Route and Spectrum Selection in Dynamic
Spectrum Networks
Qiwei Wang
Tsinghua University
Beijing,China wangqw04@https://www.doczj.com/doc/7919035504.html,
Haitao Zheng
Dept.of Computer Science Univ.of California,Santa Barbara htzheng@https://www.doczj.com/doc/7919035504.html,
Abstract—Ef?cient spectrum allocation in dynamic spectrum systems is a challenging problem,particularly for multi-hops transmissions.The inter-dependence between route selection and spectrum management makes it important to examine interaction between the two and the corresponding performance and com-plexity tradeoffs.In this paper,we explore two design method-ologies:a decoupled design where these tasks are carried out independently by different protocol layers,and a collaborative design that integrates them into a single task.Experimental results show that the collaborative design,if well-provisioned, offers signi?cant performance improvement compared to the decoupled design.
I.I NTRODUCTION
Wireless radio spectrum is a precious?nite resource.Given that the current spectrum licensing policy facing near-future threat of spectrum scarcity and the increasing crowd in unlicensed spectrum band,ef?cient spectrum management is necessary and critical to future development of system and networking[2],[9].While maximizing utilization is the primary goal,a good management scheme also needs to minimize interference and provide a degree of fairness across users.Our previous work on decentralized spectrum allocation focuses on dynamic spectrum selection which is in general the responsibility of Medium Access Control(MAC)layer[3], [14].Using a single-hop traf?c model,we show that user collaboration leads to results that closely approximate the optimal centralized allocation.
For general dynamic spectrum networks,modi?cations are required at higher layers to respond to dynamically changing spectrum availability and interference pattern.In this paper, we focus on the behavior of multi-hop transmissions where destination user is out of the transmission range of source user,and packets are routed to the destination by users in between.Unlike in single-hop transmissions,the choice of packet route heavily impacts the traf?c load on each transmis-sion link and the amount of spectrum required.Observing a strong inter-dependence between route selection and spectrum management,we examine the interaction between the two and the corresponding performance and complexity tradeoffs.In particular,we focus on two design methodologies:1)a decou-pled design where the two tasks are carried out independently by MAC and network layer,and2)a collaborative design which integrates them into a single task at the network layer-network layer selects route and schedules con?ict-free channel usage on the route.
This paper makes three contributions.First,we propose two types of interaction between route selection and spectrum management.Second,we develop detailed implementations that tradeoff performance with communication overhead and computational complexity.Finally,we use extensive simula-tions to quantify the impact of different approaches on network access.
II.B ACKGROUND AND R ELATED W ORK
In this section,we brie?y introduce the background of dynamic spectrum systems and existing work on spectrum management.In dynamic spectrum networks,channels are the fundamental units of spectrum usage,and the operational spectrum is partitioned into non-overlapping channels.Each user performs spectrum sensing to track it spectrum oppor-tunities,i.e.the set of channels it can transmit on without interfering with primary users(a set of“legacy”users with the highest priority in spectrum access,see[2],[14]).Users also experience spectrum heterogeneity,i.e.their spectrum availability?uctuates over time and location[14]. Spectrum management coordinates users’channel usage to prevent con?icts,while promoting utilization and fairness. Existing work in this context took a collaborative approach, where secondary users negotiated spectrum with neighbors in order to maximize system utility,as de?ned by optimization objectives such as fairness and utilization[3],[10],[14].The work in[10],[14]reduces the problem to a variant of graph coloring problem and proposes a labelling scheme to prioritize users in channel assignment.In[3],users negotiate spectrum assignment within local self-organized groups.Spectrum man-agement is also responsible for coordinating channel usage at each pair of transmitter and receiver to ensure successful data reception[13].
Existing work on spectrum management mainly focuses on single-hop transmissions.For large scale networks such as sensor or ad hoc networks,devices’s transmission range is con-strained by their transmission power.When destination user is out of the transmission range of source user,packets are routed to the destination by users in between-i.e.through multi-hop transmissions.To utilize spectrum ef?ciently in multi-hop transmissions,we need to jointly consider route selection and spectrum management.That problem is addressed in this paper.
III.S YSTEM M ODEL
A.Assumptions
We?rst describe our assumptions.We assume that each user keeps track a list of channels available for transmission.Each user is equipped with a single half-duplex radio and hence can only transmit or receive from one channel at a time.This assumption is consistent with the current implementations of IEEE802.11end-devices.While having multi-radio interfaces expands communication capability,it may heavily stress en-ergy resources of constrained networks such as sensor and ad hoc devices.This type of network in general consists of many low-cost low-power devices equipped with a single radio. We further assume that each radio is frequency-agile:it can fast switch across channels.Considering that channel switch consumes extra power,we restrict the frequency of switch by using a frame based transmission format and limiting the number of switch in each frame.
We assume each user transmits using a prede?ned combina-tion of operating parameters(power,modulation,etc.)Power control can also be jointly considered in spectrum management and route selection.We disable this feature since it introduces extra complexity in characterizing interference condition and performing system optimization.This leads to pseudo-static interference environment.We also assume that each channel has similar average throughput.
https://www.doczj.com/doc/7919035504.html,work Architecture
In our work,each secondary device has the following com-ponents.First a spectrum manager monitors spectrum usage in the neighborhood and identi?es available spectrum.For single-hop transmissions,the spectrum manager can choose an appropriate channel to use.Any environment change such as user movement or traf?c variation might trigger a spectrum adjustment.Each device also has a frequency agile radio module which recon?gures RF to switch to the newly selected channel,and uses the appropriate protocol and modulation on each channel.For multi-hop transmissions,modi?cations are required at higher layers to respond to dynamically changing spectrum availability.In particular,we introduce a spectrum aware routing protocol that adapts route selection to spectrum ?uctuations.The design of this spectrum aware routing pro-tocol depends on the interaction between route selection and spectrum management.
While the general route selection problem has been well studied for wireless networks,particularly for multi-channel multi-hop ad hoc networks,it is possible to apply some existing solutions to the new problem.However,existing work on general route selection does not consider the availability of multiple spectrum opportunities and?uctuations in such opportunities[7].Existing work on route selection with multi-channels assumes that each device is equipped with multi-ple radio interfaces and hence can transmit and receive on different channels concurrently[8],[1],[11].The work in [12]assumes single-radio devices but restricts the selection to one channel on each route.This restrictive assumption tradeoffs performance with design complexity.Overall,to deploy dynamic spectrum systems with single radio devices, new solutions are necessary to exploit spectrum diversity. For multi-hop transmissions,it is important to examine the interaction between route selection and spectrum management. IV.R OUTE S ELECTION AND S PECTRUM M ANAGEMENT The interaction between route selection and spectrum man-agement speci?es tradeoff between performance and signaling complexity,as well as functionality carried out by network and MAC layers.The design is complicated by the requirement of handling spectrum?uctuations and other network dynamics. In this section,we present two design methodologies.
A.Decoupled Route Selection and Spectrum Management We start from a simple approach where route selection and spectrum management on each device are carried out independently by network layer and MAC layer,respectively. An example implementation of this approach would be Route selection-The source node invokes path discovery to collect information of the nodes and selects the path using a performance metric.For example,the well-known shortest-hop routing selects the path with the least number of hops. Spectrum management-The nodes on the selected path invoke MAC coordination protocol,such as HDMAC[13]to schedule packet transmissions.In particular,each node pair coordinates time and spectrum/channel for each transmission.
The decoupled design offers a simple,modular solution to the problem of managing spectrum for multi-hop transmis-sions.One can integrate different routing schemes with MAC spectrum management schemes.It also enables quick adap-tation to network dynamics,which is essential for dynamic spectrum systems where users experience location-dependent and time-varying channel availabilities.Spectrum?uctuations can be quickly“absorbed”by spectrum management,and thus become transparent to route selection.On the other hand,the decoupled design faces several performance tradeoffs.First, optimization in spectrum management focuses on single-hop traf?cs,and thus does not address end-to-end performance. Second,it is dif?cult to predict link quality since links switch between channels frequently.This could potentially reduce the reliability of route selection.
Fig1illustrates the structure of the decoupled design. There is minimum interaction between route selection and spectrum management.To perform route selection,each source node relies on link connectivity information1to construct route.The source node can re?ne route selection by collecting information on traf?c loading and link delay,and integrating it into route selection metric[4].However,the decision is independent of how single-hop transmissions are performed, in particular,the channel used.
The task of scheduling channel for single-hop transmissions, namely spectrum management,is carried out by MAC layer. The broadcast nature of radio transmissions makes links in 1Note that in dynamic spectrum networks,two nodes can connect if they are within transmission distance and have at least one available channel in common.
close proximity interfere with each other if using the same channel.In dynamic spectrum systems,each user observes multiple available channels.By assigning interfering links with different channels,one can exploit spectrum diversity to reduce interference and improve performance.The design of spectrum management is further complicated by the single-radio con?guration.In particular,transmitter and receiver need to coordinate with each other to synchronize their channel usage.A coordination protocol is essential to enable frequent handshaking without exhausting communication resources. Section V presents the detailed implementation.
Decoupled Design Collaborative Design
Fig.1.System Design.
B.Collaborative Route and Spectrum Selection
The decoupled design distributes tasks onto MAC and network layers.While simple and modular,it can not address end-to-end optimization which is essential for multi-hop trans-missions.To address end-to-end optimization,collaboration between route selection and spectrum management is neces-sary.Next,we propose a collaborative design.
In this design,some tasks of spectrum management are integrated into route selection.In particular,each source node makes decision on both route and channel selection-the decision include not only the selected packet route,but also the channel to be used by each link on the route,and a time schedule of the channel usage(see Fig1).The objective of time-scheduling the channel usage is to approach a con?ict free channel usage.This scheduling allows for explicit and guaranteed throughput provisioning and control over packet https://www.doczj.com/doc/7919035504.html,pared to random access and coordinated channel access[13],such direct in?uence provides quality of service guarantees in an ad hoc network,especially for real-time applications.The tight control and performance enforcement allows accurate prediction of link performance,and improves accuracy and reliability of route selection.However,the trade-off is additional complexity and communication overhead.In addition,route selection is sensitive to spectrum?uctuations -any spectrum change triggers a new route and channel assignment process.
An example implementation of this approach would be Route and channel selection-Each source node uses shortest-hop based DSR routing to?nd candidate paths.It also sched-ules a time and channel usage for each hop.The information is broadcast to all the users on the path.A detailed algorithm on combined route and channel selection will be included in Section V.
Spectrum management-The nodes along the path follow the time and channel schedule to communicate with each other. There is no additional coordination except those required for time synchronization among users.
V.I MPLEMENTATIONS
We have described general methodologies of the decoupled and collaborative design.In this section,we present a detailed implementation of both approaches.
A.Decoupled Design
The decoupled design can be implemented through integrat-ing existing algorithms on routing and spectrum management. These algorithms are executed by individual nodes assuming no central management.For route discovery and selection,we use DSR[7]with the shortest hop metric.One interaction between network and MAC layers is that MAC layer needs to provide a broadcast mode for route discovery messages.
For spectrum management,we use the MAC coordination protocol in[12],[13]to select channel and schedule trans-missions.Time is divided into super-frames,each consisting of a beacon broadcast(BEACON),a coordination window (CHWIN)and a data transmission period(DATA).The single radio interface limits the device to accessing one channel at a time.Hence,the protocol uses a dedicated control win-dow CHWIN to disseminate coordination information.During CHWIN,users switch to the common control channel(e.g. channel0)to solicit transmissions and negotiate the channel for data transmissions.The coordination messages are sent during CHWIN following the CSMA/CA protocol.Each user records the number of successful negotiations on each channel by eavesdropping on coordination messages,and selects the channel with the minimum number of requests or the least traf?c load.Detailed protocol design can be found from[13]. At the beginning of DATA,users switch to the selected data channel to send/receive data packets.Note that opera-tions related to transmissions during DATA re?ect the normal operation in a single channel https://www.doczj.com/doc/7919035504.html,ers can request and con?rm packet transmissions using RTS/CTS https://www.doczj.com/doc/7919035504.html,-pared to a single-channel system,this system has the advantage of distributing data traf?c(including RTS/CTS traf?c)into multiple data channels.It should be noted that coordination is performed on the?y and not provisioned in advance.
B.Collaborative route and channel selection
The collaboration between MAC and network layer is achieved through a hierarchical route and channel selection process.A source node?nds candidate routes through stan-dard route discovery procedures,and collects information on link connectivity and quality.For each candidate route,the algorithm?nds all feasible channel assignment combinations and estimates the end-to-end throughput performance for each combination(Algorithm B).It selects the route and channel
assignment that results in the best throughput,and schedules a con?ict free channel usage for this route(Algorithm A).The source node then broadcasts the decision to all the nodes on the route.In order to predict link quality,users in close proximity should exchange their channel assignment and time schedule. For large scale networks,it is bene?cial to employ a decen-tralized architecture where each source node makes decision independently or through collaboration.While this is our ulti-mate goal,in this paper we restrict ourselves to a centralized architecture when deriving the route and channel decision. That is,both Algorithm A and B are executed by a central “gene”who has the knowledge of global network topology and large computing power.The purpose of using centralized algorithms is to obtain a upper-bound on system performance that a collaborative design can achieve.This result,when compared to that of the decoupled design,offers insights for selecting design methodologies.We are currently researching on a distributed implementation of the collaborative design. Next we will describe these algorithms in detail.
We start by modelling the network using a con?ict graph G.In particular,each single-hop link maps to a vertex in the graph.An edge exists between two vertices if the cor-responding links can not be active concurrently.Since each user is equipped with a half-duplex radio,two links sharing a common node con?ict with each other,and will have an edge in between.In addition,links in close proximity will interfere with each other if they are assigned with the same channel. These links are connected with edges.
When routes are selected and channel usage on each route is planned,a con?ict-free time and channel scheduling is required for users to communicate data https://www.doczj.com/doc/7919035504.html,ing con?ict graph,we reduce this problem into a variant of maximum independent set problem.The scheduling is generated by a re-cursive process that?nds the weighted maximum independent set of the graph.
Algorithm A:con?ict-free scheduling
1)n=1
2)Each vertex is assigned with a weight that is equal to
its degree(the number of edges it is associated with).
3)Find the weighted maximum independent set of the
con?ict graph[5],namely IS(n).
4)Delete the vertices of IS(n)and the associated edges
from the con?ict graph
5)If the con?ict graph is empty,stop;otherwise set n=
n+1,go to(2).
Given the independent set,we can schedule transmissions by dividing time into?x-length frames.During frame i,all the links in IS(i)can transmit since they will not interfere with each other.Hence,the number of time frames required to carry packets from sources to destinations is equal to the number of independent sets,i.e.n from Algorithm A.n is also referred to as the chromatic number of the con?ict graph. It is easy to show that the aggregated system throughput is inversely proportional to n.This throughput estimation can be integrated into Algorithm B to select the best route and channel combination for given network?ows.Algorithm B:route and channel selection
1)Find all possible candidate routes for existing?ows.
2)Find all possible channel assignments for each route.
3)For each route/channel combination,execute Algorithm
A to derive n.
4)Find the route/channel selection that minimizes n,and
the corresponding con?ict-free scheduling(obtained by Algorithm A).
Finding n and the maximum independent set requires a set of complex computations.Given that the scale of possible route and channel combinations increases exponentially with the number of nodes,this approach is not computationally feasible.It has been shown that a graph’s maximum clique number provides a low-complexity approximation to the chro-matic number.Hence,we propose an Algorithm C to replace step(3)of Algorithm B.
Algorithm C-estimating the maximum clique number
1)For each vertex u,delete all the vertices that are not
connected with it.
2)Construct a vertex set,V E.Set V E={u}.
3)Continue to add the remaining vertices to V E,starting
from the vertex with the largest degree.Repeat this step until all the vertices in V E become fully-connected. 4)C(u)=|V E|.
The maximum clique number is derived as max u∈G C(u).
VI.E XPERIMENTAL R ESULTS
We conduct experimental simulations to quantify the per-formance of route selection and spectrum management.We compare the aggregated network throughput of the decoupled design to that of the collaborative design assuming off-line time and channel scheduling.We also vary the number of available channels per device to examine the bene?ts of spectrum diversity.
We assume a mobile ad hoc network by placing users on an area,and use a binary interference metric-two users con?ict if they are within distance of550,two users can communicate if they are within distance of250.This corresponds to the protocol interference model[6],[10],which provides an approximation to the effects of interference in real wireless systems without delving into complex detection and decoding algorithms.
We extend NS-2simulations with CMU wireless extensions to implement the MAC coordination protocol that allows user to negotiate channel usage[13].We only simulate CBR traf?c to evaluate system performance without impacts from transport protocols,and assume that each source user is backlogged. Each frame is100ms and each CHWIN is of15ms.For the collaborative design,the CHWIN is set to0ms.Each user can only adjust its channel usage(i.e.switch from one to another) at the beginning of each frame.There is a5ms switch delay during which no transmission or reception are performed.Each single-hop link can transmit at11Mbps.
We start from a few sample topologies in Fig.2.Table I summarizes the aggregated system throughput using different design.For easy notation,we refer to the decoupled design
TABLE I
P ERFORMANCE OF DECOUPLED AND COLLABORATIVE DESIGN
(AGGREGATED SYSTEM THROUGHPUT (M BPS ))
Topo DC JNT DC JNT DC
JNT (1CH)(1CH)(2CH)(2CH)(3CH)(3CH)
I 0.750.930.68 1.770.82 2.52
II 0.790.95 1.13 1.77 1.17 2.52III 0.740.95 1.08 1.76 1.39
1.76IV 0.720.95 1.35 1.80 1.81
2.71
(I)Fig.2.Fixed topologies
as “DC”and the collaborative design as “JNT”.We examine the performance assuming 1,2and 3channels are available at each device,referred to as “1CH”,“2CH”and “3CH”,respectively.Results con?rm that increasing the number of available channels,i.e.spectrum diversity,leads to signi?cant performance improvement.With 2channels,the decoupled design achieves 45%gain compared to the same design using 1channel,and 15%gain compared to the collaborative design using 1channel.With 3channels,the gain improves to 145%and 86%,respectively.
We observe that there is still a noticeable amount of difference between the decoupled design and the collaborative design.Since the results of collaborative design are obtained assuming a centralized planning with global knowledge,the corresponding distributed implementation might lead to perfor-mance degradation.We are currently investigating distributed implementations of the collaborative design.
We also examine the performance of the collaborative design in a random network where 50nodes are randomly deployed in an area.For each simulation,we randomly choose 10pairs and route CBR traf?cs between each.Figure 3illustrates the aggregated system throughput under different node deployments.We observe that the collaboration design leads to linear throughput growth with the number of available channels.This further illustrates the importance of collabora-tion between network and MAC layers.
VII.C ONCLUSION
In this paper,we consider the problem of managing spec-trum for multi-hop wireless transmissions.We focus on the interaction between spectrum management and route selection which are in general decoupled and independently executed by MAC and network layers,respectively.We propose a col-laborative design,where route selection also includes channels
A g g r e g a t e d T h r o u g h p u t (M b p s )
Number of Channels
Fig.3.Aggregated Throughput under Random Topologies.
to be used on each hop and the time schedule of the channel usage.Experimental results show that the collaborative design,if well-provisioned,provides signi?cant improvement in end-to-end performance.
R EFERENCES
[1]A DYA ,A.,B AHL ,P.,P ADHYE ,J.,W OLMAN ,A.,,AND Z HOU ,L.
A multi-radio uni?cation protocol for ieee802.11wireless networks.In IEEE international conference on broadband networks (Broadnets)(2004).
[2]B ERGER ,R.J.Open spectrum:a path to ubiquitous connectivity.ACM
Queue 1,3(May 2003).
[3]C AO ,L.,AND Z HENG ,H.Distributed spectrum allocation via local
bargaining.In IEEE SECON (Santa Clara,CA,Sept.2005).
[4]D RAVES ,R.,P ADHYE ,J.,AND Z ILL ,B.A high-throughput path metric
for multi-hop wireless routing.In Proc.ACM SIGCOMM (Aug.2004).
[5]H ALLD ′ORSSON ,M.M.Approximations of independent sets in graphs.
In proceedings of APPROX ’98:Proceedings of the International Work-shop on Approximation Algorithms for Combinatorial Optimization,LNCS 1444(1998),Springer-Verlag,pp.1–13.
[6]J AIN ,K.,P ADHYE ,J.,V.N.P ADMANABHA ,AND Q IU ,L.Impact of
interference on multi-hop wireless network performance.In Proc.ACM Mobicom (2003).
[7]J OHNSON ,D.B.,AND M ALZE ,D.A.Mobile Computing .Kluwer
Academic Publishers,1996,ch.Dynamic Source Routing in Ad-Hoc Wireless Networks.
[8]K YASANUR ,P.,AND V AIDYA ,N.H.Routing in multi-channel multi-interface ad hoc wireless networks.Tech.rep.,UIUC,Dec.2004.
[9]M C H ENRY ,M.Spectrum white space measurements.New America
Foundation Broadband Forum (June 2003).
[10]P ENG ,C.,Z HENG ,H.,AND Z HAO ,B.Y.Utilassignmentd fairness
in spectrum assignemnt for opportunistic spectrum access.Mobile Networks and Applications (MONET)(2006).to appear.
[11]R ANIWALA ,A.,G OPALAN ,K.,AND CKER C HIUEH ,T.Centralized
channel assignment and routing algorithms for multi-channel wireless mesh networks,2004.
[12]S O ,J.,AND V AIDYA ,N.A routing protocol for utilizing multiple
channels in multi-hop wireless networks with a single transceiver.Tech.rep.,UIUC,Oct.2004.
[13]Z HAO ,J.,Z HENG ,H.,AND Y ANG ,G.Distributed coordination in
dynamic spectrum allocation networks.Accepted by Dyspan,Baltimore,Nov.2005.
[14]Z HENG ,H.,AND P ENG ,C.Collaboration and fairness in opportunistic
spectrum access.In Proc.40th annual IEEE International Conference on Communications (June 2005).
STC89C52是一种带8K字节闪烁可编程可檫除只读存储器（FPEROM-Flash Programable and Erasable Read Only Memory ）的低电压，高性能COMOS8的微处理器，俗称单片机。该器件采用ATMEL 搞密度非易失存储器制造技术制造，与工业标准的MCS-51指令集和输出管脚相兼容。 单片机总控制电路如下图4—1： 图4—1单片机总控制电路 1.时钟电路 STC89C52内部有一个用于构成振荡器的高增益反相放大器，引
脚RXD和TXD分别是此放大器的输入端和输出端。时钟可以由内部方式产生或外部方式产生。内部方式的时钟电路如图4—2(a) 所示，在RXD和TXD引脚上外接定时元件，内部振荡器就产生自激振荡。定时元件通常采用石英晶体和电容组成的并联谐振回路。晶体振荡频率可以在1.2～12MHz之间选择，电容值在5～30pF之间选择，电容值的大小可对频率起微调的作用。 外部方式的时钟电路如图4—2（b）所示，RXD接地，TXD接外部振荡器。对外部振荡信号无特殊要求，只要求保证脉冲宽度，一般采用频率低于12MHz的方波信号。片内时钟发生器把振荡频率两分频，产生一个两相时钟P1和P2，供单片机使用。 示，RXD接地，TXD接外部振荡器。对外部振荡信号无特殊要求，只要求保证脉冲宽度，一般采用频率低于12MHz的方波信号。片内时钟发生器把振荡频率两分频，产生一个两相时钟P1和P2，供单片机使用。 RXD接地，TXD接外部振荡器。对外部振荡信号无特殊要求，只要求保证脉冲宽度，一般采用频率低于12MHz的方波信号。片内时钟发生器把振荡频率两分频，产生一个两相时钟P1和P2，供单片机使用。
独立主格篇 独立主格，首先它是一个“格”，而不是一个“句子”。在英语中任何一个句子都要有主谓结构，而在这个结构中，没有真正的主语和谓语动词，但又在逻辑上构成主谓或主表关系。独立主格结构主要用于描绘性文字中，其作用相当于一个状语从句，常用来表示时间、原因、条件、行为方式或伴随情况等。除名词/代词+名词、形容词、副词、非谓语动词及介词短语外，另有with或without短语可做独立主格，其中with可省略而without不可以。*注：独立主格结构一般放在句首，表示原因时还可放在句末；表伴随状况或补充说明时，相当于一个并列句，通常放于句末。 一、独立主格结构： 1. 名词/代词+形容词 He sat in the front row, his mouth half open. Close to the bank I saw deep pools, the water blue like the sky. 靠近岸时，我看见几汪深池塘，池水碧似蓝天。 2. 名词/代词+现在分词 Winter coming, it gets colder and colder. The rain having stopped, he went out for a walk.
The question having been settled, we wound up the meeting. 也可以The question settled, we wound up the meeting. 但含义稍有差异。前者强调了动作的先后。 We redoubled our efforts, each man working like two. 我们加倍努力，一个人干两个人的活。 3. 名词/代词+过去分词 The job finished, we went home. More time given, we should have done the job much better. *当表人体部位的词做逻辑主语时，不及物动词用现在分词，及物动词用过去分词。 He lay there, his teeth set, his hands clenched, his eyes looking straight up. 他躺在那儿，牙关紧闭，双拳紧握，两眼直视上方。 4. 名词/代词+不定式 We shall assemble at ten forty-five, the procession to start moving at precisely eleven. We divided the work, he to clean the windows and I to sweep the floor.
struggle的用法和短语例句 【篇一】struggle的用法 struggle的用法1：struggle的基本意思是“奋斗”,即为实现某一目的而尽力做某事。常指遇到有力的反抗而在逆境中拼搏或努力从 束缚中解脱出来。有时含有“挣扎”的意味。 struggle的用法2：struggle也可表示“(与某人)争斗,搏斗,打斗”,还可表示“艰难地(朝某方向)行进”。 struggle的用法3：struggle是不及物动词,与介词against连用,表示“同与之对立或对抗的人或物实行斗争”; 与介词for连用,表示“为…而斗争”。 struggle的用法4：struggle可接动词不定式作目的状语。 struggle的用法5：struggle的基本意思是“斗争”,表示抽象的行为,用作不可数名词; 表示具体的“打斗,搏斗,战斗,斗争”时,可用作可数名词。 struggle的用法6：struggle作“努力,奋斗”解时,一般用单数形式。 【篇二】struggle的常用短语 struggle against (v.+prep.) struggle along1 (v.+adv.) struggle along2 (v.+prep.) struggle for (v.+prep.) struggle in (v.+prep.) struggle on (v.+adv.)
struggle out (v.+adv.) struggle with (v.+prep.) 【篇三】struggle的用法例句 1. It's a constant struggle to try to keep them up to par. 要让他们达标，需要持续努力。 2. Curiously, the struggle to survive has greatly improved her health. 奇怪的是，她拼命求生的抗争使得她的健康状况大有好转。 3. He grandly declared that "international politics is a struggle for power". 他一本正经地宣称“国际政治是一场权力之争”。 4. This age-old struggle for control had led to untold bloody wars. 这场由来已久的对控制权的争夺已经引发了无数流血的战争。 5. It is only a hobby, not a life or death struggle. 这仅仅一项爱好，不需要拼死拼活地努力。 6. There is a ceaseless struggle from noon to night. 从中午到夜晚，争斗没有停歇。 7. He is currently locked in a power struggle with his Prime Minister. 他当前陷入了一场同的权力之争当中。 8. He praised her role in the struggle against apartheid.
be动词，情态动词，助动词do/does的用法区别及练习 助动词，顾名思义就是帮助动词完成疑问及否定的，本身没有什么含义。主要的助动词有be,do,will,have等，其用法详述如下： 一、⑴由连系动词am，is，are构成的句子：变一般疑问句时把am，is，are提到句子的前面，句尾用问号即可。变否定句时直接在am，is，are后面加not即可。例如： 肯定句:He is a student. 一般疑问句: Is he a student? 否定句: He is not a student. 画线提问: 对he提问: Who is a student? 对a student 提问: What is he? (2)was 是am,is的过去式，were是are的过去式，若句子中有以上两词时，变疑问句及否定句方法与（1）相同。 二、(1) 由情态动词can, may,will ，shall等构成的句子: 变一般疑问句时把can, may,will ，shall提到句子的前面,句尾用问号即可.变否定句时直接在can,may,后面加not即可. 例如: 肯定句: She can swim. 一般疑问句: Can she swim? 否定句: She can not swim. 画线提问: 对she提问: Who can swim? 对swim提问: What can she do? （2）could,might,would,should是can,may,will,shall的过去式，若句子中有以上两词时，变疑问句及否定句方法与（1）相同。 三、（1）由行为动词构成的句子: 需要加助词do或does. 变一般疑问句时把do/does放在句子前面. 变否定句时把don’t/doesn’t放在动词的前面。要注意观察动词的形式并对号入座。一般疑问句和否定句的动词三单式都要变回原型。 play-----do plays-----does 例如: 肯定句: They play football after school. He plays football after school. 一般疑问句: Do they play football after school? Does he play football after school? 否定句: They don't (do not) play football after school. He doesn’t’ play football after school. 画线提问: 对they/he提问: Who plays football after school? 对play football提问: What do they do after school? What does he do after school? 对after school提问: When do they play football? When does he play football? （2）did是do和did的过去式，变一般疑问句时把did放在句子前面. 变否定句时把didn’t 放在动词的前面, 要注意观察动词的形式并对号入座。一般疑问句和否定句的动词都要变回原型。
英语中的五种基本句型结构 一、句型1：Subject (主语) ＋Verb (谓语) 这种句型中的动词大多是不及物动词，所谓不及物动词，就是这种动词后不可以直接接宾语。常见的动词如：work, sing, swim, fish, jump, arrive, come, die, disappear, cry, happen等。如： 1) Li Ming works very hard.李明学习很努力。 2) The accident happened yesterday afternoon.事故是昨天下午发生的。 3）Spring is coming. 4) We have lived in the city for ten years. 二、句型2：Subject (主语) ＋Link. V(系动词) ＋Predicate(表语) 这种句型主要用来表示主语的特点、身份等。其系动词一般可分为下列两类： (1)表示状态。这样的词有：be, look, seem, smell, taste, sound, keep等。如： 1) This kind of food tastes delicious.这种食物吃起来很可口。 2) He looked worried just now.刚才他看上去有些焦急。 (2)表示变化。这类系动词有：become, turn, get, grow, go等。如： 1) Spring comes. It is getting warmer and warmer.春天到了，天气变得越来越暖和。 2) The tree has grown much taller than before.这棵树比以前长得高多了。 三、句型3：Subject(主语) ＋V erb (谓语) ＋Object (宾语) 这种句型中的动词一般为及物动词, 所谓及物动词，就是这种动词后可以直接接宾语，其宾语通常由名词、代词、动词不定式、动名词或从句等来充当。例： 1) He took his bag and left.（名词）他拿着书包离开了。 2) Li Lei always helps me when I have difficulties. (代词)当我遇到困难时，李雷总能给我帮助。 3) She plans to travel in the coming May Day.（不定式）她打算在即将到来的“五一”外出旅游。 4) I don’t know what I should do next. (从句)我不知道下一步该干什么。 注意：英语中的许多动词既是及物动词，又是不及物动词。 四、句型4：Subject(主语)＋Verb(谓语)＋Indirect object(间接宾语)＋Direct object (直接宾语) 这种句型中，直接宾语为主要宾语，表示动作是对谁做的或为谁做的，在句中不可或缺，常常由表示“物”的名词来充当；间接宾语也被称之为第二宾语，去掉之后，对整个句子的影响不大，多由指“人”的名词或代词承担。引导这类双宾语的常见动词有：buy, pass, lend, give, tell, teach, show, bring, send等。如： 1) Her father bought her a dictionary as a birthday present.她爸爸给她买了一本词典作为生日礼物。 2）The old man always tells the children stories about the heroes in the Long March. 老人经常给孩子们讲述长征途中那些英雄的故事。上述句子还可以表达为： 1）Her father bought a dictionary for her as a birthday present. 2）The old man always tells stories about the heroes to the children in the Long March. 五、句型5：Subject(主语)＋Verb (动词)＋Object (宾语)＋Complement(补语) 这种句型中的“宾语＋补语”统称为“复合宾语”。宾语补足语的主要作用或者是补充、说明宾语的特点、身份等；或者表示让宾语去完成的动作等。担任补语的常常是名词、形容词、副词、介词短语、分词、动词不定式等。如： 1）You should keep the room clean and tidy. 你应该让屋子保持干净整洁。（形容词） 2) We made him our monitor.(名词)我们选他当班长。 3) His father told him not to play in the street.(不定式)他父亲告诉他不要在街上玩。
· struggle · v. ['str?gl] ( struggles; struggled; struggling ) · · 双解释义 · vi.斗争,奋斗,努力fight; try to overcome difficulties, etc.; make great efforts · 基本要点 ? 1.struggle的基本意思是“奋斗”,即为实现某一目的而尽力做某事。常指遇到有 力的反抗而在逆境中拼搏或努力从束缚中解脱出来。有时含有“挣扎”的意味。 2.struggle也可表示“(与某人)争斗,搏斗,打斗”,还可表示“艰难地(朝某方向) 行进”。 3.struggle是不及物动词,与介词against连用,表示“同与之对立或对抗的人或物 进行斗争”; 与介词for连用,表示“为…而斗争”。 4.struggle可接动词不定式作目的状语。 ? · 词汇搭配 ? ?struggle bravely 勇敢地斗争 ?struggle desperately 拼命地斗争,绝望地挣扎 ?struggle furiously 激烈地搏斗 ?struggle successfully 成功地作斗争 ?struggle wildly 疯狂地挣扎 ?struggle on 勉强支持下去,竭力维持下去 ? ?struggle against 与…作斗争
?struggle against difficulties 同困难斗争 ?struggle against tyranny 与暴政作斗争 ?struggle for 为…而斗争 ?struggle for a living 为生计而挣扎 ?struggle for freedom 为自由而斗争 ?struggle for independence 为独立而奋斗 ?struggle for recognition 为获得承认而奋斗 ?struggle in sb's arms (小孩)在母亲怀里挣扎 ?struggle through the snowstorm 冒着暴风雪行进 ?struggle to one's feet 挣扎着站起来 ?struggle with 与…作斗争 ? · 常用短语 ? struggle against(v.+prep.) 为反对…而斗争 oppose sth with difficulty ▲struggle against sb/sth He struggled against those who opposed his plan.他与那些反对他计划的人进 行了斗争。 In Chinese history the peasants kept struggling against the rule of the feudal class.在中国历史上,农民不断地与封建阶级的统治作斗争。 What is the use of trying to struggle against the system?力图反对这种制度有什么用? The swimmer struggled against the tide.游泳者逆流而上。 We must struggle against this prejudice for a more tolerant attitude to our beliefs.我们必须反对这种偏见,以争得对我们的信仰采取更为宽容的态度。 The workers have been struggling against bad conditions for too long.很久以来,工人们就在为反对恶劣的工作条件而进行斗争。
助动词在英语学习当中作了解，不需要重点把握，但是助动词也是很好理解的，希望回答能够帮到你。 1.助动词：Be 助动词不能作述语动词,要与本动词一起构成动词片语,表示时态、语态等。 1. BE作为本动词表示状态或客观存在等意思。 Your house is bigger and nicer than mine. 你的房子比我的又大又好。 2. BE+不定词连用,表示约定、义务、命令等未来的动作或状态。 I am to go abroad on business tomorrow. 明天我要去外国出公差。 表示未来的安排。 The meeting is to be held as scheduled. 会议将按原计划召开。 表示计划好的安排。 You are not to bark at my friend. 你不许对我的朋友叫。 表示命令或要求。 3. BE+现在分词构成各种进行时态。 Who are you talking to? 你在和谁说话？ I am talking to my dog. 我在和我的狗说话。 4. BE+过去分词构成被动语态。 That means I will be promoted as scheduled. 这就意味着我将要按原计划得到提升。 2. dare和used to 作为情态助动词的dare一般只能用于疑问句或否定句中,dare+原形动词表示敢。
I dare not say it is ugly. 我不敢说它丑。 How dare you say so? 你怎么敢这么说？ dare也可以作本动词,用于肯定句,后面要接带to的不定词;主词若是第三人称单数,简单现在式时,dare要变为dares。 You, you dare to talk to me like this! 你、你竟敢这样和我讲话！ used to+原形动词表示过去的习惯或状态,而现在已经不存在了。 You're not what you used to be. 你不是以前的你了。 used to构成疑问句时有两种形式。即used + 主词+ to + 其他成份？；Did + 主词+ use to+ 其他成份。 How used I to be? 我以前什么样？ How did you use to be? 你以前什么样？ used to构成否定句时有两种形式，即used not to和didn't use to。 You didn't use to say things like this. You used not to say things like this. 你以前不会说这样的话的。 be used to表示习惯于,其中used是形容词,to是介系词,后接名词、代名词或动名词等,可用于不同的时态。 You're used to hearing words of praise. 你是听好话听惯了。 3. 助动词：Do 1. DO作为助动词时的时态、人称和数的变化与它作为本动词相同,有do, does, did三种形式。 Yes, it seems he doesn't really want to have a haircut.
STC89C52RC单片机介绍 STC89C52RC单片机是宏晶科技推出的新一代高速/低功耗/超强抗干扰的单片机，指令代码完全兼容传统8051单片机，12时钟/机器周期和6时钟/机器周期可以任意选择。 主要特性如下： 增强型8051单片机，6时钟/机器周期和12时钟/机器周期可以任意选择，指令代码完全兼容传统8051. 工作电压：～（5V单片机）/～（3V单片机） 工作频率范围：0～40MHz，相当于普通8051的0～80MHz，实际工作频率可达48MHz 用户应用程序空间为8K字节 片上集成512字节RAM 通用I/O口（32个），复位后为：P1/P2/P3/P4是准双向口/弱上拉，P0口是漏极开路输出，作为总线扩展用时，不用加上拉电阻，作为 I/O口用时，需加上拉电阻。 ISP（在系统可编程）/IAP（在应用可编程），无需专用编程器，无需专用仿真器，可通过串口（RxD/,TxD/）直接下载用户程序，数秒 即可完成一片 具有EEPROM功能 具有看门狗功能 共3个16位定时器/计数器。即定时器T0、T1、T2 外部中断4路，下降沿中断或低电平触发电路，Power Down模式可由外部中断低电平触发中断方式唤醒 通用异步串行口（UART），还可用定时器软件实现多个UART 工作温度范围：-40～+85℃（工业级）/0～75℃（商业级） PDIP封装 STC89C52RC单片机的工作模式 掉电模式：典型功耗<μA,可由外部中断唤醒，中断返回后，继续执行
原程序 空闲模式：典型功耗2mA 正常工作模式：典型功耗4Ma～7mA 掉电模式可由外部中断唤醒，适用于水表、气表等电池供电系统及便携设备 STC89C52RC引脚图 STC89C52RC引脚功能说明 VCC（40引脚）：电源电压 VSS（20引脚）：接地 P0端口（～，39～32引脚）：P0口是一个漏极开路的8位双向I/O口。作为输出端口，每个引脚能驱动8个TTL负载，对端口P0写入“1”时，可以作为高阻抗输入。
独立主格结构 独立主格结构（Absolute Structure）是由名词或代词加上分词等构成的一种独立结构，用于修饰整个句子。独立主格结构中的名词或代词与其后的分词等构成逻辑上的主谓关系。这种结构与主句不发生句法上的联系，它的位置相当灵活，可置于主句前、主句末或主句中，常由逗号将其与主句分开。需特别注意的是，独立主格结构与主句之间不能使用任何连接词。 二、独立主格结构基本构成形式 名词(代词)+现在分词（过去分词；形容词；副词；不定式；名词；介词短语） 1. 名词(代词)+现在分词 Night enshrouding the earth, nobody could make out what the dark mass was from a distance． 黑夜笼罩大地，谁也看不清远处黑压压的一片是什么东西。 There being no bus, we had to walk home. 由于没有公共汽车，我们只好走回家。 2. 名词(代词)+过去分词 The workers worked still harder, their living conditions greatly improved. 由于工人们的生活条件大大提高，他们工作得更起劲了。 He was listening attentively in class, his eyes fixed on the blackboard. 他上课专心听讲，眼睛紧盯着黑板。 3. 名词(代词)+不定式 在“名词/代词+动词不定式”结构中，动词不定式和它前面的名词或代词如果存在着逻辑上的主谓关系，动词不定式则用主动的形式；如果是动宾关系，则用被动形式。 The four of us agreed on a division of labor, each to translate a quarter of the book. 我们四人同意分工干，每人翻译全书的四分之一。 Many trees, flowers, and grass to be planted, our newly-built school will look even more beautiful. 种上许多的树、花和草后，我们新建的学校看上去将更美。 4. 名词(代词)+形容词 Computers very small, we can use them widely. 电脑虽小，我们却能广泛地利用它们。 5. 名词(代词)+副词 The meeting over, our headmaster soon left the meeting room. 散会了，校长很快就离开了会议室。 The lights off, we could not go on with the work. 灯熄了，我们不能继续工作了。 6. 名词(代词)+名词 His first shot failure，he fired again． 他第一枪没击中，又打了一枪。 Two hundred people died in the accident, many of them children. 两百人死于事故，其中有许多儿童。 7. 名词(代词) +介词短语
不定式用法详解 定义：在句子中充当除了谓语以外的各种句子成分的动词形式叫做非谓语动词。 非谓语动词有三种：不定式，动名词，分词。 注：非谓语动词具有时态和语态的变化，可以同否定词not连用，构成否定形式，可以带宾语，状语，补语。非谓语动词的名词，形容词，副词的特征；可用在句中做主语，宾语，补语，状语，和定语等。 —不定式（原形动词前加to, 构成动词不定式。不定式不做谓语，属于非谓语动词） 不定式的时态，语态 一般式：不定式所表示的动作和谓语动词所表示的动作或状态同时发生或几乎同时发生或存在。但多数情况下是在谓语动词所表示的动作之后发生。 e.g :They often watch us play football. (同时) She hopes to go there again. (之后发生) 完成式：不定式所表示的动作或状态发生在谓语动词所表示动作或状态之前。 e.g : I’m glad to have seen your mother yesterday 注：1）用在intended, expected, meant, hoped. promised, wanted, planned. wished. thought, desired, was, were等后，表示过去没有实现的愿望，期待或计划的等，也用来表示先于谓语动词发生的动作或状态。 e.g: He wanted to have met (=had wanted to meet) you at the airport,but he didn’t get there in time. 他原想去机场接你（事情已经发生），但他没及时赶到那儿。（没接成） e.g: We planned to have done (had planned to do) good deeds for the poor people last month. 我们原计划上个月为贫困的人们作些好事。（没作成）。
助动词 协助主要动词构成谓语动词词组的词叫助动词（Auxiliary Verb）。被协助的动词称作主要动词（Main Verb）。构成时态和语态。助动词是语法功能词，自身没有词义，不可单独使用，它没有对应的汉译，例如：He doesn't like English. 他不喜欢英语。（does是助动词，无词义；like是主要动词，有词义) 最常用的助动词有：be, have, do, shall, will, should, would 他们表示时态，语态，构成疑问句与否定副词not合用，加强语气助动词半助动词 2 半助动词 功能介绍 在功能上介乎主动词和助动词之间的一类结构，称为半助动词。常见的半助动词有be about to, be due to, be going to, be likely to, be meant to, be obliged to, be supposed to, be willing to, have to, seem to, be unable to, be unwilling to等。......情态助动词 情态助动词1.情态助动词包括will(would), shall(should), can(could), may(might), must, need, dare, ought to, used to, had better后接原形不定词。2.情态助动词不受主语的人称和数的限制。3.两个情态助动词不能连用。中文:他将能够及时完成此事。(误)He will can finish it i...... 3 基本助动词 基本助动词基本助动词只有三个：be, do, have, 他们没有词汇意义，只有语法作用，如协助构成进行体，完成体，被动态，否定句，疑问句等。例如He is giving a lecture. 他在作报告He has made a plan. 他已经订了计划The small animals are kept in the cages. 小动物都关在笼子里。 助动词协助主要动词完成以下功用 a. 表示时态，例如： He is singing. 他在唱歌。 He has got married. 他已结婚。 b. 表示语态，例如： He was sent to England. 他被派往英国。 c. 构成疑问句，例如： Do you like college life? 你喜欢大学生活吗？ Did you study English before you came here? 你来这儿之前学过英语吗？ d. 与否定副词not合用，构成否定句，例如： I don't like him. 我不喜欢他。 e. 加强语气，例如： Do come to the party tomorrow evening. 明天晚上一定来参加晚会。 He did know that. 他的确知道那件事。 3）最常用的助动词有：be, have, do, shall, will, should, would 4 具体用法 have的用法 一、have作助动词 形式 主要变化形式：have，has，had 动名词/现在分词：having 1）have +过去分词，构成完成时态，例： He has left for London.他已去了伦敦。
STC89C52单片机用户手册 [键入作者姓名] [选取日期]
STC89C52RC单片机介绍 STC89C52RC单片机是宏晶科技推出的新一代高速/低功耗/超强抗干扰的单片机，指令代码完全兼容传统8051单片机，12时钟/机器周期和6时钟/机器周期可以任意选择。 主要特性如下： 1.增强型8051单片机，6时钟/机器周期和12时钟/机器周期可以任 意选择，指令代码完全兼容传统8051. 2.工作电压：5.5V～ 3.3V（5V单片机）/3.8V～2.0V（3V单片机） 3.工作频率范围：0～40MHz，相当于普通8051的0～80MHz，实 际工作频率可达48MHz 4.用户应用程序空间为8K字节 5.片上集成512字节RAM 6.通用I/O口（32个），复位后为：P1/P2/P3/P4是准双向口/弱上 拉，P0口是漏极开路输出，作为总线扩展用时，不用加上拉电阻， 作为I/O口用时，需加上拉电阻。 7.ISP（在系统可编程）/IAP（在应用可编程），无需专用编程器，无 需专用仿真器，可通过串口（RxD/P3.0,TxD/P3.1）直接下载用 户程序，数秒即可完成一片 8.具有EEPROM功能 9.具有看门狗功能 10.共3个16位定时器/计数器。即定时器T0、T1、T2 11.外部中断4路，下降沿中断或低电平触发电路，Power Down模式
可由外部中断低电平触发中断方式唤醒 12.通用异步串行口（UART），还可用定时器软件实现多个UART 13.工作温度范围：-40～+85℃（工业级）/0～75℃（商业级） 14.PDIP封装 STC89C52RC单片机的工作模式 ●掉电模式：典型功耗<0.1μA,可由外部中断唤醒，中断返回后，继续执行原 程序 ●空闲模式：典型功耗2mA ●正常工作模式：典型功耗4Ma～7mA ●掉电模式可由外部中断唤醒，适用于水表、气表等电池供电系统及便携设备
一、 独立主格结构是一个名词或代词（作为逻辑主语），加上一个形容词、副词、介词短语、分词、不定式等在句中作状语。它有以下三个特点： (1)结构的逻辑主语与句子的主语不同，它独立存在。 (2)名词或代词与后面的形容词、副词、介词短语、分词、不定式等存在逻辑上的主谓关系。 (3)独立主格结构一般用逗号与主句分开，但与主句之间不能使用任何连接词。 二、 英语中，的形式是：名词或代词跟形容词、副词、介词短语、非谓语动词连在一起，构成。 1.名词/ 代词+ 不定式。如： A house to be built, we must save every cent. 由于要建一座房子，我们必须节省每一分钱。 Now here is Li Lei, Wei Fang to come tomorrow. 现在李蕾来了，魏方明天到。 2.名词/ 代词+ -ing分词。如： The bus coming here soon, we should get everything ready. 汽车很快就要来了，我们应该把一切事情准备好。 Mother being ill, Li Lei was very worried. 母亲病了，李蕾非常焦急。 3.名词/ 代词+ 动词的过去分词。如： His cup broken, he used his bowl instead. 茶杯破了，他就用碗来代替。 4.名词/ 代词+ 形容词。如： The ground muddy, we should be careful. 地面泞泥，我们应该小心。 5.名词/ 代词+ 副词。如： The class over, we all went out to play. 下课后，我们都出去玩。 6. Glasses in his hand, he asked where his glasses were. 手里拿着眼镜，他问他的眼镜哪去了。 三、 在句子中作状语，表示时间、原因、条件和伴随等情况。 1.作时间状语 School over, the students went home. 放学后，学生们都回家了。 The ceremony ended, the games began. 仪式结束后，比赛开始了。 2.作条件状语 It being fine tomorrow, we’ll go boating. 如果明天天气好的话，我们就去划船。
2020年初中英语10大词类详解+用法+考点(总结详细篇) 一、词性得分类 词类又叫词性，英语单词凭据其在句子中得功用，可以分红十个大年夜类。 1.名词 noun n. student 学生 2.代词 pronoun pron. you 你 3.形容词 adjective adj. happy 雀跃得 4.副词 adverb adv. quickly 敏捷地 5.动词 verb v. cut 砍、割 6.数词 numeral num. three 三 7.冠词 article art. a 一个 8.介词 preposition prep. at 在... 9.连词 conjunction conj. and 和 10.慨叹词 interjection interj. oh 哦 前六类叫实词，后四类叫虚词。 二、名词 名词单数得规则改变
名词得格 在英语中有些名词可以加“‘s”来示意所有关系，带这种词尾得名词形式称为该名词得所有格，如：a teacher’s book。 名词所有格得规则如下： (1)单数名词词尾加“'s”，单数名词词尾没有s，也要加“'s”，如the boy‘s bag 男孩得书包，men’s room 男厕所。 (2)若名词已有单数词尾-s ，只加“'”，如：the workers’ struggle工人得斗争。 三、代词 大年夜少数代词具有名词和形容词得功用。 英语中得代词，按其意义、特征及在句中得作用分为：人称代词、物主代词、指导代词、反身代词、彼此代词、疑问代词、关系代词、毗邻代词和不定代词九种。 人称代词得用法：
指导代词 指导代词分单数（this / that）和单数（these / those）两种形式，既可作限制词又可做代词。 疑问代词 指人：who， whom， whose 指物：what 既可指人又可指物：which 四、冠词 冠词是位于名词或名词词组之前或以后，在句子里首如果对名词起限制作用得词。冠词是一种虚词。 不定冠词a (an)与数词one 同源，是“一个”得意思。 定冠词得用法 定冠词the与指导代词this，that同源，有“那（这）个”得意思。 (1)特指双方都熟悉打听得人或物：Take the medicine.把药吃了。 (2)上文提到过得人或事：He bought a house. I’ve been to the house. (3)指世上独一物二得事物：the sun， the sky， the moon， the earth（敲黑板，这个是常常会在选择题中考到得常识点） (4)单数名词连用示意一类事物，如：the dollar 美元；the fox 狐狸；
助动词（Auxiliary Verb）：协助主要动词构成谓语动词词组的词。自身没有词义，不可单独使用。 主要动词（Main Verb）：是被协助的动词，构成时态和语态。 He doesn't like English. 他不喜欢英语。（doesn't是助动词，无词义；like是主要动词，有词义) 最常用的助动词有：be, have, do, shall, will, should, would 等。 基本助动词只有三个：be, do, have 他们没有词汇意义，只有语法作用，如协助构成进行时，完成时，被动态，否定句，疑问句等。 一、be 动词的用法 既可作系动词，又可作助动词，做助动词有人称和数的变化，第一人称用am,第二人称及复数用are,第三人称及单数用is，am,is 过去式
为was, are的过去式为were,它与现在分词构成进行时态和过去分词一起构成被动语态。 a. 表示时态be+doing（现在分词）表示现在进行的动作 He is singing. 他正在唱歌。 b. 表示语态be+done(过去分词）表被动语态 He was sent to England. 他被派往英国。 c. be+to do（动词不定式）表示计划安排命令。 We are to plant trees next week. 下周我们将要去植树。 You are to explain this 。对此你要做出解释。 二、do的用法 Do主要帮助实意动词构成否定和疑问句，后跟动词原形，有时放在实意动词前起强调作用，还可代替前文出现的动词，避免重复。Do 有人称和数的变化，第一、二人称及复数用do，第三人称及单数用does，过去式为did。 1）构成一般疑问句。DO +主语+动词原形+其他 I like singing 变为疑问句为Do you like singing ? 2）do + not 构成否定句。主语+do +not +动词原形。 I do not want to be criticized.我不想挨批评。 He doesn't like to study.他不想学习。 Many students didn’t know the importance of English before.过去好多学生不知道英语的重要性. 3)构成否定祈使句。
STC89C52单片机学习开发板介绍 全套配置： 1 .全新增强STC89C5 2 1个【RAM512字节比AT89S52多256个字节FLASH8K】 2 .优质USB数据线 1条【只需此线就能完成供电、通信、烧录程序、仿真等功能，简洁方便实验，不需要USB 转串口和串口线，所有电脑都适用】 3 .八位排线 4条【最多可带4个8*8 LED点阵，从而组合玩16*16的LED点阵】 4 .单P杜邦线 8条【方便接LED点阵等】 5 .红色短路帽 19个【已装在开发箱板上面，短路帽都是各功能的接口,方便取用】 6 .实验时钟电池座及电池 1PCS 7 .DVD光盘 1张【光盘具体内容请看页面下方，光盘资料截图】 8 .全新多功能折叠箱抗压抗摔经久耐磨 1个【市场没有卖,专用保护您爱板的折叠式箱子，所有配件都可以放入】 9 .8*8（红+绿）双色点阵模块 1片【可以玩各种各样的图片和文字,两种颜色变换显示】 10.全新真彩屏SD卡集成模块 1个【请注意：不包含SD卡，需要自己另外配】 晶振【1个方便您做实验用】 12.全新高速高矩进口步进电机 1个【价格元/个】 13.全新直流电机 1个【价值元/ 个】 14.全新红外接收头 1个【价格元/ 个】 15.全新红外遥控器（送纽扣电池） 1个【价格元/个】 16.全新18B20温度检测 1个【价格元/只】 17.光敏热敏模块 1个（已经集成在板子上）【新增功能】 液晶屏 1个 配件参照图：
温馨提示：四点关键介绍，这对您今后学习51是很有帮助的） 1.板子上各模块是否独立市场上现在很多实验板，绝大部分都没有采用模块化设计，所有的元器件密 密麻麻的挤在一块小板上，各个模块之间PCB布线连接，看上去不用接排线，方便了使用者，事实上是为了降低硬件成本，难以解决各个模块之间的互相干扰，除了自带的例程之外，几乎无法再做任何扩展，更谈不上自由组合发挥了，这样对于后继的学习非常不利。几年前的实验板，基本上都是这种结构的。可见这种设计是非常过时和陈旧的，有很多弊端，即便价格再便宜也不值得选购。 HC6800是采用最新设计理念，实验板各功能模块完全独立，互不干扰，功能模块之间用排线快速连接。 一方面可以锻炼动手能力，同时可加强初学者对实验板硬件的认识，熟悉电路，快速入门；另一方面，因为各功能模块均独立设计，将来大家学习到更高级的AVR，PIC，甚至ARM的时候，都只