Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L1/00—Arrangements for detecting or preventing errors in the information received
  • H04L1/12—Arrangements for detecting or preventing errors in the information received by using return channel
  • H04L1/16—Arrangements for detecting or preventing errors in the information received by using return channel in which the return channel carries supervisory signals, e.g. repetition request signals
  • H04L1/18—Automatic repetition systems, e.g. Van Duuren systems
  • H04L1/1803—Stop-and-wait protocols
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L1/00—Arrangements for detecting or preventing errors in the information received
  • H04L1/0001—Systems modifying transmission characteristics according to link quality, e.g. power backoff
  • H04L1/0002—Systems modifying transmission characteristics according to link quality, e.g. power backoff by adapting the transmission rate
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L1/00—Arrangements for detecting or preventing errors in the information received
  • H04L1/12—Arrangements for detecting or preventing errors in the information received by using return channel
  • H04L1/16—Arrangements for detecting or preventing errors in the information received by using return channel in which the return channel carries supervisory signals, e.g. repetition request signals
  • H04L1/18—Automatic repetition systems, e.g. Van Duuren systems
  • H04L1/1867—Arrangements specially adapted for the transmitter end
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L1/00—Arrangements for detecting or preventing errors in the information received
  • H04L1/12—Arrangements for detecting or preventing errors in the information received by using return channel
  • H04L1/16—Arrangements for detecting or preventing errors in the information received by using return channel in which the return channel carries supervisory signals, e.g. repetition request signals
  • H04L1/18—Automatic repetition systems, e.g. Van Duuren systems
  • H04L1/1806—Go-back-N protocols
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L1/00—Arrangements for detecting or preventing errors in the information received
  • H04L1/12—Arrangements for detecting or preventing errors in the information received by using return channel
  • H04L1/16—Arrangements for detecting or preventing errors in the information received by using return channel in which the return channel carries supervisory signals, e.g. repetition request signals
  • H04L1/18—Automatic repetition systems, e.g. Van Duuren systems
  • H04L1/1809—Selective-repeat protocols

Definitions

• the present invention relates to a method of and apparatus for retransmission in wireless communications systems and is particularly concerned with packet retransmission.
• An object of the present invention is to provide an improved method and apparatus for wireless communications systems packet retransmission.
• a method of wireless data communications comprising the steps of: transmitting the data packet at a first rate; and upon receipt of a request to retransmit the data packet, transmitting the data packet at a second rate lower than the first rate.
• Figs, la, lb, lc, Id illustrate in block diagrams a wireless communications system
• Fig. 2 illustrates in a block diagram a wireless communications system with forward error detection (FED);
• FED forward error detection
• Fig. 3 illustrates in a block diagram a wireless communications system with forward error detection (FED) and retransmissions according to prior art
• Fig. 4 illustrates in a flow chart a method of wireless communication in accordance with an embodiment of the present invention.
• Fig. 5 illustrates in a block diagram a wireless communications system for implementing the method of Fig. 4.
• the wireless communications system 10 includes two wireless stations (terminals) 12 and 14.
• Fig. lb illustrates a wireless station of the wireless communications system 10 in further detail.
• the wireless station 12 (or 14) includes a data input 16 and a data output 18, a transmit path 20 and a receive path 30 and an antenna port 40.
• Fig. lc illustrates the transmit path of the wireless station of the wireless communications system 10 in further detail.
• the transmit path 20 typically includes a data randomization block 22, a forward error correction (FEC) coding block 24, a modulation block 26 and an RF conversion & amplification block 28.
• FEC forward error correction
• the typical transmit path 20 for a wireless communication system uses the data randomization block 22 to ensure that the modulated signal will have a uniform spectrum.
• the FEC coding block 24 improves the communication robustness to noise or channel imperfections. Examples of FEC schemes are Reed-Solomon, convolutional or turbo coding. Depending on the FEC schemes this block may include one or more data interleaving schemes to improve the FEC performance.
• the modulation block 26 transforms data bits in base-band digital or analog signals.
• Modulation schemes may (but they are not limited to) be single carrier or multi- carrier (e.g. OFDM) with different constellation mappings (e.g. BPSK, QPSK, QAM) and may include spectrum spreading techniques such as direct sequence spread- spectrum (DSSS) or frequency hopping (FH).
• DSSS direct sequence spread- spectrum
• FH frequency hopping
• Fig. Id illustrates the receive path of the wireless station of the wireless communications system 10 in further detail.
• the receive path 30 typically includes an RF amplification conversion block 32, a FEC decoding block 34, a demodulation, equalizer & sheer block 36 and a data de-randomization block 38. These functions operate in reverse fashion to the corresponding blocks in the transmit section, with the exception of the equalizer which is necessary to compensate for the channel impulse response.
• a combination of coding and modulation is referred to as a "rate", since the overall data rate (coding rate multiplied by the number of bits per symbol and by the symbol rate) describes the system outcome.
• wireless communication systems can automatically change the "rate” with which they operate in order to compensate for changes in the channel, i.e. attenuation, multipath, natural noise and interference, changes in channel are perceived by the wireless system as changes in the overall signal-to-noise-ratio (SNR).
• SNR signal-to-noise-ratio
• data rate i.e. attenuation, multipath, natural noise and interference
• PER packet-error-rate
• Wireless communication systems that are capable of adapting their "rate” to the channel characteristics, set their “rate” enough low such that the overall BER or PER falls within quality requirements for the link. Typical requirements for overall BER are in the order 10 "4 to 10 "12 depending on the application.
• each data packet is sent only once. If transmission encountered errors (detected either at the level of the wireless system or at a higher level in the protocol) the corresponding data is discarded or, in other words, is lost.
• the "rate” is set enough low so that the BER/PER meets the quality requirements for the link. Such a strong condition on BER/PER leads to poor usage of the wireless channel since the "rate” is significantly reduced below the potential rate (by 20% to 50%) to meet 10 "4 to 10 "12
• Fig. 2 there is illustrated another form of wireless station of the wireless communications system 10 in further detail.
• Fig. 2 includes a forward error detection (FED) function 50 that includes a FED add block 52, a FED check block 54 and a data valid/invalid output 56.
• FED forward error detection
• Many wireless systems monitor the BER/PER to ensure proper operation. This is usually performed using error-detection techniques.
• FED redundant information is appended or inserted in the data stream by the FED add block 52.
• the FED information is extracted from the data stream by the FED check block 54 and used to verify if the received data is valid or not. Invalid data packets are eventually removed from the received data stream.
• FED forward-error-detection
• CRC cyclic-redundant-codes
• error detection may be combined with error correction like with Reed-Solomon FEC/FED or other error detecting and correcting cods.
• the presentation discusses the case of separate FEC and FED.
• results of the error detection are exchanged between terminals systems in the form of acknowledge (ACK) and not-acknowledge (NACK) messages. This is to enable the sender of the packet to know if the packet was received properly.
• ACK acknowledge
• NACK not-acknowledge
• systems that implement both FED and ACK/NACK also employ packet retransmission.
• the sender of a packet stores data until an ACK or a NACK is received for that packet or, in other words, until it knows if the packet was or was not properly transmitted to the receiver. If the sender receives ACK, it proceeds with the next packet. If the sender receives NACK, it tries to retransmit the stored packet. Depending of the system, one or more retransmissions may be allowed.
• Fig. 3 there is illustrated a further form of wireless station of the wireless communications system 10 in further detail.
• Fig. 3 includes an ACK/NACK Extractor 60 coupled to the output of the Rx path 30 and an ACK/NACK Generator 62 coupled to the input of the Tx path 20.
• Fig. 3 also includes a transmit data buffer 64 and a switch 66. The ACK/NACK extractor 60 is coupled to the switch.
• each packet sent is buffered in the transmit data buffer 64.
• the ACK/NACK Extractor causes the switch 66 to switch from a transmit position to a retransmit position to cause the missing packet to be resent.
• the ACK/NACK generator 62 based on output from the FED check block 54 appends the appropriate ACK or NACK to transmitted packets.
• the transmitted data is grouped in packets (also called frames, bursts, etc) and the receiving station acknowledges each received data packet (implicitly or explicitly) to the transmitting station. The transmitting station can then retransmit the erroneous packets.
• PERo is significant. For example allowing up to 3 retransmissions improves the BER/PER from 10 "4 to 10 "16 and from 10 "2 to 10 "8 . Therefore, wireless systems with retransmission can operate with higher PERj which in terms means increased "rate”.
• the rate back off in wireless systems with retransmission can theoretically be much smaller (e.g. 5-10%) that in systems without retransmission.
• the increase in rate facilitated by retransmission is much higher that the difference between R0 and Rl, and thus systems with retransmission utilize more efficiently the channel.
• the communications link is effectively lost and packets accumulate at both stations until either the channel returns to normal or the "rate" adaptation algorithm reacts. This further increases the packet delay to a degree, which in many cases, may be unacceptable (i.e. the accumulated packets must be discarded and thus they are lost).
• the method includes a step of initially transmitting a data packet at a first rate, Ri and represented by a block 100.
• the first rate Rj being a rate, chosen to provide a predetermined level of packet error-ratio (PER). If a request for retransmission is received, as represented by a decision block 102, then resending the packet at a second rate, R 2 , which is lower than the first rate Rj. Otherwise, transmitting the next packet.
• Ri a rate, chosen to provide a predetermined level of packet error-ratio (PER).
• PER ⁇ PER 2 «PER, 2 This occurs because PER 2 /PER ⁇ is typically lO o 10 '8 for a 20% decrease in rate. Therefore the system using the retransmission method of Fig. 4 has overall BER and rate characteristics that are similar to systems with multiple retransmissions in constant channels.
• a wireless communications station incorporating the method of Fig. 4.
• the wireless communications station of Fig. 5 is similar to that of Fig. 3, except the data buffer 64' provides data for retransmission at a second rate R 2 that is lower that the transmission rate Rj. This accomplished by connection 68 that takes the output of the ACK NACK Extractor 60 and passes as control input to the Tx Path 20 for the purpose of switching between rates Ri and R 2 .
• the system with dual- rate transmission can achieve BER and rate characteristics of a system with n retransmissions while maintaining a maximum delay similar to that of a system that allows only one retransmission at the same rate as the initial transmission.
• the second rate R 2 can be further reduced so that it provides an acceptable overall BER for the worst case.
• the dual rate system of Figs. 4 and 5 can automatically avoid extreme packet loss and/or packet delays during accidental (rare)
• a wireless system using the dual-rate retransmission algorithm of Fig. 4 has the advantage of providing performance similar to multiple retransmission algorithms, while incurring a delay similar to that of single-retransmission algorithms.
• a further advantage to the dual-rate retransmission algorithm of Fig. 4 is that it can operate with very little back-off for Ri and very large back-off for R 2 . Thus it optimizes the overall rate while it optimizes the immunity to channel variations. Some implementations may measure PER over a predetermined number of packets or a combination of both.

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• Engineering & Computer Science (AREA)
• Computer Networks & Wireless Communication (AREA)
• Signal Processing (AREA)
• Quality & Reliability (AREA)
• Mobile Radio Communication Systems (AREA)
• Detection And Prevention Of Errors In Transmission (AREA)

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• 2002
  • 2002-08-13 EP EP02754029A patent/EP1419604A1/de not_active Withdrawn
  • 2002-08-13 WO PCT/CA2002/001252 patent/WO2003017560A1/en not_active Ceased

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