WO2002032162A2 - System and method for adaptive communication - Google Patents
System and method for adaptive communication Download PDFInfo
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- WO2002032162A2 WO2002032162A2 PCT/JP2001/008985 JP0108985W WO0232162A2 WO 2002032162 A2 WO2002032162 A2 WO 2002032162A2 JP 0108985 W JP0108985 W JP 0108985W WO 0232162 A2 WO0232162 A2 WO 0232162A2
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- signal
- quality
- communications network
- humanly
- network
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Classifications
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L47/00—Traffic control in data switching networks
- H04L47/10—Flow control; Congestion control
- H04L47/18—End to end
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L47/00—Traffic control in data switching networks
- H04L47/10—Flow control; Congestion control
- H04L47/24—Traffic characterised by specific attributes, e.g. priority or QoS
- H04L47/2416—Real-time traffic
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04W—WIRELESS COMMUNICATION NETWORKS
- H04W28/00—Network traffic management; Network resource management
- H04W28/02—Traffic management, e.g. flow control or congestion control
- H04W28/0205—Traffic management, e.g. flow control or congestion control at the air interface
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04W—WIRELESS COMMUNICATION NETWORKS
- H04W28/00—Network traffic management; Network resource management
- H04W28/02—Traffic management, e.g. flow control or congestion control
- H04W28/10—Flow control between communication endpoints
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04W—WIRELESS COMMUNICATION NETWORKS
- H04W72/00—Local resource management
- H04W72/50—Allocation or scheduling criteria for wireless resources
- H04W72/54—Allocation or scheduling criteria for wireless resources based on quality criteria
- H04W72/542—Allocation or scheduling criteria for wireless resources based on quality criteria using measured or perceived quality
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04W—WIRELESS COMMUNICATION NETWORKS
- H04W72/00—Local resource management
- H04W72/20—Control channels or signalling for resource management
- H04W72/21—Control channels or signalling for resource management in the uplink direction of a wireless link, i.e. towards the network
Definitions
- the present invention relates to the transmission of signals over a communications network, and more particularly to the use of metrics identifying humanly perceptible variations in the transmission of a signal to adjust the quality level of the signal.
- Communications networks of various types are becoming increasingly popular in our society as the rapid and accurate exchange of information becomes increasingly important to our economy.
- These networks include the Internet, wherein data is broken into packets, transmitted and reassembled in the proper order by the location receiving the information, traditional telephone systems using Public Switched Telephone Network (PSTN), and wireless mobile communications networks.
- PSTN Public Switched Telephone Network
- a number of environmental factors can significantly affect the transmission of real time data, such as streaming video and voice, over communication networks. For instance, as a cellular wireless network approaches its transmission capacity, the probability that a call may be dropped or that frames of voice or data may be lost increases significantly. Furthermore, the location of a cellular receiver relative to a transmission station may cause frames of data to not be received. In IP networks, buffer overflow in routers and switching stations may cause them to refuse to accept and forward packets of data. These environmental factors can and do often result in degradation in the quality of a transmission that is perceptible to the users of a network. The perceptual quality of services can directly affect providers ' ability to attract and retain customers. Furthermore, the perceptual quality of the service relates to the fees the provider can charge in the market place.
- Most communications networks provide mechanisms for selecting the transmission parameters between two or more connected devices. For instance, in a Code Division Multiple Access (CDMA) cellular network, the transmitter will, based on information received from the receiver, for example, adjust the power level of its transmission to the receiver in order to preserve the signal integrity at the receiver.
- CDMA Code Division Multiple Access
- Two modems in a PSTN network will set transmission protocols when establishing a connection therebetween based on the condition of the transmission line.
- the receiver of packetized data will signal a transmitter to retransmit packets it has received containing errors or packets it has failed to receive.
- these types of adaptations do not directly relate to the transmissions' perceptual quality relative to a user of the receiving device.
- the frame erasure rate does not directly relate to perceived quality. For instance, a high FER may not degrade the perceived quality of a transmission if the frames are dropped in isolation relative to each other; whereas , a transmission with a low overall FER may cause a perceivable portion of a voice transmission to be lost if a concentration of frames are dropped consecutively or in close proximity to each other.
- the dropped, lost or incomplete packets are resent by the transmitter in response to an indication by the receiver, yet the retransmission of a packet does not ensure that the packet will be received in a timely manner to obviate any potential perceived quality problems that the missing packet and other associated missing packets may cause.
- UDP User Datagram Protocol
- a system, method and apparatus for dynamically maintaining perceptual quality of service are described. Using metrics gathered in real time by a receiver correlated with a user's perceived quality, a transmitter adjusts parameters to maintain a predetermined level of transmission quality. Network providers dynamically allocate limited network resources by adjusting transmission parameters in real time, thereby maximizing resource allocation, and by offering subscribers different prices correlated to quality, maximizing the revenue generation of the network.
- FIGURE 1 is a generalized illustration of a wireless cellular network
- FIGURE 2 graphically represents how the real time perceptual quality level of a voice transmission may vary over time
- FIGURE 3 is a block diagram of a base transceiver and a mobile transceiver in communication over a wireless network according to one embodiment of the present invention
- FIGURE 4 is a flow diagram illustrating a method for adaptively adjusting transmission protocols from a network device based upon perceptible metrics ;
- FIGURE 5 illustrates in a graphical format how a signal can be adjusted to a selected level according to embodiments of the present invention
- FIGURE 6 graphically illustrates how the transmission rate of a voice signal may vary during a call according to an embodiment of the present invention.
- a transmitter Based on metrics gathered in real time by a receiver that can be directly correlated to a user's perceived quality, a transmitter adjusts its transmission parameters to either increase or decrease the quality level of the transmission to maintain the quality level as close as possible to a predetermined level.
- network providers can offer differing service plans based on a user's particular needs, wherein users requiring the best possible service pay more than users who are willing to sacrifice quality for a reduced cost .
- Network providers can dynamically allocate limited network resources by adjusting transmission parameters in real time to maximize the revenue generating potential of the network.
- FIG. 1 of the Drawings a portion of a CDMA cellular network, as can be utilized with CDMA and W-CDMA technology, is diagrammatically illustrated and generally designated by the reference numeral 100.
- the network 100 is divided into a number of cells 110 that indicate the typical area of coverage for a base transceiver 120 located in the middle of each cell.
- Each base transceiver 120 within the cellular network is coupled to one or more central stations 130 for communication therewith.
- Each central station 130 facilitates communication between base transceiver 120 to coordinate operations concerning certain tasks , such as handing off a mobile transceiver 150 from one cell to another.
- Each central station 130 includes or is coupled to a switching center 140. The switching center is typically coupled to a PSTN 145.
- voice signals are transmitted between a mobile transceiver 150, such as a cellular phone, and the base transceiver 120 located within an associated cell 110.
- a mobile transceiver 150 such as a cellular phone
- the base transceiver 120 located within an associated cell 110.
- the signal is transmitted between the mobile and the base transceivers 120 and 150 over the entire available bandwidth.
- Each signal is identified by a code, which specifies a specific order of frequency sequencing within the band, such that the base or mobile transceivers 120 and 150 can decipher the voice signal and relay it to the user.
- a relatively large number of distinct coded signals can be transmitted simultaneously in any particular cell 110 over the available spectrum.
- the base transceivers 120 associated with the overlapping regions 160 or 170 can transmit to and receive signals from the mobile transceiver 150 simultaneously.
- the mobile transceiver 150 leaves the overlapping regions 160 and 170 and at least one of the cells 110, the mobile transceiver 150 will continue to communicate with the base transceiver 120 associated with the cell 110 in which it is located and communication with the other base transceiver(s) 120 will cease. This process is known as a "soft handoff".
- the probability that a call will be dropped as the mobile transceiver 150 moves from one cell 110 to another is reduced since the mobile transceiver 150 establishes communication with the cell 110 into which it is moving before ending communication with the cell 110 it is leaving. Conversely, the probability that the call will be dropped in a hard handoff situation is greater since the "new" cell 110 that the user is moving into may not have capacity for the user that it would been able to set aside had the base transceiver 120 associated with the cell been aware of the possibility of the user's arrival therein.
- the use of a "soft handoff" can act to limit the overall capacity of the CDMA network 100 slightly since a portion of the capacity of each of the overlapping cells 110 is being utilized by a single user concurrently.
- the signal Prior to transmitting the voice signal over a wireless connection to a transceiver 120 or 150, the signal is typically compressed by a vocoder (voice coder) , It can be appreciated that by compressing the size of a voice signal prior to transmission, the total number of signals that may be supported in a limited bandwidth environment can be increased. However, as a voice signal is compressed, its perceptual quality degrades as well. In first generation CDMA networks , the voice signal is typically compressed for transmission at a rate of about 8 kbs.
- the vocoders in W-CDMA and CDMA2000 permit several different compression levels of voice encoding ostensibly to allow network operators to offer plans to customers based on a chosen level of encoding.
- the prior art does not provide a means for adjusting the encoding compression levels dynamically to improve perceptual QoS. Since CDMA involves the transmission of many signals over the same frequency spectrum simultaneously, the strength of each transmission signal received by a transceiver 120 or 150 must be similar to the strengths of the other signals being transmitted over the frequency spectrum simultaneously.
- the strength of a signal received at a receiving transceiver is dependent on several factors, including a transmitting transceiver's distance from the receiving transceiver, obstacles located between the transmitting and receiving transceivers, and the power level at which the transmitter is broadcasting. Accordingly, by sampling the signal strength received by the receiving transceiver from a transmitting transceiver and adjusting the transmission power relative to the signal strength, a base and mobile transceiver 120 and 150 can maintain associated signal strengths at a fairly constant level relative to other signals being transmitted within the cell 110.
- the number of users in a CDMA network can be varied depending on how the cell is being affected by interference within the cell and from surrounding cells .
- the ability to vary the capacity of the cell dynamically is referred to as "soft capacity".
- the cell when the interference from neighboring cells is low because of low usage in the neighboring cells, the ultimate capacity of a cell may be increased. Conversely, when usage is high in neighboring cells and that usage is causing interference in a cell, the cell can reduce its maximum capacity.
- the soft capacity does not only relate to the number of simultaneous users active within a cell, rather it also relates to total bit rate transmission capacity available from a cell as an aggregate of all users within that cell. Accordingly, a cell can support a greater number of low bit rate users (i.e., users having their voice data streams more highly compressed by the vocoder) than high bit rate users (users having less compressed voice data streams or users streaming data at high bit rates) for a given maximum soft capacity.
- the cell is capable of dynamically reducing its area of coverage.
- the ability to vary the geographic coverage of the cell dynamically is referred to as "soft coverage.” It can be appreciated that a base transceiver 120 of a cell 110 with a limited amount of transmission power to be distributed among simultaneous mobile transceivers 150 can support a greater number of mobile transceivers when most of the mobile transceivers are close to the base transceiver than when most of the mobile transceivers are located far from the base transceiver.
- the mechanisms described above permit the CDMA and/or W-CDMA network provider to dynamically vary the transmission parameters for the network 100 as a whole, for all the mobile transceivers 150 within a particular cell 110, or for a single mobile transceiver 150.
- Current networks do not utilize metrics that can be related to the QoS as it will be perceived by a user of the network. Rather, typical communications networks rely on a handful of simple indicators to adjust the performance of a network that in isolation do not relate to the perceptual QoS of a signal.
- One of the primary metrics utilized by the base transceiver 120 in a CDMA or W-CDMA network 100 to determine the relative strength of a signal being received both by and from a mobile transceiver 150 is a measure of the frame erasure rate (FER) .
- the FER describes the number of data frames that were erased by a receiving transceiver due to bit errors during transmission that could not be recovered or due to frames that were never received as may be the cause in regards to streaming traffic from a server over the Internet. As the FER increases, a decrease in received signal strength relative to the "noise" generated by the other signals within the cell is indicated.
- the frame erasure rate does not directly relate to the real time quality level perceived by the user of a mobile transceiver 150.
- MOS Mean Opinion Score
- a MOS is the result of a subjective listening test, wherein listeners compare various samples generated from voice streams and assign a quality value of 1 to 5 thereto.
- the MOS is an arithmetic mean determined from a large number of samples for a particular voice stream.
- MOSs have been used to characterize the relative quality of vocoder compressed voice streams, although MOSs can also be used to determine the perceived transmission quality for wireless voice streams, subject to a variety of conditions.
- land lines transmitting at 16 kbps typically have MOSs of around 3.6-3.7; whereas, voice signals compressed to 4.75 kbps for wireless transmission have MOSs around 3.2.
- MOSs are therefore typically limited to use in developing best practices scenarios for voice streams transmitted under certain predetermined conditions.
- Line 210 represents MOS quality levels 220 that may be perceived by a user at certain points along a timeline 230 during a CDMA network voice transmission.
- Environmental factors such as the location of the user relative to a transceiver shadowing and other fading effects, can significantly affect perceptual QoS.
- the perceptual quality may drop as indicated at time 240 when a user is deep within a building where the signal has difficulty penetrating through walls.
- the noise level may increase, along with the FER, thereby reducing the perceptual QoS.
- the perceptual QoS may increase to relatively high levels as shown at time 250 when a user is located close to the base transceiver 120 and there are a minimal number of simultaneous users within the cell 110.
- FIGURE 3 there is illustrated a block diagram of a system for (1) measuring metrics that correlate to a user's perceptual QoS and (2) making adjustments to the transmission protocols to adaptively change the perceptual QoS of the voice stream in real time according to one embodiment of the invention.
- FIGURE 4 is a flow diagram illustrating a method for adaptively adjusting the transmission protocols of a network device based on metrics that correlate with the user's perceptual QoS according to one embodiment of the invention.
- communication channels are established between a base transceiver 120 and a mobile transceiver 150, as illustrated in FIGURE 1.
- At least two channels 310 and 320 are established on initiation of a telephone call, as illustrated in FIGURE 3.
- One channel serves to transmit a voice stream from the base transceiver 120 to the mobile transceiver 150 and is referred to herein as the downlink channel 310.
- the other channel serves to transmit voice stream data and data to the base transceiver 120 from the mobile transceiver 150, and is known as the uplink channel 320.
- At least two channels are required for a full duplex communication, although in alternative embodiments communication between the transceivers can be established over additional channels.
- a third or fourth channel can be established solely for the purpose of transmitting channel control data between the transceivers , where the uplink 320 and downlink channels 310 are used only to transmit voice signals.
- the base transceiver 120 transmits a compressed voice signal to the mobile transceiver 150.
- the voice signal may originate from a PSTN signal sent to the base transceiver 120 through a switching center station 140, which is ultimately in communication with a device, such as a telephone connected to the PSTN network, generally designated in FIGURE 4 by the reference numeral 145, or the call may originate from another mobile transceiver 150 in communication with the base transceiver 120 through other channels.
- the signal is received by the mobile transceiver 150, as indicated in block 415.
- the voice signal is transmitted at constantly varying frequencies over the entire bandwidth available to the CDMA, W-CDMA or CDMA2000 network, as is well understood in this art. Each channel is identified by a code, which uniquely specifies its signal over the shared band.
- the voice signal upon receipt, is forwarded to and decompressed by a vocoder 330 in the mobile transceiver 150, as indicated by block 420 in FIGURES 3 and 4.
- the vocoder 330 is preferably resident in software that utilizes a processor or processing unit, generally designated by the reference numeral 390, contained within the mobile transceiver 150, although it should be understood that in alternative embodiments the vocoder 330 can be resident in dedicated hardware.
- the decompressed signal is then forwarded to and transmitted by a speaker 340 for audio transmission to the user of the mobile transceiver 150, as shown in block 425.
- the decompressed voice signal is analyzed by a voice signal analyzer 350 to determine perceptual QoS metric values concerning anomalies in the voice stream that are humanly perceptible.
- Software to measure perceptual voice quality has been recently made available, known as MultiVQ and DualVQ, developed by Genista Corp. of Japan, the assignee of the present invention.
- Dual-VQ for example, is a voice quality tool that is run on a windows platform to analyze voice files for various numerical or digital characterizations of audio anomalies that a user would perceive.
- Dual-VQ detects and measures voice choppiness, delay variation (jitter), and variations in active speech levels.
- Dual-VQ then provides a variety of metrics describing the quality of the analyzed voice stream, including an MOS metric.
- the MOS metric generated by Dual-VQ has been found to exhibit a 97% correlation with MOSs derived by traditional methods. It is within the ordinary level of skill of someone in the software and programming arts to port a version of the voice tools for operability within a cell phone for use by a microprocessor contained within the cell phone.
- voice signals can be analyzed in real time with metrics being generated that directly correlate to humanly perceptible variations in the quality of a voice signal.
- the resulting metric may be a single number indicating the overall quality of the voice signal similar to a MOS, or a more sophisticated set of data may be obtained that individually describes the various anomalies within the voice stream.
- the resulting data concerning perceptual QoS is transmitted over the uplink channel 320 to the base transceiver 120.
- the base transceiver 120 uses the data to adjust channel and network parameters based on the data, as indicated by step or block 440.
- the data may be transmitted to the base transceiver 120 over a dedicated data link instead of over the uplink channel 320 with voice signals,
- the data is utilized to maximize the quality of a voice signal sent to a particular mobile transmitter 150.
- the data is utilized to adjust the quality of the voice signal either upwardly or downwardly to match a particular level associated with a user of a particular mobile transceiver 150.
- a business person may desire the best possible level with regard to his mobile phone, and may be willing to pay a premium for a guarantee that he will have the best possible signal.
- the base transceiver 120 will adjust the transmission characteristics concerning the business person to maximize quality.
- a college student may be willing to live with a lower level of signal quality in return for an inexpensive rate plan.
- the base transceiver may adjust the college student's signal quality downwardly, e.g., upon network congestion and/or when the quality level exceeds the student ' s subscribed level .
- Line 510 indicates the relative quality level of the signal in terms of a metric, as it would be experienced without the use of a perceptual QoS adaptive system.
- Line 520 indicates the level after the adjustment of applicable transmission parameters, such as, but not limited to, the power level of the voice signal transmission, the soft capacity of the cell, the soft coverage of the cell, and the compression level of the voice signal.
- the vocoder 330 in the base transceiver 120 may be directed by the base transceiver's processing unit, generally designated by the reference numeral 390, to increase the level of compression of the voice signal to lower the user's, thereby freeing up capacity within that particular cell for use by other users.
- the processing unit 390 within base transceiver 120 would initiate parameter adjustments to increase the user's perceptual QoS, such as directing a power controller 360 within the base transceiver 120 to boost the power of the signal transmitted to the user, and/or decreasing the amount of vocoder 330 compression.
- the entire cell 110 may be experiencing greater than acceptable levels of noise as compared to neighboring cells, in which case the base transceiver's processing unit 390 may direct a capacity controller 380 and a soft coverage controller 370 to decrease the geographic coverage and capacity of the cell to improve the perceptual QoS of all the active mobile transceivers 150 located within the cell 110.
- the base transceiver's processing unit 390 may direct a capacity controller 380 and a soft coverage controller 370 to decrease the geographic coverage and capacity of the cell to improve the perceptual QoS of all the active mobile transceivers 150 located within the cell 110.
- FIGURE 6 there is illustrated how the compression level used by the base transceiver vocoder 330 may vary during a telephone call in a W-CDMA network.
- the vocoder 330 utilized in a W-CDMA is capable of compressing a voice signal into one of a plurality of rates from 4.75 kbs to 12.2 kbs.
- the base transceiver can dynamically adjust the compression rate accordingly.
- line 610 shows the voice signal rate at various points during a 20 second timespan of a telephone call.
- CDMA 2000 another version of a CDMA network, also utilizes a vocoder 330 capable of adjusting compression levels of a voice stream before transmission thereof.
- the perceptual QoS of a voice transmission is continuously monitored in real time to detect variations in the quality of the signal and make the proper adjustments before any change in the perceptual QoS becomes noticeable to a user.
- the sampling rate of the voice signals pursuant to the principles of the present invention can be varied as can the duration of the sampled portions of a voice transmission. Although sampling short duration portions of the voice stream is within the capability of current art electronics, it is preferred in embodiments of the invention that analyzed portions of the voice stream be of sufficient duration, typically a second or greater, such that problems that would be perceptible to a human can be identified.
- metrics can be utilized to both (i) make a determination whether to adjust the perceptual QoS of a single mobile transceiver within a cell, and (ii) make a determination whether to adjust the transmission characteristics of the cell 110 as a whole.
- the metrics can be utilized by a central base station controller 130 to adjust the relative levels in one cell compared to another neighbor's cell.
- the central base station controller 130 can direct the poorly-performing first cell to contract and reduce its capacity, while directing the second cell to expand its coverage and capacity to include those users that will be placed outside of the first cell's new coverage area. Accordingly, the levels of users in both cells can be harmonized as the levels in the first cell increase and the levels of the users in the second cell decrease.
- a cellular base station adjusts its transmission parameters based on humanly perceptual metrics received from a mobile transceiver regarding voice signals sent from the base transceiver.
- Embodiments of the invention may, however, be also applied in reverse, wherein the mobile transceiver 150 adjusts it voice signal transmission parameters based on metrics received from respective base stations 120.
- the use of humanly perceptible metrics can be utilized in conjunction with communications networks of many different types and is not limited to use in the CDMA and WCDMA networks described herein.
- embodiments of the present invention are contemplated for use with any type of signal transmitted over a communications network wherein variations in the receipt of the signal are humanly perceptible, such as but not limited to any multimedia signal that is experienced in real time.
- a first and second device such as the base transceiver 120 and mobile transceiver 150 in FIGURES 1 and 3, may instead constitute two devices or nodes in a landline system or combined landline/wireless system.
- High bandwidth communications such as visual images , movie images and multimedia data, may therefore be streamed employing the techniques of the present invention, i.e., (1) identification of at least one humanly perceptual or perceptible variation in the signal stream, whether audio or visual, e.g., in relation to a predetermined level of transmission quality, and (2) modification or adjustment of the transmitter's parameters or protocols to correct or ameliorate the humanly perceptual variation, e.g., to better accord with the predetermined level of transmission quality. Conformation of the transmitted signal to the baseline or target level of quality reduces the perceived flaws in transmission, which a customer may pay extra for. Conversely, deviation from an optimal or target measure may be in order for other users less desirous of optimal quality.
- the present invention is applied to the transmission of signals over a communications network, and more particularly to the use of metrics identifying humanly perceptible variations in the transmission of a signal to adjust the quality level of the signal.
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AU2001294243A AU2001294243A1 (en) | 2000-10-13 | 2001-10-12 | System and method for adaptive communication |
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US24053000P | 2000-10-13 | 2000-10-13 | |
US60/240,530 | 2000-10-13 |
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Citations (1)
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---|---|---|---|---|
US4381546A (en) * | 1979-03-02 | 1983-04-26 | Paradyne Corporation | System for the quantitative measurement of impairments in the communication channel of a quadrature amplitude modulation data communication system |
-
2001
- 2001-10-12 WO PCT/JP2001/008985 patent/WO2002032162A2/en active Application Filing
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US4381546A (en) * | 1979-03-02 | 1983-04-26 | Paradyne Corporation | System for the quantitative measurement of impairments in the communication channel of a quadrature amplitude modulation data communication system |
Non-Patent Citations (2)
Title |
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CHRISTIANSON L ET AL: "Rate adaptation for improved audio quality in wireless networks" MOBILE MULTIMEDIA COMMUNICATIONS, 1999. (MOMUC '99). 1999 IEEE INTERNATIONAL WORKSHOP ON SAN DIEGO, CA, USA 15-17 NOV. 1999, PISCATAWAY, NJ, USA,IEEE, US, 15 November 1999 (1999-11-15), pages 363-367, XP010370735 ISBN: 0-7803-5904-6 * |
HONG D P ET AL: "Performance of ATM available bit rate for bursty TCP sources and interfering traffic" COMPUTER COMMUNICATIONS AND NETWORKS, 1997. PROCEEDINGS., SIXTH INTERNATIONAL CONFERENCE ON LAS VEGAS, NV, USA 22-25 SEPT. 1997, LOS ALAMITOS, CA, USA,IEEE COMPUT. SOC, US, 22 September 1997 (1997-09-22), pages 298-305, XP010245763 ISBN: 0-8186-8186-1 * |
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