LTE与IEEE 802.16的退避算法研究

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"这篇论文主要探讨了在UMTS-LTE(通用移动通信系统-长期演进)和IEEE 802.16(无线城域网标准，也称为WiMAX)系统中的退避算法设计与分析，用于随机接入信道。文章在2011年10月发表于IEEE Transactions on Vehicular Technology期刊，由Jun Seo和Victor C.M. Leung等人撰写。该研究对提高无线网络的效率和性能具有重要意义，已被引用47次，阅读量达到166次。两位作者还参与了其他相关项目，如MIMO多工系统的性能评估和VENDNET（车载命名数据网络）项目。" 退避算法是无线通信系统中解决冲突和提高信道利用率的关键技术，特别是在多用户共享同一信道的情况下。在LTE和IEEE 802.16系统中，随机接入信道（Random Access Channel, RACH）是设备连接到网络并请求服务的第一步，比如建立连接、发送数据包或请求资源。退避算法通过在设备尝试访问信道之前引入随机等待时间，减少了多个设备同时发送数据导致的碰撞。在LTE系统中，随机接入过程包括几个步骤，如 preamble传输、竞争解决阶段以及随后的随机接入响应。退避算法在竞争解决阶段起着重要作用，通过增加再次尝试接入的时间间隔来降低碰撞概率。例如，当一个设备检测到冲突时，它会根据一定的规则（如二进制指数退避）选择一个新的随机等待时间，然后再次尝试。对于IEEE 802.16的WiMAX系统，随机接入过程也包含类似的机制，但可能有其特定的退避策略。WiMAX使用了一种称为“预约请求（Request To Send, RTS）/清除发送（Clear To Send, CTS）”的协议，设备在发送数据前先发送RTS帧，若收到CTS响应，则表明可以安全发送。若发生冲突，退避算法将指导设备如何重新尝试。这篇论文详细分析了这两种技术中的退避算法设计，可能涵盖了算法的优化、性能评估和仿真结果。通过这样的研究，可以改善网络的吞吐量、延迟以及资源分配的公平性，确保无线网络在高用户密度环境下的稳定性和效率。论文作者的研究工作不仅限于此，他们还关注了MIMO多工系统在异构网络（HetNets）中的性能评估，这涉及到在不同类型的基站和用户设备共存的环境中，如何利用多天线技术提高无线网络的容量和可靠性。此外，VENDNET项目则关注的是车辆通信网络，可能涉及车联网的新型架构和通信协议，以支持未来智能交通系统的需求。这篇论文及其相关研究对理解并改进现代无线通信系统中的关键机制，尤其是退避算法在LTE和WiMAX系统中的应用，提供了宝贵的理论和实践洞见。

SEO AND LEUNG: BACKOFF ALGORITHM FOR RANDOM ACCESS CHANNEL IN UMTS-LTE AND IEEE 802.16 SYSTEM 3977

there can be F random access channels in the frequency domain

within one access period, among which an accessing terminal

may randomly select one. At the same time, a terminal picks

one of P RAPs and transmits it in the selected random access

channel. If the same RAP is transmitted by more than one

terminal over the same random access channel, a code collision

occurs, or if the transmitted RAP is not acknowledged by the

BS within a speciﬁed time window, e.g., the random access

response (RAR) window in UMTS-LTE, it is also declared as a

collision. This latter condition may occur due to the transmitted

RAP not being successfully decoded under severe MAI. When-

ever the terminals’ RAP transmissions are not successful, they

retransmit after a delay determined by the UB algorithm (in

UMTS-LTE) or BEB algorithm (in an IEEE 802.16 system).

These algorithms will be explained in the next sections. In

both systems, if the number of retransmissions exceeds the

maximum retry limit, the RAP is dropped in the MAC layer.

This is reported to a higher layer. Once the RAP is successfully

transmitted, the terminal can transmit a bandwidth request mes-

sage for bandwidth reservation on the uplink channel that the

BS has granted to it via the RAR message in UMTS-LTE or the

uplink map (UL-MAP) in the IEEE 802.16 system. This proce-

dure is triggered at the time when an end-user makes a phone

call or uses an application that needs to access the Internet.

Accordingly, fast resolution of collisions in RAP transmissions

is expected to enable fast bandwidth reservations. Additionally,

in UMTS-LTE, the window size of the UB algorithm, which

is denoted by U hereafter and is assigned by the BS in the

ﬁrst RAR message, can be dynamically chosen according to

network congestion. Thus, it is expected that appropriately

controlling U can efﬁciently resolve collisions under some

time-varying trafﬁc condition.

In what follows, we assume that the transmission power

and timing of RAPs are perfectly known to terminals, which

simpliﬁes the power-ramping scheme in UMTS-LTE. This is

equivalent to the assumption that initial and periodic ranging

processes are sufﬁciently well performed in an IEEE 802.16

system. We further assume that perfect orthogonality among

RAPs simultaneously transmitted by multiple terminals is pre-

served. Owing to this perfect orthogonality assumption, a sys-

tem with P RAPs and F random access channels is equivalent

to a system with P · F RAPs. Note that, in UMTS-LTE, each

BS provides 64 RAPs, some of which can be reserved for the

use of noncontention-based access. Additionally, in the frame

structure of type-1 frequency-division duplex (FDD) mode, one

random access channel in the frequency domain appears every

two slots [15]. For analytical simplicity, we set F =1 every

slot and assume a slotted system with FDD, in which each slot

corresponds to one RAP transmission (one packet) time. The

number of RAPs will be given later in Section V. Hereafter, we

interchangeably use the terms RAP and packet. In the system

model, we assume a ﬁnite population of M terminals. Each

terminal can hold only one packet for transmission or retrans-

mission until it is successfully received. A terminal that has no

packet to transmit is said to be idle. An idle terminal generates a

packet in every slot based on a Bernoulli trial with probability ,

i.e., unsaturated trafﬁc condition. When a terminal has a packet

to send, it immediately becomes backlogged by performing the

backoff algorithm, i.e., it employs a delayed-ﬁrst-transmission

protocol. We ﬁnally assume that terminals can get feedback

information for the RAP transmitted, e.g., success or failure,

from the BS right before the next slot with no error, i.e., no

feedback delay is taken into account.

B. Uniform Backoff Algorithm in UMTS-LTE

Each backlogged terminal randomly draws a backoff counter

between 0 and U − 1. The backoff counter is then decremented

by one at every slot, until it reaches zero. When the backoff

counter is zero, the terminal selects a random real number

between 0 and 1. As in [11], if this number is less than the

persistence probability (or persistence value) p, the terminal

transmits its packet; otherwise, it postpones the packet trans-

mission by repeating the preceding procedure in the next slot,

until the packet is transmitted. When the packet is not success-

fully transmitted, i.e., the transmission encounters a collision,

the terminal delays its retransmission again by a random time

newly chosen between 0 and U − 1. This trial is repeated

up to L times more, until the retransmission succeeds, or the

terminal drops the packet and goes back to idle if the L +1

retransmission remains unsuccessful.

C. BEB Algorithm in IEEE 802.16

Upon an unsuccessful packet transmission, the BEB algo-

rithm exponentially increases the backoff window by doubling

its size. Let W

denote the backoff window size of the ith

backoff stage, which is determined by

for 0 ≤ i ≤ K (1)

where W

is the minimum backoff window size. At the ith

backoff stage, the backlogged terminal randomly chooses a

backoff counter in the range [0,W

− 1] for 0 ≤ i ≤ K. Once

chosen, the backoff counter is decremented every slot as in the

UB algorithm. When it reaches zero, the terminal transmits

its packet. Upon transmission failure, the terminal increments

the backoff stage by one and chooses the next backoff counter

again. Although the retry limit L is independent of K in [1], we

assume here that the BEB algorithm retries L times additionally

at the Kth stage. Thus, upon the retransmission failure after

a total of L +1 retransmissions at the Kth stage, it drops

the packet. One can ﬁnd more details of the random access

procedure for IEEE 802.16 in [16].

D. Access Prioritization by Persistence Value

To provide access priority, UMTS includes eight access

priority classes [11], each of which has a different persistence

value p = P

(N)=s

· 2

−(N−1)

for 0 ≤ i ≤ 7. Note that the

value of i can indicate a certain partition of the random access

channels. The persistence scaling factor s

is assumed to be

a value between 0.2 and 0.9 by step of 0.1, and the dynamic

persistence level N from 1 to 8 is regularly broadcast. If the

persistence scaling factors are not broadcast, a default value

of one is assumed. Once the backoff counter reaches zero, the

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LTE与IEEE 802.16的退避算法研究

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