WO2009023986A1 - The method and the device for triggered repeat in the multicast system - Google Patents

The method and the device for triggered repeat in the multicast system Download PDF

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Publication number
WO2009023986A1
WO2009023986A1 PCT/CN2007/002504 CN2007002504W WO2009023986A1 WO 2009023986 A1 WO2009023986 A1 WO 2009023986A1 CN 2007002504 W CN2007002504 W CN 2007002504W WO 2009023986 A1 WO2009023986 A1 WO 2009023986A1
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Prior art keywords
threshold
retransmission
enb
nack signaling
condition
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PCT/CN2007/002504
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English (en)
French (fr)
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WO2009023986A8 (zh
Inventor
Yonggang Wang
Bernd Haberland
Pingping Xing
Bijun Zhang
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Alcatel Shanghai Bell Co., Ltd.
Alcatel Lucent
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Application filed by Alcatel Shanghai Bell Co., Ltd., Alcatel Lucent filed Critical Alcatel Shanghai Bell Co., Ltd.
Priority to US12/674,287 priority Critical patent/US8667356B2/en
Priority to JP2010521277A priority patent/JP5361888B2/ja
Priority to EP07800725A priority patent/EP2182666A4/en
Priority to PCT/CN2007/002504 priority patent/WO2009023986A1/zh
Priority to CN200780100172.4A priority patent/CN101809921B/zh
Priority to KR1020107006060A priority patent/KR101342424B1/ko
Publication of WO2009023986A1 publication Critical patent/WO2009023986A1/zh
Publication of WO2009023986A8 publication Critical patent/WO2009023986A8/zh

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    • G—PHYSICS
    • G08—SIGNALLING
    • G08C—TRANSMISSION SYSTEMS FOR MEASURED VALUES, CONTROL OR SIMILAR SIGNALS
    • G08C25/00—Arrangements for preventing or correcting errors; Monitoring arrangements
    • G08C25/02—Arrangements for preventing or correcting errors; Monitoring arrangements by signalling back receiving station to transmitting station
    • 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
    • H04L1/0015—Systems modifying transmission characteristics according to link quality, e.g. power backoff characterised by the adaptation strategy
    • H04L1/0019—Systems modifying transmission characteristics according to link quality, e.g. power backoff characterised by the adaptation strategy in which mode-switching is based on a statistical approach
    • H04L1/0021—Systems modifying transmission characteristics according to link quality, e.g. power backoff characterised by the adaptation strategy in which mode-switching is based on a statistical approach in which the algorithm uses adaptive thresholds
    • 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
    • H04L1/1825—Adaptation of specific ARQ protocol parameters according to transmission conditions
    • 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
    • H04L1/1829—Arrangements specially adapted for the receiver end
    • H04L1/1835—Buffer management
    • H04L1/1838—Buffer management for semi-reliable protocols, e.g. for less sensitive applications such as streaming video
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L5/00—Arrangements affording multiple use of the transmission path
    • H04L5/003—Arrangements for allocating sub-channels of the transmission path
    • H04L5/0053—Allocation of signaling, i.e. of overhead other than pilot signals
    • H04L5/0055—Physical resource allocation for ACK/NACK
    • 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
    • H04L2001/0092—Error control systems characterised by the topology of the transmission link
    • H04L2001/0093—Point-to-multipoint

Definitions

  • the present invention relates to a multicast system, and more particularly to a retransmission triggering method in a multicast system and a system for implementing the same.
  • E-MBMS is an evolution of multimedia broadcast and multicast services in 3GPP LTE.
  • MBMS transmission can be performed in two ways: multi-cell transmission and single-cell transmission.
  • the present invention generally relates to a single cell transmission mode.
  • MTCH MBMS Traffic Channel
  • DL-SCH Down Ink Synchronization Channel
  • the DL-SCH is mapped primarily through a dedicated traffic channel (DTCH).
  • DTCH dedicated traffic channel
  • technologies such as HARQ, adaptive modulation and transmission power, dynamic or semi-static resource allocation, and feedback CQI.
  • the MTCH When the MTCH is mapped to the DL-SCH, this part of the DL_SCH should be broadcast throughout the cell.
  • 3GPP applies a procedure similar to that used by the DL-SCH unicast service, such as HARQ (Hybrid Automatic Repeat Request) ⁇ and HSDPA (High Similar to Speed Downlink Packet Access), physical HARQ can reduce the block error rate of the receiver to achieve higher transmission performance. But the price is to reduce part of the transmission efficiency.
  • the BLER of the first transmission is between 10% and 30%, which can achieve the optimal performance-efficiency ratio.
  • the probability theory if n users are located in the same cell, the error probability for all UEs in the first transmission is ⁇ , then the probability that at least one UE receives the error block is 1-(1- ⁇ ) ⁇ . If the UE The number is large and the probability becomes very large. Therefore, certain mechanisms should be used to limit the feedback of NACK signaling, or to trigger retransmission of data blocks.
  • the goal is to first limit the UE with a BLER less than 30% to feed back the NACK, and secondly, limit the eNB's retransmission probability, for example, to 50%. On the other hand, if the UE reports too many NACKs, the uplink will be overloaded, so the number of NACK signaling should be limited when the UE receives the error block.
  • the technology of the present invention relates to a method for triggering retransmission when HARQ is applied to a multicast system, regardless of the manner in which the UE feeds back NACK signaling and the manner in which the eNB retransmits.
  • a retransmission triggering method in a multicast system including the steps of: the base station eNB sets a retransmission condition according to a channel state indication, and sends the set retransmission condition to the user equipment UE; When the UE receives the error block, the channel state indication of the current UE is compared with the received retransmission condition. When the retransmission condition is met, the UE feeds back the negative acknowledgement NACK signaling to the eNB to trigger the retransmission.
  • a retransmission triggering system comprising: a retransmission condition setting device at a base station eNB, setting a retransmission condition according to a channel state indication, and transmitting the set retransmission condition to a user The device UE; the retransmission condition comparing device at the user equipment UE, when the UE receives the error block, compares the channel state indication of the current UE with the received retransmission condition, and when the retransmission condition is met, the UE feeds back to the eNB. Negatively acknowledge NACK signaling to trigger a retransmission.
  • the triggering of the NACK signaling is restricted by restricting the condition that the UE feeds back the NACK signaling. Therefore, the feedback signaling in the uplink channel is limited, and the retransmission probability is limited to an appropriate region in a manner similar to Rel 6 HSDPA, thereby achieving an optimum ratio of efficiency to power. Uplink overload is avoided due to the limited number of NACK signaling. At the same time, the optimal ratio of efficiency to power is achieved by limiting the probability of retransmission.
  • FIG. 1 is a schematic view showing the present invention by taking BLER as an example
  • Figure 2 is a schematic view for explaining a threshold definition according to the present invention.
  • FIG. 3 is a flow chart of a retransmission triggering method in accordance with the present invention.
  • FIG. 4 is a schematic block diagram of a retransmission triggering system in accordance with the present invention. detailed description
  • the technology is designed to extend the cell edge or increase throughput.
  • the main point is to extend the cell edge and provide transmission performance that meets the error block rate requirement at the cell edge.
  • the cell edge is indicated by a solid line.
  • the BLER is 0.01 and the coverage is 70 3 ⁇ 4.
  • the BLER is 0.3.
  • the purpose of using HARQ is to extend the cell edge from 70% to 90%, or to reduce the BLER at the cell boundary of 90% coverage from 0.3 to 0.01 o.
  • the retransmission trigger system 400 may include: a feedback condition setting means 401 located at the eNB and a feedback condition comparing means 402 located at the UE.
  • the feedback condition setting device 401 includes a static threshold setting module 403, a dynamic threshold setting module 405, and an output module 407.
  • the output module 407 sends the thresholds generated by the static threshold setting module 403 and the dynamic threshold setting module 405 to the receiving module 408 of the feedback condition comparing means 402 via the control channel.
  • the dynamic threshold setting module 405 is optional in the static domain value scheme.
  • the feedback condition comparison means 402 further includes a random number generation module 410, a threshold comparison module 412, a probability calculation module 414, and a NACK feedback module 416.
  • the threshold comparison module 412 compares the received threshold with the current instantaneous channel state parameter of the UE to determine whether to feed back NACK signaling to the eNB to trigger retransmission.
  • a condition for feeding back NACK signaling when the UE receives the error block is set.
  • This condition can be two thresholds A and 8.
  • the threshold can be limited to:
  • threshold A and the threshold B can be configured by the eNB. As shown in FIG. 2, for example, the actual meaning of threshold A and threshold B may be corresponding to the boundary of coverage in the cell. Other channel state indications can also be used to set thresholds, such as signal to noise ratios and the like. Assuming a measured value or statistical value is X, X can be BER or BLER in different cases. So, you can use the following detailed steps:
  • X is defined as BER or BLER
  • the UE does not feed back NACK signaling.
  • the UE may feed back NACK signaling. Therefore, as shown in FIG. 2, the UE far from the eNB is located outside the threshold A (for example, UE3), and there is no need to feed back NACK signaling. Because it is assumed that under the same channel conditions, even if retransmitted, the probability of correctly receiving the previous error block is very low. Since the BLER requirement (less than 1%) has been met and the UE close to the eNB is located inside the threshold B (e.g., UE1), there is no need to feed back NACK signaling. Since HARQ will correct the error block received by the UE in the area, only the UE (e.g., UE2) located between the threshold A and the threshold B should feed back the NACK signaling.
  • the threshold A for example, UE3
  • BLER is taken as an example for description.
  • condition for feeding back NACK signaling when the UE receives the error block can be set as:
  • step 302 when an error block is received, the UEs satisfying the above conditions feed back NACK signaling.
  • the lower limit is extended from 0.1 to 0.05, and the effective UE that feeds back NACK signaling is in the region of 0.05 to 0.3.
  • the present invention proposes two solutions to achieve the above objectives:
  • Scheme 1 Static threshold, but the dynamic feedback probability factor of the UE receiver is p ;
  • Scheme 2 Dynamic Threshold, but the eNB should know the instantaneous BLER of each UE that feeds back the NACK and set the threshold.
  • a probability factor p is set. Once the UE detects an error block, the NACK signaling in response to HARQ is fed back based on the probability factor.
  • the value of the probability factor can vary depending on the UE measurement or BLER.
  • the probability calculation module 414 can be utilized to set a higher probability factor p. If the UE is far away from the eNB and the measured BLER is large, a lower probability may be set.
  • the detailed steps are as follows: When the UE receives the error block, when the instantaneous BLER of the UE is greater than Th-A or less than Tk-B, The UE does not feed back NACK signaling.
  • the random number generating module 410 When the instantaneous BLER of the UE is between Th_A and Th_B, the random number generating module 410 generates a random number X between 0 and 1, wherein if X ⁇ p, the UE does not feed back NACK signaling; if X > p, the UE feeds back NACK signaling through the NACK feedback module 416.
  • BP BP
  • UE and eNB may not be able to know the total number of UEs actually receiving multicast services in the cell, ie n. If it is not known, in order to solve this problem, a method of adaptively selecting the p value is proposed.
  • the UE calculates the appropriate one according to the following equation through the probability calculation module 414.
  • ⁇ is the BLER measured by the current UE. Therefore, when the UE receives the error block, the UE generates a random number X between 0 and 1, where: if X ⁇ A, the UE does not feed back NACK signaling; If x>A, the UE feeds back NACK signaling.
  • the UE uses the statistical total retransmission packet and adjusts /c:
  • D is the number of packets transmitted during this period
  • d is the data of the total retransmission packet during this period
  • A is the adjustment factor, and >1.
  • the eNB dynamic threshold setting module 405 sets the dynamic thresholds to Th-A and Th-B, respectively, and transmits them to the UE through the control channel. Since the eNB should know the instantaneous BLER in each UE, in the scheme, the UE not only feeds back NACK signaling, but also should simultaneously feed back the BLER in the uplink.
  • the dynamic threshold scheme according to the present invention will be described below:
  • the UE When the UE receives the error block, when the instantaneous BLER of the UE is greater than Tk_A or less than Th_B, the UE does not feed back NACK signaling. When the instantaneous BLER of the UE is between 73 ⁇ 4_ and 73 ⁇ 4, the UE feeds back NACK signaling in the following format:
  • the eNB uses the statistics of all BLER feedback from the UE.
  • the dynamic threshold does not limit the probability of eNB retransmission.
  • the retransmission probability factor q in the eNB can be set, for example, ⁇ 50%, and the eNB attempts to retransmit the error transmission block based on the probability factor.
  • the eNB receives NACK signaling, the eNB generates a random number x between 0 and 1, where: if x ⁇ q, the eNB retransmits the error transport block; if X >, the eNB does not retransmit the error transport block.
  • the present invention proposes a method for triggering a NACK and performing retransmission in a multicast system for HARQ. Since HARQ is introduced in the multicast system, the conditional retransmission mechanism is used, thereby achieving a relatively high transmission performance and efficiency.
  • the triggering of the NACK signaling is restricted by restricting the condition that the UE feeds back the NACK signaling. Therefore, the feedback signaling in the uplink channel is limited, and the retransmission probability is limited to an appropriate region in a manner similar to Rel 6 HSDPA, thereby achieving an optimum ratio of efficiency to power. Uplink overload is avoided due to the limited number of NACK signaling. At the same time, the optimal ratio of efficiency to power is achieved by limiting the probability of retransmission.

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Description

多播系统中的重传触发方法及实现该方法的装置
技术领域
本发明涉及一种多播系统， 更具体地， 涉及一种多播系统中的重传触发方法 及实现该方法的系统。 背景技术
E- MBMS是 3GPP LTE中多媒体广播和多播业务的一种演进。 可以按照以下两 种方式来执行 MBMS传输： 多小区传输和单小区传输。 本发明主要涉及单小区传 输模式。
在单小区传输的情况下， 即， 小区专用于多点传输， 每一个小区中多播业务 的内容彼此独立。 无需同步地传输来自多个小区的 MBMS数据。 在 3GPP规范中， 可以将 MTCH (MBMS Traffic Channel )映射到 DL-SCH ( Downl ink Synchronization Channel )。
在 E- UTRAN 中， 主要通过专用业务信道 （DTCH) 来映射 DL- SCH。 为了改进 UE单播业务的峰值速率以及系统吞吐量， DL-SCH釆用了诸如 HARQ、 自适应调制 编码和传输功率、 动态或半静态资源分配、 以及反馈 CQI等技术。
当将 MTCH映射到 DL- SCH时， 应当在整个小区中广播这一部分 DL_SCH。为了 提供小区边缘处所需的数据率和误块率（BLER)， 3GPP应用了与 DL- SCH的单播业 务所使用的相似的过程， 例如 HARQ (Hybrid Automat ic Repeat Request )^ 与 HSDPA ( High Speed Downlink Packet Access ) 相似， 物理 HARQ可以减 小接收机的误块率，从而实现更高的传输性能。但代价在于降低了部分传输效率。 根据 HSDPA部署的经验， 第一次传输的 BLER在 10%到 30%之间， 能够实现最优 的性能-效率比。
然而， 当将 HARQ应用于多播系统时， 由于多个 UE同时接收下行多播业务， 原则上只要有一个 UE接收到差错块并返回 NACK信令， 基站 （eNB ) 应当重传该 数据块。
根据概率理论， 如果 n个用户位于同一个小区中， 在第一次传输中对于所有 UE的差错概率是 α , 则至少一个 UE接收到差错块的概率是 1-(1-α)η。 如果 UE的 数目较多， 该概率变得非常大。 因此， 应当将某些机制用于限制 NACK信令的反 馈， 或触发数据块的重传。
目标在于， 首先限制 BLER小于 30%的 UE可反馈 NACK， 其次， 限制 eNB 的重传概率， 例如， 限制为 50%。 另一方面， 如果 UE反馈的 NACK过多， 则 上行链路会过载， 因此应当在 UE接收到差错块时限制 NACK信令的数目。
本发明的技术涉及将 HARQ应用于多播系统时触发重传的方法，与 UE反馈 NACK 信令的方式和 eNB重传的方式无关。 发明内容
根据本发明的一方面， 提出了一种多播系统中的重传触发方法， 包括步骤： 基站 eNB根据信道状态指示来设置重传条件， 并将所设置的重传条件发送到用户 设备 UE; 当 UE接收到差错块时， 将当前 UE的信道状态指示与接收到的重传条件进 行比较， 当满足重传条件时， UE向 eNB反馈否定应答 NACK信令， 以便触发重传。
根据本发明的另一方面， 提出了一种重传触发系统， 包括： 基站 eNB处的重传 条件设置装置， 根据信道状态指示来设置重传条件， 并将所设置的重传条件发送 到用户设备 UE; 用户设备 UE处的重传条件比较装置， 当 UE接收到差错块时， 将当 前 UE的信道状态指示与接收到的重传条件进行比较，当满足重传条件时， UE向 eNB 反馈否定应答 NACK信令， 以便触发重传。
当 UE接收到差错块时， 通过限制 UE反馈 NACK信令的条件， 限制了 NACK信 令的触发。 因此， 限制了上行链路信道中的反馈信令， 并且按照与 Rel 6 HSDPA 类似的方式将重传概率限制在适当的区域中， 从而实现了效率与功率的最优比。 由于限制了 NACK信令的数目， 避免了上行链路过载。 同时， 由于限制了重传概 率， 实现了效率与功率的最优比。 附图说明
图 1是以 BLER为例说明本发明的示意图；
图 2是用于说明根据本发明的阈值定义的示意图；
图 3是根据本发明的重传触发方法的流程图；
图 4是根据本发明的重传触发系统的示意方框图。 具体实施方式
对于 E-MBMS，其技术的目的在于扩展小区边缘或提高吞吐量。对于 HARQ， 主要点在于扩展小区边缘， 在小区边缘提供符合误块率要求的传输性能。 在图 1 中， 当不使用 HARQ时， 小区边缘由实线表示， 作为示例， BLER为 0.01， 覆盖 率为 70 ¾。 在覆盖率达到 90%的边界处， BLER为 0.3。 使用 HARQ的目的在于 将小区边缘从 70%扩展到 90%， 或者是说将 90%覆盖的小区边界处的 BLER从 0.3减小为 0.01 o
当进行重传时， 如果小区中 UE的接收是错误的， 假设小区中存在数十个预 订的 UE， 则导致 100%的重传。 在 Rel 6 HSDPA中， 重传的概率小于 30 %， 从 而实现了效率和功率的最优比。下文中给出了采用 eNB来有条件地触发重传的方 法。
图 4是根据本发明的重传触发设备的方框图。 如图 4所示， 根据本发明的重 传触发系统 400可以包括：位于 eNB处的反馈条件设置装置 401和位于 UE处的 反馈条件比较装置 402。其中，反馈条件设置装置 401包括静态阈值设置模块 403、 动态阈值设置模块 405以及输出模块 407。 输出模块 407将由静态阈值设置模块 403和动态阈值设置模块 405产生的阈值通过控制信道发送到反馈条件比较装置 402的接收模块 408。 动态阈值设置模块 405在静态域值方案中是可选的。 反馈 条件比较装置 402还包括随机数产生模块 410、 阈值比较模块 412、 概率计算模 块 414和 NACK反馈模块 416。 当 UE接收到差错块时， 阈值比较模块 412将接 收到的阈值与 UE当前的瞬时信道状态参数进行比较， 以便确定是否向 eNB反馈 NACK信令， 以便触发重传。
下面将参考图 1一 4来详细描述根据本发明的重传触发方法。
首先， 在步骤 301 , 设置当 UE接收到差错块时反馈 NACK信令的条件。 该 条件可以是两个阈值 A和8。 可以将阈值限定为：
1 ) 物理层中的误比特率 （BER); 或
2 ) 层 2中的误块率 ( BLER ) o
这里， 可以由 eNB来配置阈值 A和阈值 B。 如图 2所示， 例如， 阈值 A和阈 值 B的实际意义可以是对应于小区中覆盖的边界。 还可以使用其它信道状态指示 来设置阈值， 例如信噪比等。 假设一个测量值或统计值为 X, X可以是不同情况下的 BER或 BLER。 于是， 可以釆用以下详细步骤：
当用户接收到差错块时， X被定义为 BER或 BLER, 则
如果 X〉A， 则 UE不反馈 NACK信令；
如果 X<B， 则 UE不反馈 NACK信令
如果 X<= A且 X〉=B， 则 UE可能会反馈 NACK信令。 因此， 如图 2所示， 远离 eNB的 UE位于阈值 A的外部 （例如 UE3 ) , 则无需 反馈 NACK 信令。 因为假设在同等信道条件下， 即使重传， 正确接收之前错误块 的概率也很低。 由于已满足 BLER要求 （小于 1 % ) , 靠近 eNB的 UE位于阈值 B 的内部 （例如 UE1 ) , 则无需反馈 NACK信令。 由于 HARQ会校正该区域中 UE接收 到的差错块， 只有位于阈值 A和阈值 B之间的 UE (例如 UE2 ) 才应当反馈 NACK 信令。
下文将描述如何选择阈值以及如何实现根据本发明的方法， 其中以 BLER为 例进行描述。
假设在小区中存在 n个 UE接收多播业务， 瞬时 BLER是 则在 一次分组传输期间， 至少一个 UE接收到差错块的概率是-
Figure imgf000006_0001
不失一般性， 设 ≤α2 如果 eNB能够接受的、 在 RRC CON ECTTION中可同时用于 UE进行反馈 的上行链路 （时隙） 的数目是 m， 则应满足条件：
'•=1
因此， 可以将 UE收到差错块时反馈 NACK信令的条件设置为：
1 ) 将重传概率限制在 50 %以下；
2 ) 限制为 30 %以下的 UE反馈 NACK;
3 ) 限制同时传输的 NACK信令的数目。
即， 可以将条件和阈值限制为：
Figure imgf000007_0001
& » j≤i≤ k o
在步骤 302，当接收到差错块时，满足以上条件的这些 UE反馈 NACK信令。 这里， 将下限从 0.1扩展到 0.05 , 反馈 NACK信令的有效 UE处于 0.05〜0.3的 区域内。 当然， 不必将重传概率局限为 50 %和 30 ? 的 UE反馈 NACK， 该值取决于 实际的业务性能要求和所提供的无线电资源。
根据上述条件， 本发明提出了两种方案来实现上述目标：
方案 1 : 静态阈值， 但 UE接收机的动态反馈概率因子为 p ;
方案 2 : 动态阈值， 但 eNB应当已知每一个反馈 NACK的 UE的瞬时 BLER 并设置阈值。 对于方案 1， eNB通过静态阈值设置模块 403将静态阈值分别设为 Th—A=Q.3 且 777一^=0.05， 并将其通过控制信道传送到 UE。 对于 UE反馈 NACK信令， 设 置概率因子 p。 一旦 UE检测到差错块， 则基于该概率因子反馈响应 HARQ 的 NACK信令。 概率因子的值可以根据 UE测量或 BLER进行变化。 例如， 如果 UE靠近 eNB , 测量到的 BLER较小， 则可以利用概率计算模块 414来设置较高 的概率因子 p。 如果 UE远离 eNB , 测量到的 BLER较大， 则可以设置较低的 概率因 详细步骤如下所述： 当 UE接收到差错块时， 当 UE的瞬时 BLER大于 Th—A或小于 Tk—B时， UE 不反馈 NACK信令。 当 UE的瞬时 BLER处于 Th—A 和 Th—B之间时， 随机数产 生模块 410产生 0和 1之间的随机数 X， 其中 如果 X < p， 则 UE不反馈 NACK信令； 如果 X > p， 则 UE通过 NACK反馈模块 416反馈 NACK信令。 如何选择概率因子 P : 通常， 将根据本发明的方法应用于小区内存在大量 UE 的情况。 为了避免由 07002504 于反馈导致的上行链路过载， 即使 UE检测到差错块， 也应当尝试基于概率因子 来反馈响应 HARQ的 NACK信令。 假设 UE反馈 NACK信令的概率是 A， eNB 至少接收到一个 NACK的概率是 ' = 1-Π(ΐ-αΛ)
/=1 如果 eNB能够接受的、 在 RRC CONNECTTION中同时用于 UE进行反馈的 上行链路 （时隙） 的数目是 m， 则条件是
Figure imgf000008_0001
考虑到重传概率， 期望 P 是最小值。 根据 Cauchy 不等式， 仅在 α,Α = 2ρ2 =… = αηρ„的情况下， Ρ取最小值。 通过限制 ?,≤1， 则反馈概率因子 ?应当为
m
α, >
«,· · n
1， 但这里存在一个问题， BP, UE和 eNB未必能知道小区内实际接收多播业务 的 UE的总数， 即 n。 如果不能知晓， 为了解决该问题， 提出了一种自适应选择 p值的方法。
(1) 在业务传输的开始处， UE设定初始 / 值， 例如， /。 =0.05;
(2) —旦第 i个 UE接收到 / ， 则 UE通过概率计算模块 414， 根据以下等式计 算适当的
Figure imgf000008_0002
其中 α, 是当前 UE测量的 BLER。 因此， 当 UE接收到差错块时， UE产生- 个 0和 1之间的随机数 X， 其中： 如果 X<A， 则 UE不反馈 NACK信令； 如果 x> A， 则 UE反馈 NACK信令。
(3) 在预配置期间， UE采用统计总重传分组， 并调整 /c:
[λκ,
κΜ =\ 3 ， ^^<0.5
[1 // Kj > Ϊ D
Kj+] others
Figure imgf000009_0001
其中， D是这期间传输的分组数目， d,r 是这期间的总重传分组的数据， A是 调节系数， 且 >1。
(4) UE根据步骤 （2) 进行操作。
另外一种解决该 UE和 eNB不能知晓 n的问题的最简单方法，是采用固定 p值 的方法， 如/ ? = 0.3。
方案 2 eNB动态阈值设置模块 405将动态阈值分别设置为 Th—A和 Th—B， 并将其通 过控制信道传送到 UE。 由于 eNB应当已知每一个 UE中的瞬时 BLER， 在方案中， UE不仅会反馈 NACK信令， 还应当在上行链路中同时反馈 BLER。 下面将描述根据本发明的动 态阈值方案：
( 1 ) eNB 分别将初始阈值设置为 Th—A=0.3， ΓΛ— ^=0.05并将其通过控制信道传 送到 UE;
(2) 当 UE接收到差错块时， 当 UE的瞬时 BLER大于 Tk—A或小于 Th_B时， UE不反馈 NACK信令。 当 UE的瞬时 BLER处于 7¾_ 和 7¾一 之间时， UE反 馈以下格式的 NACK信令：
UE id NACK BLER
(2) 在预配置期间， eNB釆用来自 UE的所有 BLER反馈的统计值
并根据以下等式调整 Th A 和 Th B:
Figure imgf000010_0001
如果^ ,.≥w， 则
Figure imgf000010_0002
如果^ ,.</"， 则
Th_Anm, =A k, ak = a,
Figure imgf000010_0003
其中， A是调节系数， 且 1>1 ;
(4)动态阈值并没有局限 eNB重传的概率。 据此， 可以设置 eNB中的重传概率 因子 q, 例如 ίρ50%， eNB 尝试基于该概率因子来重传差错传输块。 当 eNB接收到 NACK信令时， eNB产生 0和 1之间的随机数 x, 其中： 如果 x<q, 则 eNB重传差错传输块； 如果 X> , 则 eNB不重传差错传输块。 本发明提出了一种针对 HARQ在多播系统中触发 NACK并进行重传的方法。 由 于在多播系统中引入了 HARQ， 使用.了条件重传机制， 从而实现了较髙的传输性能 和效率。
当 UE接收到差错块时， 通过限制 UE反馈 NACK信令的条件， 限制了 NACK信 令的触发。 因此， 限制了上行链路信道中的反馈信令， 并且按照与 Rel 6 HSDPA 类似的方式将重传概率限制在适当的区域中， 从而实现了效率与功率的最优比。 由于限制了 NACK信令的数目， 避免了上行链路过载。 同时， 由于限制了重传概 率， 实现了效率与功率的最优比。

Claims

权 利 要 求
1. 一种多播系统中的重传触发方法， 包括步骤- 基站 eNB根据信道状态指示来设置重传条件， 并将所设置的重传条件发送到 用户设备 UE;
当 UE接收到差错块时， 将当前 UE的信道状态指示与接收到的重传条件进行 比较， 当满足重传条件时， UE向 eNB反馈否定应答 NACK信令， 以便触发重传。
2.根据权利要求 1所述的方法， 其特征在于， 所述重传条件包括第一阈值和 大于第一阈值的第二阈值， 当当前 UE的信道状态指示小于第一阈值或大于第二阈 值时， UE不向 eNB反馈 NACK信令。
3.根据权利要求 1所述的方法， 其特征在于， 所述重传条件包括第一阈值和 大于第一阈值的第二阈值， 当当前 UE的信道状态指示处于第一阈值和第二阈值之 间时， 如果小区内实际接收多播业务的 UE的数目已知， 则根据小区内实际接收多 播业务的 UE的数目、可用于反馈 NACK信令的最大同时建立的上行链路数目以及当 前 UE的信道指示状态来确定是否反馈 NACK信令；
如果小区内实际接收多播业务的 UE的数目未知， 则根据统计总重传分组的 个数以及当前 UE的信道指示状态来确定是否反馈 NACK信令。
4. 根据权利要求 3所述的方法， 其特征在于， UE利用小区内实际接收多播业 务的 UE的数目或统计总重传分组个数、 以及可用于反馈 NACK信令的最大同时建立 的上行链路数目和当前 UE的信道指示状态来得到一个概率值， 产生一个介于 0和 1 之间的随机数， 并将所产生的随机数与所述概率值进行比较， 当随机数大于概率 值时， LIE向 eNB反馈 NACK信令， 以触发 eNB重传， 否则不反馈 NACK信令。
5.根据权利要求 1所述的方法， 其特征在于， 所述重传条件包括第一阈值和 大于第一阈值的第二阈值， 当当前 UE的信道状态指示处于第一阈值和第二阈值之 间时， UE向 eNB反馈 NACK信令， 向 eNB通知 UE当前的信道状态指示；
eNB根据接收到的 NACK信令和信道状态指示来更新第一阈值和第二阈值， 并 将更新的阈值发送给 UE;
UE根据更新的阈值来确定是否发送 NACK信令。
6.根据权利要求 5所述的方法， 其特征在于， 当 eNB接收到 NACK信令时， 根据 预设的重传概率因子来确定是否重传差错传输块。
7.根据权利要求 6所述的方法， 其特征在于， eNB产生一个 0到 1之间的随机数， 当预设的重传概率因子大于所述随机数时， eNB不重传差错传输块， 否则， eNB向 UE重传差错传输块。
8. 根据权利要求 1到 7之一所述的方法， 其特征在于， 所述信道状态指示包括 误码率、 误块率或信噪比。
9. 根据权利要求 1到 7之一所述的方法， 其特征在于， eNB通过控制信道向 UE 发送重传条件。
10. 一种重传触发系统， 包括：
基站 eNB处的重传条件设置装置， 根据信道状态指示来设置重传条件， 并将 所设置的重传条件发送到用户设备 UE ;
用户设备 UE处的重传条件比较装置， 当 UE接收到差错块时， 将当前 UE的信 道状态指示与接收到的重传条件进行比较， 当满足重传条件时， UE向 eNB反馈否 定应答 NACK信令， 以便触发重传。
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