中英對照頻譜效率

1、頻譜效率頻譜效率（ Spectral efficiency 、Spectrum efficiency ）是指在數(shù)位通信 系統(tǒng)中的帶寬限制下， 可以傳送的資料總量。 在有限的波頻譜下， 物理層通信協(xié) 議可以達(dá)到的使用效率有一定的限度。? 鏈路頻譜效率數(shù)字通信系統(tǒng)的鏈路頻譜效率（ Link spectral efficiency ）的單位是 bit/s/Hz ，或（bit/s）/Hz（較少用，但更準(zhǔn)確）。其定義為凈比特率（有用信息速率，不包括糾錯碼）或最大吞吐量除以通信信道或數(shù)據(jù)鏈路的帶寬（單位：赫 茲）。調(diào)制效率定義為凈比特率（包括糾錯碼）除以帶寬。 頻譜效率通常被用于分析數(shù)字調(diào)制方式的效率，

2、有時也考慮前向糾錯碼 （ forward error correction, FEC和其他物理層開銷。在后一種情況下，1個“比特”特 指一個用戶比特，F(xiàn)EC的開銷總是不包括在內(nèi)的。例1： 1kHz帶寬中可以傳送毎秒lOOObit的技術(shù)，其頻譜效率或調(diào)制效率均為1 bit/s/Hz 。例2:電話網(wǎng)的V.92調(diào)制解調(diào)器在模擬電話網(wǎng)上以56,000 bit/s的下行速率和48,000 bit/s的上行速率傳輸。經(jīng)由電話交換機(jī)的濾波，頻率限制在300Hz到3,400Hz 之間，帶寬相應(yīng)為 3400 - 300 = 3100 Hz 。頻譜效率或調(diào)制效率為 56,000/3,100 = 18.1 bit/

3、s/Hz （下行）、48,000/3,100 = 15.5 bit/s/Hz （上 行）。使用 FEC 的架空調(diào)變方式可達(dá)到最大的頻譜效率可以利用標(biāo)本化定理來求 得，信號的字母表（計算機(jī)科學(xué)）利用符號數(shù)量M來組合、各符號使用N = log2 M bit 來表示。此情況下頻譜效率若不使用編碼間干涉的話， 無法超過 2Nbit/s/Hz 的效率。舉例來說，符號種類有 8種、每個各有 3bit 的話，頻譜效率最高不超過 6 bit/s/Hz 。在使用前向錯誤更正編碼的情形時頻譜效率會降低。 比如說使用 1/2 編碼率的FEC時，編碼長度會變?yōu)?.5倍，頻譜效率會降低50%頻譜效率降低的 同時FEC可

4、以改善信號的SN比（并非一定會有改善）。對某個SN比通信回來說、 在完全沒有傳輸錯誤， 且編碼與調(diào)變方式皆處于理想的狀況時， 其頻譜效率的上 限可哈特利定理得出。比如說 SN比1即分貝為0時，無論編碼與調(diào)變方式如何變 化，頻譜效率不會超過1 bit/s/Hz 。Goodput (應(yīng)用層情報使用的量)比一般在 此計算的吞吐量還小，其原因?yàn)橛蟹獍俅蝹魉?、超傳輸協(xié)議的架空造成的。頻譜效率這個用語， 會產(chǎn)生數(shù)值越大的話可以使周波數(shù)頻譜產(chǎn)生更有效的誤 解產(chǎn)生。比如手機(jī)因?yàn)轭l譜擴(kuò)散與使用 FEC技術(shù)使得頻譜效率低下，但SN比不 好有時還是可以正常通信。 因此可以使用到比周波帶寬數(shù)還多的鏈結(jié)、 以整體來

5、看其效果可以彌補(bǔ)頻譜效率低下的缺點(diǎn)還有過之。 如同后面會提到的， 具有較為 合適尺度代表”單位帶寬利用率”單位的 bit/s/Hz 存在，這是屬于分碼多工 (CDMA的技術(shù)并已成為數(shù)位手機(jī)的基本構(gòu)成技術(shù)。但是電話線路與有線電視網(wǎng)等 由于沒有頻道相互干擾的問題，其使用的基本上皆為其SN比下最大頻譜效率。? 系統(tǒng)頻譜效率無線網(wǎng)絡(luò)是以系統(tǒng)頻譜效率 在有限的無線周波數(shù)帶寬下可以同時支援的客 戶數(shù)與服務(wù)進(jìn)行量化。其單位為 bit/s/Hz/area unit、bit/s/Hz/cell、bit/s/Hz/site 等進(jìn)行計量。有可以把系統(tǒng)能同時支援使用者的吞吐量與 goodput 的總量以通信回路的帶寬

6、 (Hz) 來表示。 這并不單影響使用單一通信回路 的技術(shù)，多元連接手法與無線資源管理技術(shù)也受到影響， 特別是動態(tài)無線資源管 理可以得到改善。定義最大 goodput 時，會排除掉通信回路間的相互干渉與沖突， 高階通訊協(xié)定的架空也是忽略不計的。手機(jī)網(wǎng)絡(luò)的容量也是以 1 MHz 周波數(shù)帶寬上可以同時最大連接線數(shù)來表示， 即 Erlang/MHz/cell 、Erlangs/MHz/sector 、Erlangs/M H z/km 2 等單位。這個數(shù) 值也影響到訊息編碼技術(shù)(數(shù)據(jù)壓縮) 、在類比電話網(wǎng)絡(luò)也有使用。例：以頻分多址(FDMA)與固定頻道分配(FCA)為基礎(chǔ)的手機(jī)系統(tǒng)在頻率再 利用系數(shù)是

7、 4的時候、各基地局可以利用的是所有頻譜的 1/4 。根據(jù)此推算、最 大系統(tǒng)頻譜效率 (bit/s/Hz/site )是鏈結(jié)頻譜效率的 1/4 。各基地局使用 3個扇 形天線將訊號分為 3扇區(qū)時，被稱為 4/12再利用模式。各部份可以使用全頻譜的 1/12 ，因此系統(tǒng)的頻譜效率 (bit/s/Hz/cell 或 bit/s/Hz/sector )為鏈結(jié)頻譜 效率的 1/12 。即使鏈結(jié)頻譜效率( bit/s/Hz )偏低，以”系統(tǒng)頻譜效率”的観點(diǎn) 來看，并不一定代表編碼效率不好。例如、分碼多工 (CDMA) 頻譜擴(kuò)散為單一通信回路 (即只有依未使用者 )時， 頻譜效率是不好的， 但是由于在同一

8、帶寬中有復(fù)數(shù)的通信回路存在， 因次系統(tǒng)頻譜效率非常好。例：以W-CDM3G手機(jī)系統(tǒng)來說、打電話時最大壓縮 8,500 bit/s時、會造 成5 MHz 帶寬的擴(kuò)散，此時此連接的吞吐量為8,500/5,000,000 = 0.0017bit/s/Hz 。在這情形下同扇區(qū)內(nèi)可以有同時容納100通電話(有聲音)的進(jìn)行。由 于各基地局以3個方向的扇形天線區(qū)分為3個扇區(qū)，在頻譜擴(kuò)散后、頻率再利用系 數(shù)會變的比1還小。此時的系統(tǒng)頻譜效率為 1 100 0.0017 = 0.17bit/s/Hz/site 亦或 0.17/3= 0.06 bit/s/Hz/cell (也可 換算成bit/s/Hz/secto

9、r )。頻譜效率可以使用固定/動態(tài)頻道分配、電力控制、即被稱為Link Adaptatio的無線資源管理技術(shù)來進(jìn)行改善。比較表如下表1：一般通信系統(tǒng)的頻譜效率數(shù)值一般通信系統(tǒng)每秒頻道的帶寬R(Mbit/s)頻道的帶寬B(MHz)鏈結(jié)頻譜效系統(tǒng)頻譜效率一般R/B/K數(shù)值(bit/s/Hz/site)率R/B(bit/s/Hz)典型的頻率再利用系數(shù)1/K的頻譜效率服務(wù)規(guī)格0.013 8第二世代手GSM1993時隙=:0.20.521/70.17機(jī)(2G)0.104最大0.384最大 1.922.75GGSM+ EDGE0.21/70.33通常0.20通常1.00IS-136HS +最大0.384最

10、大 1.922.75G0.21/70.45EDGE通常0.27通常1.35第三世代手W-CDMAFDD傳到手機(jī)時傳到手機(jī)時51/70.51機(jī)(3G)1997最大0.384最大0.077傳到手機(jī)時傳到手機(jī)時3.5GHSDPA200751/70.71最大14.4最大2.883.5GHSOPAOFDMA傳到手機(jī)時10傳到手機(jī)時1/70.71最大100最大5第三世代攜CDMA20001X傳到手機(jī)時1.25傳到手機(jī)時1/70.51帯電話（3G）IEEE最大0.144最大0.115Wi-Fi802.11a/g200最大5420最大2.71/30.93IEEE 802.11 nWi-FiDraft 2.0最

11、大144.420最大7.221/32.4200720IEEE(1.75,WiMAX802.162004963.5,4.81/41.27.)0.5760.34數(shù)位廣播DAB1.1521.7120.671/50.080.170.5760.34數(shù)位廣播DAB+ SFN1.1521.7120.67最大31.67最大 4.0數(shù)位電視DVB-T通常22.08通常2.81/50.55最大31.67最大 4.0數(shù)位電視DVB-T+ SFN通常22.08通常2.80.68 數(shù)位電視DVB-H5.5 1181.41/50.140.280.68 數(shù)位電視DVB-H+ SFN5.5 1181.4光纖用數(shù)位256-QA

12、M3866.3316.33電視TVSpectral efficiencySpectral efficiency, spectrum efficiency or bandwidth efficiency refers to the information rate that can be transmitted over a given bandwidth in a specific communication system. It is a measure of how efficiently a limited frequency spectrum is utilized by the ph

13、ysical layer protocol, and sometimes by the media access control (the channel access protocol).Link spectral efficiencyThe link spectral efficiency of a digital communication system is measured 1in bit/s/Hz , or, less frequently but unambiguously, in (bit/s)/Hz . It is the net bitrate (useful inform

14、ation rate excluding error-correctingcodes) or maximum throughput divided by the bandwidth in hertz of a communication channel or a data link. Alternatively, the spectral efficiency may be measured in in bit/symbol , which is equivalent to bits per channel use ( bpcu), implying that the net bit rate

15、 is divided by the symbol rate (modulation rate) or line code pulse rate.Link spectral efficiency is typically used to analyse the efficiency of a digital modulation method or line code, sometimes in combination with a forward error correction (FEC) code and other physical layer overhead. In the lat

16、ter case, a bit refers to a user data bit; FEC overhead is always excluded.The modulation efficiency in bit/s is the gross bitrate (including any error-correcting code) divided by the bandwidth.Example 1: A transmission technique using one kilohertz of bandwidth to transmit 1,000 bits per second has

17、 a modulation efficiency of 1 (bit/s)/Hz.Example 2: A V.92 modemfor the telephone network can transfer 56,000 bit/s downstream and 48,000 bit/s upstream over an analog telephone network.Due to filteringin the telephone exchange, the frequency range is limitedto between 300 hertz and 3,400 hertz, cor

18、responding to a bandwidth of 3,400 - 300 = 3,100 hertz. The spectral efficiency or modulation efficiency is 56,000/3,100 = 18.1 (bit/s)/Hz downstream, and 48,000/3,100 = 15.5 (bit/s)/Hz upstream.An upper bound for the attainable modulation efficiency is given by the Nyquist rate or Hartleys law as f

19、ollows: For a signaling alphabet with Malternative symbols, each symbol representsN = log 2 Mbits. Nis themodulation efficiency measured inbit/symbol or bpcu. In the case ofbaseband transmission (line coding or pulse-amplitude modulation) with a baseband bandwidth (or upper cut-off frequency) B, the

20、 symbol rate can not exceed 2B symbols/s in view to avoid intersymbol interference. Thus,the spectral efficiency can not exceed 2N(bit/s)/Hz in the basebandtransmission case. In the passband transmission case, a signal with passband bandwidth Wcan be converted to an equivalent baseband signal (using

21、 undersampling or a superheterodyne receiver), with upper cut-off frequency W/2. If double-sideband modulation schemes such as QAM, ASK, PSKor OFDMare used, this results in a maximumsymbol rate of Wsymbols/s, and in that the modulation efficiency can not exceedN (bit/s)/Hz. Ifdigital single-sideband

22、 modulation is used, the passband signal with bandwidth Wcorresponds to a baseband message signal with baseband bandwidth W, resulting in a maximum symbol rate of 2Wand an attainablemodulation efficiency of 2 N (bit/s)/Hz.Example 3: An 16QAMmodemhas an alphabet size of M= 16 alternative symbols, wit

23、h N= 4 bit/symbol or bpcu. Since QAM is a form of double sideband passband transmission, the spectral efficiency cannot exceedN = 4(bit/s)/Hz.Example 4: The 8VSB (8-level vestigial sideband) modulation scheme used in the ATSC digital television standard givesN=3 bit/symbol or bpcu.Since it can be de

24、scribed as nearly single-side band, the modulation efficiency is close to 2N= 6 (bit/s)/Hz. In practice, ATSC transfers agross bit rate of 32 Mbit/s over a 6 MHz wide channel, resulting in a modulation efficiency of 32/6 = 5.3 (bit/s)/Hz.Example 5: The downlink of a V.92 modemuses a pulse-amplitude

25、modulation with 128 signal levels, resulting inN= 7 bit/symbol. Since thetransmitted signal before passband filtering can be considered as baseband transmission, the spectral efficiency cannot exceed 2N = 14(bit/s)/Hz over the full baseband channel (0 to 4 kHz). As seen above, a higher spectral effi

26、ciency is achieved if we consider the smaller passband bandwidth.If a forward error correction code is used, the spectral efficiency isreduced from the uncoded modulation efficiency figure.Example 6: If a forward error correction(FEC) code with code rate 1/2 isadded, meaning that the encoder input b

27、it rate is one half the encoder output rate, the spectral efficiency is 50%of the modulation efficiency. In exchange for this reduction in spectral efficiency, FECusually reduces the bit-error rate, and typically enables operation at a lower signal to noise ratio (SNR).An upper bound for the spectra

28、l efficiency possible without bit errors in a channel with a certain SNR, if ideal error coding and modulation is assumed, is given by the Shannon-Hartley theorem.Example 7: If the SNR is 1 times expressed as a ratio, corresponding to 0 decibel, the link spectral efficiency can not exceed 1 (bit/s)/

29、Hz for error-free detection (assuming an ideal error-correcting code) according to Shannon-Hartley regardless of the modulation and coding.Note that the goodput (the amount of application layer useful information) is normally lower than the maximum throughput used in the above calculations, because

30、of packet retransmissions, higher protocol layer overhead, flow control, congestion avoidance, etc. Onthe other hand, a data compression scheme, such as the V.44 or V.42bis compression used in telephone modems, may however give higher goodput if the transferred data is not already efficiently compre

31、ssed.The link spectral efficiency of a wireless telephony link may also be expressed as the maximum number of simultaneous calls over 1 MHz frequency spectrum in erlangs per megahertz, orE/MHz. This measure isalso affected by the source coding (data compression) scheme. It may be applied to analog a

32、s well as digital transmission.In wireless networks, the link spectral efficiency can be somewhat misleading, as larger values are not necessarily more efficient in their overall use of radio spectrum. In a wireless network, high link spectralefficiency may result in high sensitivity to co-channel i

33、nterference(crosstalk), which affects the capacity. For example, in a cellular telephone network with frequency reuse, spectrum spreading and forward error correction reduce the spectral efficiency in (bit/s)/Hz but substantially lower the required signal-to-noise ratio in comparison to non-spread s

34、pectrum techniques. This can allow for much denser geographical frequency reuse that compensates for the lower link spectral efficiency, resulting in approximately the samecapacity (the samenumber of simultaneous phone calls) over the same bandwidth, using the same number of base station transmitter

35、s. As discussed below, a more relevant measure for wireless networks would besystem spectral efficiencyinbit/s/Hz per unit area. However, in closed communication links such as telephone lines and cable TV networks, and in noise-limited wireless communication system where co-channel interference is n

36、ot a factor, the largest link spectral efficiency that can be supported by the available SNR is generally used.System spectral efficiency or area spectral efficiencyIn digital wireless networks, the system spectral efficiency or area spectral efficiency is typically measured in (bit/s)/Hz per unit a

37、rea, (bit/s)/Hz per cell, or (bit/s)/Hz per site. It is a measure of the quantity of users or services that can be simultaneously supported by a limited radio frequency bandwidth in a defined geographic area. It may for example be defined as the maximumthroughput or goodput, summedover all users in

38、the system, divided by the channel bandwidth. This measure is affected not only by the single user transmission technique, but also by multiple access schemes and radio resource management techniques utilized. It can be substantially improved by dynamic radio resource management. If it is defined as

39、 a measure of the maximum goodput, retransmissions due to co-channel interference and collisions are excluded. Higher-layer protocol overhead (above the media access control sublayer) is normally neglected.Example 8:In a cellular system based on frequency-division multiple access (FDMA) with a fixed

40、 channel allocation (FCA) cellplan using a frequency reuse factor of 4, each base station has access to 1/4 of the total available frequency spectrum. Thus, the maximum possible system spectral efficiency in (bit/s)/Hz per site is 1/4 of the link spectral efficiency. Each base station may be divided

41、 into 3 cells by means of 3 sector antennas, also known as a 4/12 reuse pattern. Then each cell has access to 1/12 of the available spectrum, and the system spectral efficiency in (bit/s)/Hz per cell or (bit/s)/Hz per sector is 1/12 of the link spectral efficiency.The system spectral efficiency of a

42、 cellular network may also be expressed as the maximumnumber of simultaneous phone calls per area unit over 1 MHz frequency spectrum in E/MHz per cell, E/MHz per sector, E/MHz 2per site, or (E/MHz)/m . This measure is also affected by the source coding (data compression) scheme. It may be used in an

43、alog cellular networks as well.Low link spectral efficiency in (bit/s)/Hz does not necessarily mean that an encoding schemeis inefficient from a system spectral efficiency point of view. As an example, consider Code Division Multiplexed Access (CDMA) spread spectrum, which is not a particularly spec

44、tral efficient encoding schemewhenconsidering a single channel or single user. However, the fact that one can layer multiple channels on the samefrequency band meansthat the system spectrum utilization for a multi-channel CDMAsystem can be very good.Example 9: In the W-CDM3AGcellular system, every p

45、hone call is compressed to a maximum of 8,500 bit/s (the useful bitrate), and spread out over a 5 MHz wide frequency channel. This corresponds to a link throughput ofonly 8,500/5,000,000= 0.0017 (bit/s)/Hz. Let us assume that 100simultaneous (non-silent) simultaneous calls are possible in the same c

46、ell. Spread spectrum makes it possible to have as low a frequency reuse factor as 1, if each base station is divided into 3 cells by means of 3 directional sector antennas. This corresponds to a system spectrum efficiencyof over 1 x 100 x 0.0017 = 0.17 (bit/s)/Hzper cell or sector.The spectral effic

47、ie ncy can be improved by radio resource man ageme nttech niq ues such as efficie nt fixed or dyn amic cha nnel allocati on, power con trol, li nk adaptati on and diversity schemes.A combined fairness measure and system spectral efficiencymeasure is thefairly shared spectral efficie ncy.Comparis on

48、tableExamples of nu mericalspectral efficie ncy values of some com moncom muni cati on systems can be found in the table below.Spectral efficie ncy of com mon com muni catio n systems.ServiceSta ndard 回La unc hed yea raNet bitrate Rpe r carrier (Mbit/s) aBan dwid thBper carrier(MHz)SLi nk spectral efficie ncy R/B(bit/s)/Hz) 0Typical reuse factor1/K3System spectr al efficie ncyAppro x.(R/B)/ K)(bit/s) /Hz persite)198IGcellularNMT 45010.00120.0250.450.0641980.0096citatio耳IGcellularAMPS3n needed0.030