base transceiver stations (BTSs) 20. Each BTSs 20 includes numerous TRXs that employ the uplink and downlink RF channels to serve a particular common geographic area. The BTSs 20 primarily offer the RF links for the transmission and reception of data bursts to the mobile stations 12 and from said mobile stations within their designated cells. In an exemplary embodiment, numerous BTSs 20 are incorporated into a radio base station (RBS) 22. The RBS 22 can be configured in accordance with a family of RBS-2000 products, offered by Ericsson, the beneficiary of the present invention. With reference to Figure 3, an RF channel 26 (uplink or downlink) is divided into repetitive time frames 27 during which the information is communicated. Each frame 27 is in turn divided into time segments 28 carrying information packets. The voice or data is transmitted during segments of time designated as traffic channels (TCHi, ...., TCHn). All signaling functions pertaining to the handling of calls in the system, including initiation, transfer and termination, are handled through the control information transmitted in control channels. The mobile stations 12 employ slow associated control channels (SACCHs) to transmit associated control signals, such as an RX-LEV signal, which corresponds to the strength of the signal received at the mobile station and RX-QUAL signal, which is a measurement of several frequency levels of errors in the bits in the mobile station 12 in accordance with that defined by the GSM standard. Fast associated control channels (FACCHs) perform control functions, for example transfers, by stealing assigned time segments for TCCHs. The BSC 16 instructs the RBS 22 based on measurements of RF link channel characteristics between mobile stations 12 to the RBS 22. According to what is described in more detail below, the channel characteristics can be measured based on various parameters, including the strength of the received signal, the frequency of errors in the bits, the multipath propagation property of the uplink RF channel, for example, time dispersion, or a combination of them. The system 10 carries out the transmission of the information during a segment of time in a burst that contains a predefined number of encoded bits. The GSM specification defines several types of burst: normal burst (NB), frequency correction burst (FB), burst of synchronization (SB), burst of access (AB), as well as fictitious outburst. The normal burst, which lasts 576μs, is used both during traffic signaling channels and some control signaling channels. The remaining bursts are used primarily to access and maintain signal and frequency synchronization within the system. As shown in Figure 4, a normal burst 29 includes 2 separate data portions 30 during which digital data bits are communicated. The normal burst also includes tail and guard sections 31 and 32 as shown. Among other things, the protection section 32 is used to allow increase of the burst and decrease of the bursts. The tail section 31 is used for demodulation purposes. All burst transmissions, except fictitious burst transmissions, include training sequences. The training sequences have a design with predefined autocorrelation characteristics. During the demodulation process, the autocorrelation characteristic of the training sequence helps to synchronize the bit sequences received in an RF channel. In normal burst 29, a training sequence 33 is positioned in the middle part of the burst between its data portions. In order to compensate for two propagation delays in the RF links, the communication system 10 employs a time alignment process by which the mobile stations 12 align their burst transmissions to arrive at the BTSs in an appropriate time relationship with relation to other bursts transmissions. As described below, the mobile station 12 and the RBS 22 incorporate pedestals that correlate sequences of baseband bits received in the uplink or downlink RF channels with the training sequences, in order to provide answers of correlator that correspond to the propagation properties of multiple paths. Based on the correlator responses, the receiver section of the BTS generates a timing advance parameter (TA). The mobile station 12 employs the parameter of TA that is transmitted from the RBS 22 to advance or delay its bursts transmissions in relation to a time reference. With reference to Figure 5, the block diagram of a mobile station 12 is illustrated. The mobile station 12 includes a receiver section 34 and a transmitter section 36, connected to an antenna 38 through a duplexer 39. The antenna 38 is used to receive and transmit RF signals to the BTS 20 and from the BTS 20 on assigned uplink and downlink RF channels. The receiver section 34 includes an RF receiver 40 that includes a local oscillator 41, a mixer 42, and selectivity filters 43 arranged in a well-known manner to down-convert and demodulate received signals at a baseband level. The RF receiver 40, which is tuner by the local oscillator 41 to the downlink channel, also offers an RX-LEV signal on the line 44 corresponding to the strength of the signal received in the mobile station 12. The receiver of RF provides a baseband signal upon demodulation 46 that demodulates bits of encoded data representing the voice, data- and signaling information received. According to the type of mobile station 12, the demodulator 46 can support one or more demodulation schemes corresponding to schemes LLM1, LLM2, and HLM. For example, the demodulator of a mobile station 12 was subscribed to an operator that supports an LLMl scheme that can demodulate only signals modulated with LLMl. On the other hand, the demodulator of a mobile station 12 subscribed to an operator supporting the 3 modulation schemes is preferably capable of demodulating schemes LLM1, LLM2, and HLM.

In accordance with what has been described above, the demodulator 46 includes an equalizer (not shown) that processes the coded bit pattern placed in the training sequences, to provide a correlator response that is used to demodulate the baseband signal. The peer uses the correlator responses to determine the most probable bit sequence for demodulation. In accordance with the definition of the GSM specification, a channel coder / deinterleaver 50 also provides an RX-QUAL signal on line 48, which is a multi-level measurement of error errors in the mobile station 12. The mobile station 12 reports the RX-QUAL signal and the RX-LEV signal to the BSC 16 on an SACCH channel. Channel modifier / deinterleaver 50 decodes and deinterleaves the demodulated signal. The channel decoder / deinterleaver 50 can employ a wide variety of channel decoding schemes including decoding schemes CS1-CS4. The voice data bits are applied to a speech decoder 52 that decodes the speech pattern using one of several supported speech decoding schemes. After the decoding, the speech decoder 52 applies an analogue voice signal to an output device 53, for example a speaker, through an audio amplifier 54. The channel decoder 50 offers the decoded data and the decoder. signaling information to a microprocessor 56 for further processing, for example, for displaying the data to a user The transmitter section 36 includes an input device 56, eg, a microphone and / or a keyboard, for inputting voice information or data In accordance with a specified voice / data coding technique, a voice coder 58 digitizes and encodes speech signals in accordance with various supported speech coding schemes.A channel coder / interleaver 62 encodes the data of uplink in accordance with specified coding / interleaving algorithms, including coding schemes CS1-CS4. channel 62 offers an uplink baseband signal to a modulator 64. Modular 64 modulates the uplink baseband signal in accordance with one or more supported modulation schemes. In a similar manner to the demodulator 46, the modulator 64 of the mobile station 12 can support one or more schemes LLM1, LLM2 and HLM. The modulator 64 applies the encoded signal to a rising converter 67, which receives a bearer signal from the local oscillator 41 of the up converted signal. An RS65 amplifier amplifies the upconverted signal for transmission through the antenna 38. A well-known frequency synthesizer 66, under the control of the microprocessor 56, supplies the operating frequency information to the local oscillator 41. The microprocessor 56 causes the mobile station 12 to transmit the RX-QUAL and RX-LEV parameters to the RBS 22 in the SACCH. With reference to Figure 6, an exemplary diagram of the RBS 22 is illustrated which includes a plurality of BTSs 20 serving different geographic areas. Through a timing bus 72, the BTSs 20 are synchronized with each other. Voice and data information are offered to the RBS 22 and from the RBS 22 via a traffic bus 74 which may be connected, through the A-BIS interface, to the public voice and data transmission line or private, such as an IT line (not illustrated). Each BTS includes TRXs 75 and 76 that communicate with the mobile station 12. As shown, two antennas designated 24 (A) and 24 (B) are spaced in accordance with coverage cells 77 and 78. The TRXs 76 are connected to the antennas 24 through combiner / duplexers 80 which combine downlink transmission signals from the TRXs 76 and distribute the uplink received signals from the mobile station 12. The RBSs 22 also include a function block 68 common base station (BCF) that controls the operation and maintenance of the RBS 22. With reference to figure 7, a block diagram of a TRX 76 is illustrated. The TRX 76 includes a transmitter section 86, a receiver section 87, a baseband processor 88 and a TRX controller 90. Via a corresponding antenna 24 (illustrated in Figure 6), the receiver section 87 receives uplink signals from the mobile station 12. A downlink conversion block 91 downconverts the received signal. After the downward conversion of the received signals, the receiver section 87 samples its phase and magnitude, through a sampler block 92, to provide a bit sequence received to the baseband processor 88. An RSSI estimator 94 offers an RSSI signal on line 95, which is the measurement of the strength of the received signal. The RSSI 94 estimator can also measure noise disorder levels during inactive channels. The TRX controller 90, connected to the traffic bus 74, processes the commands received from BS16 and transmits the information related to TRX, such as, for example, various TRX measurements to the BSC 16. According to this arrangement, the TRX 76 periodically reports the signal of RSSI and noise disorder levels at BSC 16. Baseband processor 88 includes a demodulator 96 that receives uplink baseband data from receiver section 87. Scrambler 96 generates correlator responses that are processed in a well-known manner for recovering uplink baseband data. Similar to the mobile station 12, the demodulator can support the demodulation of modulated signals by the use of one or more of the schemes LLM1, LLM2, or HLM. The uplink baseband data is applied to a channel decoder 97 which decodes the baseband signal in accordance with one or more supported channel decoding schemes, including decoding schemes CS1-CS4. The channel decoder 97 places the decoded baseband signal on the traffic bus 78 for further processing by the BSC 16. When downlink baseband data is transmitted, the baseband processor 88 receives appropriately encoded data or information of speech digitized from BSC 16 on the traffic bus 74 and applies them to a channel encoder 102 that encodes and interleaves voice and data according to one or more of the supported channel coding schemes, including the channel coding schemes CS1-CS4 The transducer section includes a modulator 104 that modulates the data bits supplied in accordance with one or more of the schemes LLM1, LLM2, and H LM. The modulator 104 offers downlink baseband signals to an uplink conversation block 106 for uplink conversation. A power amplifier 108 amplifies the upconverted signal for transmission through a corresponding antenna. The system 10, for example, uses a parameter or a combination of parameters RX-QUAL, RX-LEV or time dispersion, which are measurements of link quality parameters of an RF link, in order to select an optimal combination of channel coding and modulation in an RF link. The system 10 also uses these parameters to decide whether or not to initiate a link adaptation procedure. The BSC 16 compares the channel characteristic parameter to corresponding thresholds to determine whether a link adaptation procedure is initiated within the coverage areas supporting the LLM1, LLM2, and HLM schemes. With reference to Figure 8, there is shown a flow diagram of a method for selecting a combination of channel coding and modulation schemes in an RF link in accordance with an exemplary embodiment of the present invention. In this exemplary embodiment, the system 10 is considered to offer a non-transparent data service, for example, a packet data service according to GPRS, where data blocks, the smallest retransible units, are transmitted and erroneously received blocks are retransmitted in accordance with an Automatic Repetition Request (ARQ) scheme.

The selection method begins by measuring the link quality parameters of an RF link in a receiver that may be in the mobile station 12 or a BTS 20, block 801. If more than one RF link are available, The selection method can measure the link quality parameters of all available links as well. Examples of link quality parameter measurements include C / I ratio, received signal strength, burst level time dispersion, as well as raw BER at block level. The measurements are processed to determine the distribution of the channel characteristic measurements. For example, the channel characteristic measurements that can be made by the distribution of link quality parameters can be calculated as average values and variances of the link quality parameters of block 803. The results of the processed measurement are reported to an estimator of link drop, block 805. In a preferred embodiment, the link quality estimator performs a display function fi, which maps the channel characteristics measurements with estimated user quality values of each of the supported combinations of channel coding and modulation schemes and, block 807. For example, the mapping function fi calculates the mean value and the variance of gross BERi based on measurement results, and the variance value estimates BERi.

Mapping functions can be implemented using a table initially constructed based on empirical results, such as simulation results, or experimental results, such as laboratory results, of the various combinations of channel coding scheme and modulation. Alternatively, the table may include results tuned based on actual measurements during the normal operation of the system 10. In this exemplary mode, BLER estimates are used to calculate user quality values in terms of data outputs if for each of the combinations of modulation and coding schemes, block 809. User quality values are used to select an optimal combination of channel coding and modulation schemes in an RF link by comparing data outputs Si, block 811. If production of data of a new combination, another that a combination currently used is significantly higher, a link adaptation procedure to commute with the new combination is initiated. In order to select the combination of channel coding scheme and modulation in an uplink RF link, the present invention performs all the steps specified above in the RBS 22. For the selection of the combination of channel coding scheme and modulation in a downlink RF link, the mobile station 12 performs the steps of measuring link quality parameters and calculating the mean values and variances and reporting the channel characteristic measurements to the RBS 22. The RBS 22 then performs the link quality estimation function and decides whether a new combination of channel coding and modulation schemes in an RF link should be selected or not. For the downline, the link quality estimate can obviously also be carried out in the mobile station. Figure 9 shows an exemplary block diagram of a means for estimating data production for N combinations of coding and modulation schemes. A channel characteristic estimator block 112 receives the link quality parameter measurements, for example, the C / I ratio, the received signal strength, gross BER, and the time dispersion parameter. Based on the measured link quality parameters, the channel characteristic estimator block 112 offers its average values and variances. A block of user quality value estimator 114, which operates based on statistical link performance results previously obtained or actual system measurements, offers BLERi estimation through BLERN. Based on bit rates of nominal data Ri, a converter block 106 converts the BLERi to BLERN estimates to estimates of Si to SN by using equation (1): (1) YES = RI (1-BLERI) With Based on the produced data Yes, a selector block 118 selects an optimal combination of channel coding and modulation schemes in an RF link. In accordance with another aspect of the invention, a power control scheme is used in combination with the link selection method described above. Considering that a transmitter has a dynamic power range (Pmin and Pmax, this aspect of the invention selects an optimum power level P0ptE [Pmin max] for each combination of channel coding and modulation schemes. white C / I (C / Ides) for each combination, which can be based on a white user quality value, such as BLER white (BLERes) • With reference to figure 10, a flow diagram of the system is shown. power control of this aspect of the invention The system 10 measures the average value of the C / I ratio (or other link quality parameters), for example using the reduction obtained from step 803 of figure 8, block 101. Based on the average C / I ratio, the system 10 calculates an optimal power • opt using equation (2): (2) Popt (i) = P + (C / Ides (i) = mean C / I) , where P is the transmission power in time t and the ratio C / Ides (I) is a White C / I ratio to achieve a desired user quality value for a combination of modulation channel coding schemes and block 103. For example, the C / Ides ratio may be a proportion that BLER offers ti) white for different combinations of channel coding and modulation schemes. Then, the optimal Popt power is truncated for each combination of channel coding scheme and modulation using equation (3): (3) Popt (ij = min [Pmax, max (Pmin, Popt tu)] The truncation step , block 105, allows the selection of a combination of channel coding scheme and modulation that offers the best user quality value, provided that the transmitter can produce the selected Popt without exceeding its Pma ?. If the calculated Popt is higher that the Pmax, the system 10 establishes the power of the transducer in Pma? On the other hand, if the calculated Popt is smaller than the Pmin, the system 10 establishes the power of the transmitter at Pm-n. modulation and schema change channels, system 10 calculates, block 107, the ratio C / Ii mean using equation 4: (4) C / Ii mean = C / I mean + (Popt <i) -P) - This step estimates a corresponding average C / Ii for each combination of channel coding and modulation scheme, taking into account the dynamic range of the transmission power between Pmax and Pmin. Once an optimal combination of channel coding and modulation schemes has been selected, using, for example, the steps described in blocks 805-811, the system 10 transmits on a selected RF link by using the optimal power combination. optimal Pops, blocks 109 and 11. With reference to figure 11, a link performance graph of two combinations of channel coding scheme and modulation is shown to describe the exemplary power control scheme in accordance with the described aspect above the invention. At a given time, the transmission energy of the transmitter, which, for example, has a dynamic range between Pmin = 5 dBm and Pmax = 33 dBm, is considered to be at Pt = 20 dBm. The measured C / It ratio is found to be 8dB. The proportion C / Ides white is determined which provides a desired user quality. For example, the C / Ides ratio is 12 dB for the first combination (illustrated with graph 1), and it is 27 dB for the second combination (illustrated with graph 2). In order to achieve the C / Ides ratio for the first combination and the second combination, the transmission power must be increased by 4 dB and 19 dB, respectively. Therefore, for the first combination Popt is equal to 24 dBm, and for the second combination Popt is 39 dbm, which is beyond Pmax. In this case, the system 10 establishes the transmission power in Pma of 33 dBm and calculates the C / I ratio in accordance with equation (4). Based on the C / I ratio measured in Pma ?, a link is selected that provides the best user quality value. From the foregoing it will be appreciated that the present invention significantly facilitates the RF link selection process in systems that support multiple coding and modulation schemes. Through the statistical characterization of RF links in terms of distribution and variances of link quality parameters, the present invention offers a more effective link selection process. In this way, the present invention improves the communication quality of systems that support multiple combinations of coding and modulation schemes. Although the invention has been described in detail with reference to only one preferred embodiment, those skilled in the art will note that various modifications can be made without departing from the invention. Accordingly, the invention is defined only by the following claims which have the purpose of encompassing all of their equivalents.

Claims (3)

1. CLAIMS In a communication system, a method for selecting a combination of channel coding scheme and modulation from a plurality of combinations of channel coding and modulation schemes, comprising the steps of: measuring at least one quality parameter for linking an RF link; calculating at least one channel characteristic measurement based on at least one measured link quality parameter; Based on the measured channel characteristic measurement, estimate user quality values for each of the combinations of channel coding and modulation schemes in order to determine how changing a channel coding scheme and modulation would affect a quality link; and selecting a combination of channel coding and modulation schemes in an RF link that provides the best user quality value. The method according to claim 1, wherein the at least one link quality parameter is selected from one of the following: C / I ratio, BER, received signal strength, or time dispersion. The method according to claim 1, wherein the step of calculating the at least one channel characteristic measurement includes the step of calculating a variance of the at least one measured link quality parameter. The method according to claim 3, wherein the step of calculating the at least one channel characteristic measurement includes the step of calculating a mean value of the at least one measured link quality parameter. The method according to claim 1, wherein the step of estimating the user quality values includes the step of representing the at least one calculated channel characteristic measurement with estimated user quality values of supported combinations of channel coding and modulation. 6. The method according to claim 1, wherein the user quality values are estimated using simulation results or laboratory results. The method according to claim 1, wherein the user quality values are estimated using derived results during the normal operation of the communication system. The method according to claim 1, wherein the user quality values include a production of user data. The method according to claim 8, wherein the step of estimating the user quality values includes the step of estimating block error frequencies. The method according to claim 9, wherein the step of estimating user quality values includes the step of calculating estimates of the production of user data based on the estimated frequencies of block errors as well as nominal speeds of bits. The method according to claim 1, wherein the user quality values include speech quality values. The method according to claim 11, wherein said step of estimating the user quality values includes the step of estimating the voice quality values that come from the use of different voice coding schemes. 3. The method according to claim 1, further comprising the step of determining an optimum transmit power for each combination of channel coding and modulation schemes based on the at least one measured link quality parameter, wherein the Optimal transmission power is limited by a dynamic range of a power transmitter. . The method according to claim 13, further comprising the step of transmitting on an RF link at the optimum transmit power. . The method according to claim 1, wherein the step of selecting a combination of channel coding and modulation schemes is carried out during inactive states or wait states. A communication system, a method for selecting a combination of channel coding and modulation schemes from a plurality of combinations of channel coding and modulation schemes, comprising the steps of: communicating data by using a non-service transparent in an RF link; measure at least one link quality parameter in the RF link; calculating at least one channel characteristic measurement based on the at least one measured link quality parameter; estimating the production of user data for each combination of channel coding schemes and modulation based on the calculated channel characteristic to determine how a change in channel coding scheme and modulation would affect a link quality; and selecting a combination of channel coding schemes and RF link modulation from the plurality of combinations of channel coding and modulation schemes that provide the best production of user data. . The method according to claim 16, wherein the at least one link quality parameter is selected from one of the following: C / I ratio, BER, received signal strength, or time dispersion. The method according to claim 16, wherein the step of calculating the at least one channel characteristic measurement includes the step of calculating a variance of at least one measured link quality parameter. The method according to claim 18, wherein the step of calculating the at least one channel characteristic measurement includes the step of calculating a mean value of the at least one measured link quality parameter. The method according to claim 16, wherein the step of estimating user quality values includes the step of representing the calculated channel characteristic measurement with estimated production of user data from supported combinations of channel coding and modulation schemes . 21. The method according to claim 16, wherein the production of user data is estimated using simulation results or laboratory results. 22. The method according to claim 16, wherein the production of user data is estimated by the use of derived results during the normal operation of the communication system. The method according to claim 16, wherein said step of estimating user data production includes the step of estimating block error sequences and calculating the production of user data based on the estimated error frequencies of the user. block and the nominal bit rates. The method according to claim 16, further including the step of determining an optimum transmit power for each combination of channel coding and modulation schemes in the measured link quality parameter and, where the optimal transmit power is limited by a dynamic range of power transmitter. 25. The method according to claim 24, further including the step of transmitting on the RF link with the optimum transmit power. 26. A communication system that communicates on RF links that support different combinations of channel coding and modulation schemes, comprising: a means for measuring at least one link quality parameter of an RF link; means for calculating at least one channel characteristic measurement based on the at least one measured link quality parameter; means for estimating the user quality values based on the calculated channel characteristic and the corresponding combinations of the channel and modulation coding schemes in order to determine how a change in channel coding scheme and modulation could accept a link quality; and a means for selecting a combination of channel coding and modulation schemes in an RF link that offers the best user quality value. The communication system according to claim 26, wherein the at least one link quality parameter is selected from the following: a * C / I ratio, BER, received signal pitch, or else time dispersion. The communication system according to claim 26, wherein the means for calculating the at least one channel characteristic measurement calculates a variance of the at least one measured link quality parameter. 29. The communication system according to claim 28, wherein the means for calculating the at least one channel characteristic measurement- calculates a mean value of the at least one measured link quality parameter. The communication system according to claim 26, wherein the means for estimating the user quality values includes a means for representing the at least one channel characteristic measurement calculated with the supported combinations of channel coding schemes and modulation. 31. The communication system according to claim 26, wherein the user quality values are estimated using simulation results. 32. The communication system according to claim 26, wherein the user quality values are estimated using the results derived from the normal operation of the communication system. 33. The communication system according to claim 26, wherein the user quality values include a production of user data. 34. The communication system according to claim 26, wherein the means for estimating the user quality values estimates the error sequences in the blocks. . The communication system according to claim 26, wherein the means for estimating the user quality values calculates the estimates of user data production based on the estimated error rates in the blocks and nominal bit rates. The communication system according to claim 26, wherein the user quality values include speech quality values. The communication system according to claim 36, wherein the means for estimating the user quality values estimates the voice quality values that come from the use of different voice coding schemes. The communication system according to claim 36, further including a power transmitter for transmitting on the RF link and a means for determining an optimum transmit power for each combination of channel coding and modulation schemes, based on the measured link quality parameter, where the optimum transmission power is limited by a dynamic range of the power transmitter. In a communication system that provides communication between a mobile station and a base station in uplink and downlink RF links, a method for selecting a combination of channel coding and modulation schemes from a plurality of combinations of channel coding and modulation schemes, comprising the steps of: measuring at least one link quality parameter of an RF link in the base station; calculating at least one measurement of channel characteristic in the link quality parameter measured in the base station; estimating user quality values for each of the combinations of channel coding and modulation schemes based on the calculated channel characteristic and the corresponding combinations of channel coding scheme and modulation supported in the base station to determine how a Changing channel coding scheme and modulation would affect a link quality; and selecting a combination of channel coding schemes and modification in the RF link that provides the best user quality value. In a communication system that provides communication between a mobile station and a base station in uplink and downlink RF links, a method for selecting a combination of channel coding and modulation schemes from a plurality of combinations of channel coding and modulation schemes, comprising the steps of: measuring at least one link quality parameter of an RF link in the mobile station; calculating at least one channel characteristic measurement based on the at least one link quality parameter measured in the mobile station; report the calculated channel characteristic measurement to the base station; estimate the user quality values for each of the combinations of channel coding and modulation schemes based on the calculated channel characteristic and the corresponding combinations of channel coding scheme and modulation supported in the base station to determine how a change of channel coding scheme and modulation will affect a link quality; and selecting a combination of channel coding and modulation schemes in the RF link that provides the best user quality value. In a communication system that provides communication between a mobile station and a base station in uplink and downlink RF links, a method for selecting a combination of channel coding and modulation schemes from a plurality of combinations of channel coding and modulation schemes, comprising the steps of: measuring at least one link quality parameter of an RF link in the mobile station; calculating at least one channel characteristic measurement based on the at least one link quality parameter measured in the mobile station; estimating user quality values for each of the combinations of channel coding and modulation schemes in the calculated channel characteristic and the corresponding combinations of channel coding and modulation schemes in the base station to determine how a schema change channel coding and modulation will affect a link quality; and selecting a combination of channel coding and modulation schemes in the RF link that provides the best user quality value.