通信工程专业外文翻译码分多址.docx

1、通信工程专业外文翻译码分多址外文原文Code division multiple accessCode division multiple access (CDMA) is a channel access method used by various radio communication technologies. It should not be confused with the mobile phone standards called cdmaOne, CDMA2000 (the 3G evolution of cdmaOne) and WCDMA (the 3G standard

2、 used by GSM carriers), which are often referred to as simply CDMA, and use 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 u

3、sers to share a band of frequencies (see bandwidth). This concept is called multiple access. CDMA employs spread-spectrum 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, time div

4、ision multiple access (TDMA) divides access by time, while frequency-division multiple access (FDMA) divides it by frequency. CDMA is a form of spread-spectrum signalling, since the modulated coded signal has a much higher data bandwidth than the data being communicated.Steps in CDMA ModulationEach

5、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 separation between the signal of a desired user and the signals of other users

6、. The separation of the signals is made by correlating 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 extract that signal. If the desired users code has nothing in com

7、mon 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 correlation should be as close to zero as possible. This is referred t

8、o 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)Synchronous CDMA exploits mathematical properties of orthogonality

9、between vectors 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 dot product, by summing the products of their respective components (for example, if u = (a, b) and v = (c, d), then their dot product u

10、v = 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 W-CDMA works.Each user in synchronous CDMA uses a code orthogonal to the others codes to modulate their signal. An example of four mutually o

11、rthogonal 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 Walsh codes are used to encode the signal to separate different users. Since each of the 64 Walsh codes are or

12、thogonal 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 handsets, a differ

13、ent 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 systems. A PN c

14、ode 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). These PN s

15、equences 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 central limit theorem in statistics). Gold codes are an example of a PN suitable for this purpose, as t

16、here is low correlation 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

17、to the signal of interest and interfere slightly with the desired signal in proportion to number of users.All forms of CDMA use spread spectrum process gain to allow receivers to partially discriminate against unwanted signals. Signals encoded with the specified PN sequence (code) are received, whil

18、e signals with different codes (or the same code but 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 th

19、eory completely reject arbitrarily strong signals 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

20、 desired signal, they will overwhelm it. This leads 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

21、 transmit power.edit Efficient Practical utilization of 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 t

22、he case of FDMA.TDMA systems must carefully synchronize 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 red

23、uces the probability that users will interfere, but decreases the spectral efficiency. Similarly, FDMA systems must use a guard-band between adjacent channels, due to the unpredictable doppler shift of the signal spectrum because of user mobility. The guard-bands will reduce the probability that adj

24、acent channels will interfere, but decrease the utilization of the spectrum.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

25、 of simultaneous orthogonal codes, time slots and frequency slots respectively 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 rem

26、ain underutilized due to the bursty nature of telephony and packetized 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

27、 Ratio) varies inversely with the number of users. In a bursty traffic 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 percentag

28、e of utilization. Suppose there are 2N users that only talk half of 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,

29、 whereas it is a random quantity (with the same mean) for 2N users talking 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), T

30、DMA, and FDMA systems cannot recover the underutilized resources inherent to bursty traffic due to the fixed number of orthogonal 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 h

31、alf of the time, then half of the time there will be more than N users needing 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 tran

32、smitters simply send when they have something to say, and go off the air when they 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

33、 limited resource. However, spread spectrum techniques use a transmission bandwidth 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 beca