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1、精选优质文档-倾情为你奉上外文原文Code division multiple accessCode division multiple access (CDMA) is a used by various radio communication technologies. It should not be confused with the called , (the evolution of cdmaOne) and (the 3G standard used by carriers), which are often referred to as simply CDMA, and use
2、 CDMA as an underlying channel access method.One of the concepts in data communication is the idea of allowing several transmitters to send information simultaneously over a single communication channel. This allows several users to share a band of frequencies (see ). This concept is called . CDMA e
3、mploys technology and a special coding scheme (where each transmitter is assigned a code) to allow multiple users to be multiplexed over the same physical channel. By contrast, (TDMA) divides access by , while (FDMA) divides it by . CDMA is a form of signalling, since the modulated coded signal has
4、a much higher than the data being communicated.Steps in CDMA ModulationEach user in a CDMA system uses a different code to modulate their signal. Choosing the codes used to modulate the signal is very important in the performance of CDMA systems. The best performance will occur when there is good se
5、paration between the signal of a desired user and the signals of other users. The separation of the signals is made by the received signal with the locally generated code of the desired user. If the signal matches the desired users code then the correlation function will be high and the system can e
6、xtract that signal. If the desired users code has nothing in common with the signal the correlation should be as close to zero as possible (thus eliminating the signal); this is referred to as cross correlation. If the code is correlated with the signal at any time offset other than zero, the correl
7、ation should be as close to zero as possible. This is referred to as auto-correlation and is used to reject multi-path interference. In general, CDMA belongs to two basic categories: synchronous (orthogonal codes) and asynchronous (pseudorandom codes).Code division multiplexing (Synchronous CDMA)Syn
8、chronous CDMA exploits mathematical properties of between representing the data strings. For example, binary string 1011 is represented by the vector (1, 0, 1, 1). Vectors can be multiplied by taking their , by summing the products of their respective components (for example, if u = (a, b) and v = (
9、c, d), then their dot product uv = ac + bd). If the dot product is zero, the two vectors are said to be orthogonal to each other. Some properties of the dot product aid understanding of how works.Each user in synchronous CDMA uses a code orthogonal to the others codes to modulate their signal. An ex
10、ample of four mutually orthogonal digital signals is shown in the figure. Orthogonal codes have a cross-correlation equal to zero; in other words, they do not interfere with each other. In the case of IS-95 64 bit are used to encode the signal to separate different users. Since each of the 64 Walsh
11、codes are orthogonal to one another, the signals are channelized into 64 orthogonal signals. The following example demonstrates how each users signal can be encoded and decoded. Asynchronous CDMAWhen mobile-to-base links cannot be precisely coordinated, particularly due to the mobility of the handse
12、ts, a different approach is required. Since it is not mathematically possible to create signature sequences that are both orthogonal for arbitrarily random starting points and which make full use of the code space, unique pseudo-random or pseudo-noise (PN) sequences are used in asynchronous CDMA sys
13、tems. A PN code is a binary sequence that appears random but can be reproduced in a deterministic manner by intended receivers. These PN codes are used to encode and decode a users signal in Asynchronous CDMA in the same manner as the orthogonal codes in synchronous CDMA (shown in the example above)
14、. These PN sequences are statistically uncorrelated, and the sum of a large number of PN sequences results in multiple access interference (MAI) that is approximated by a Gaussian noise process (following the in statistics). are an example of a PN suitable for this purpose, as there is low correlati
15、on between the codes. If all of the users are received with the same power level, then the variance (e.g., the noise power) of the MAI increases in direct proportion to the number of users. In other words, unlike synchronous CDMA, the signals of other users will appear as noise to the signal of inte
16、rest and interfere slightly with the desired signal in proportion to number of users.All forms of CDMA use to allow receivers to partially discriminate against unwanted signals. Signals encoded with the specified PN sequence (code) are received, while signals with different codes (or the same code b
17、ut a different timing offset) appear as wideband noise reduced by the process gain.Since each user generates MAI, controlling the signal strength is an important issue with CDMA transmitters. A CDM (synchronous CDMA), TDMA, or FDMA receiver can in theory completely reject arbitrarily strong signals
18、using different codes, time slots or frequency channels due to the orthogonality of these systems. This is not true for Asynchronous CDMA; rejection of unwanted signals is only partial. If any or all of the unwanted signals are much stronger than the desired signal, they will overwhelm it. This lead
19、s to a general requirement in any asynchronous CDMA system to approximately match the various signal power levels as seen at the receiver. In CDMA cellular, the base station uses a fast closed-loop power control scheme to tightly control each mobiles transmit power. Efficient Practical utilization o
20、f Fixed Frequency SpectrumIn theory, CDMA, TDMA and FDMA have exactly the same spectral efficiency but practically, each has its own challenges power control in the case of CDMA, timing in the case of TDMA, and frequency generation/filtering in the case of FDMA.TDMA systems must carefully synchroniz
21、e the transmission times of all the users to ensure that they are received in the correct time slot and do not cause interference. Since this cannot be perfectly controlled in a mobile environment, each time slot must have a guard-time, which reduces the probability that users will interfere, but de
22、creases the spectral efficiency. Similarly, FDMA systems must use a guard-band between adjacent channels, due to the unpredictable of the signal spectrum because of user mobility. The guard-bands will reduce the probability that adjacent channels will interfere, but decrease the utilization of the s
23、pectrum.Flexible Allocation of ResourcesAsynchronous CDMA offers a key advantage in the flexible allocation of resources i.e. allocation of a PN codes to active users. In the case of CDM (synchronous CDMA), TDMA, and FDMA the number of simultaneous orthogonal codes, time slots and frequency slots re
24、spectively is fixed hence the capacity in terms of number of simultaneous users is limited. There are a fixed number of orthogonal codes, time slots or frequency bands that can be allocated for CDM, TDMA, and FDMA systems, which remain underutilized due to the bursty nature of telephony and packetiz
25、ed data transmissions. There is no strict limit to the number of users that can be supported in an asynchronous CDMA system, only a practical limit governed by the desired bit error probability, since the SIR (Signal to Interference Ratio) varies inversely with the number of users. In a bursty traff
26、ic environment like mobile telephony, the advantage afforded by asynchronous CDMA is that the performance (bit error rate) is allowed to fluctuate randomly, with an average value determined by the number of users times the percentage of utilization. Suppose there are 2N users that only talk half of
27、the time, then 2N users can be accommodated with the same average bit error probability as N users that talk all of the time. The key difference here is that the bit error probability for N users talking all of the time is constant, whereas it is a random quantity (with the same mean) for 2N users t
28、alking half of the time.In other words, asynchronous CDMA is ideally suited to a mobile network where large numbers of transmitters each generate a relatively small amount of traffic at irregular intervals. CDM (synchronous CDMA), TDMA, and FDMA systems cannot recover the underutilized resources inh
29、erent to bursty traffic due to the fixed number of codes, time slots or frequency channels that can be assigned to individual transmitters. For instance, if there are N time slots in a TDMA system and 2N users that talk half of the time, then half of the time there will be more than N users needing
30、to use more than N time slots. Furthermore, it would require significant overhead to continually allocate and deallocate the orthogonal code, time slot or frequency channel resources. By comparison, asynchronous CDMA transmitters simply send when they have something to say, and go off the air when t
31、hey dont, keeping the same PN signature sequence as long as they are connected to the system.Spread-spectrum characteristics of CDMAMost modulation schemes try to minimize the bandwidth of this signal since bandwidth is a limited resource. However, spread spectrum techniques use a transmission bandw
32、idth that is several orders of magnitude greater than the minimum required signal bandwidth. One of the initial reasons for doing this was military applications including guidance and communication systems. These systems were designed using spread spectrum because of its security and resistance to j
33、amming. Asynchronous CDMA has some level of privacy built in because the signal is spread using a pseudo-random code; this code makes the spread spectrum signals appear random or have noise-like properties. A receiver cannot demodulate this transmission without knowledge of the pseudo-random sequenc
34、e used to encode the data. CDMA is also resistant to jamming. A jamming signal only has a finite amount of power available to jam the signal. The jammer can either spread its energy over the entire bandwidth of the signal or jam only part of the entire signal. CDMA can also effectively reject narrow
35、 band interference. Since narrow band interference affects only a small portion of the spread spectrum signal, it can easily be removed through notch filtering without much loss of information. and can be used to assist in recovering this lost data. CDMA signals are also resistant to multipath fadin
36、g. Since the spread spectrum signal occupies a large bandwidth only a small portion of this will undergo fading due to multipath at any given time. Like the narrow band interference this will result in only a small loss of data and can be overcome.Another reason CDMA is resistant to multipath interf
37、erence is because the delayed versions of the transmitted pseudo-random codes will have poor correlation with the original pseudo-random code, and will thus appear as another user, which is ignored at the receiver. In other words, as long as the multipath channel induces at least one chip of delay,
38、the multipath signals will arrive at the receiver such that they are shifted in time by at least one chip from the intended signal. The correlation properties of the pseudo-random codes are such that this slight delay causes the multipath to appear uncorrelated with the intended signal, and it is th
39、us ignored.Some CDMA devices use a , which exploits multipath delay components to improve the performance of the system. A rake receiver combines the information from several correlators, each one tuned to a different path delay, producing a stronger version of the signal than a simple receiver with
40、 a single correlation tuned to the path delay of the strongest signal. Frequency reuse is the ability to reuse the same radio channel frequency at other cell sites within a cellular system. In the FDMA and TDMA systems frequency planning is an important consideration. The frequencies used in differe
41、nt cells must be planned carefully to ensure signals from different cells do not interfere with each other. In a CDMA system, the same frequency can be used in every cell, because channelization is done using the pseudo-random codes. Reusing the same frequency in every cell eliminates the need for f
42、requency planning in a CDMA system; however, planning of the different pseudo-random sequences must be done to ensure that the received signal from one cell does not correlate with the signal from a nearby cell. Since adjacent cells use the same frequencies, CDMA systems have the ability to perform
43、soft hand offs. Soft hand offs allow the mobile telephone to communicate simultaneously with two or more cells. The best signal quality is selected until the hand off is complete. This is different from hard hand offs utilized in other cellular systems. In a hard hand off situation, as the mobile te
44、lephone approaches a hand off, signal strength may vary abruptly. In contrast, CDMA systems use the soft hand off, which is undetectable and provides a more reliable and higher quality signal. Collaborative CDMAIn a recent study, a novel collaborative multi-user transmission and detection scheme cal
45、led Collaborative CDMA has been investigated for the uplink that exploits the differences between users fading channel signatures to increase the user capacity well beyond the spreading length in multiple access interference (MAI) limited environment. The authors show that it is possible to achieve
46、this increase at a low complexity and high performance in at fading channels, which is a major research challenge for overloaded CDMA systems. In this approach, instead of using one sequence per user as in conventional CDMA, the authors group a small number of users to share the same spreading seque
47、nce and enable group spreading and despreading operations. The new collaborative multi-user receiver consists of two stages: group multi-user detection (MUD) stage to suppress the MAI between the groups and a low complexity maximum-likelihood detection stage to recover jointly the co-spread users da
48、ta using minimum Euclidean distance measure and users channel gain coefcients. In CDM signal security is high.中文译文码分多址码分多址（CDMA）和各种无线电通信技术所使用的通道存取方法。它不应该被称为cdmaOne在内，CDMA2000（cdmaOne在内的3G演进）和WCDMA（GSM运营商所使用的3G标准）手机标准，这通常被称为简单的CDMA混淆，它们使用的CDMA作为底层通道访问方法。在数据通信的概念，一个是允许多个发射机同时发送一个单一的沟通渠道，信息的想法。这允许多个用户共
49、享一个频带（带宽）。这个概念被称为多址接入。 CDMA采用扩频技术和特殊的编码方案（其中每个发射机分配一个代码），以允许多个用户在同一物理信道复用。相比之下，时分多址（TDMA）分为按时间的访问，而频分多址（FDMA）除以频率。 CDMA是扩频信号的一种形式，因为调制的编码信号具有更高的数据带宽比数据传达。CDMA调制的步骤在CDMA系统中的每个用户使用不同的代码来调节他们的信号。选择使用的代码，以调节信号在CDMA系统的性能是非常重要的。最佳的性能会发生时所需的用户和其他用户的信号，信号之间有良好的分离。关联与本地生成的代码所需的用户接收到的信号是由信号的分离。如果信号匹配所需的用户代码，然后相关的功能将是高，系统可以提取该信号。如果所需的用户代码具有与信号相关毫无共同之处，应该尽可能接近零（从而消除了信号），这被称为交叉相关。如果代码被抵消为零以外的任何时间与信号相关，相关性，应该尽可能接近零。这被称为自动相关，用于拒绝多径干扰。在一般情况下，CDMA属于两个基本类别：同步（