《MIMO多进多出多天线》课程教学资源（文献资料）A Simple Transmit Diversity Technique for Wireless Communications.pdf_大学文库

IEEE JOURNAL ON SELECT AREAS IN COMMUNICATIONS,VOL 16,NO.OCTOBER 199 A Simple Transmit Diversity Technique for Wireless Communications Siavash M.Alamouti This pape hranch tran environment however.may requi up to 10 dB imp emen the schem in SNR.The improvement in SNR may not be achieved by higher transmit power or additional bandwidth,as it is contran o tn or next genera on systems.It rder of 2M.Th both the remote units and the base stations without additiona power or any sacrifice in bandwidth. heoretically,the most effective technique to mitigate mu com Index Term roces baseband proce by the power contro side of the link are known at the transmitter on the other side gh fa g enna the transmitter can predistort the signal in order to overcome he effect of the channel at the receiver.There are tw lems wiu this approa THE NEXT gen n wireless systems are require by that same evel,which in most cases is not practical bec mobile radio standards and provide high bit rate data ser. f radiation power limitations and the size and cost of the pro vices (up to 2 Mbits/s) At the same time,the remote units supposed be st mmunicato nt in svsteme where the u e operate ent typeso ban.and rural:indoor ndoutdoor.n other words,the next e ame frequency Hence,the channel information has to be generation systems are supposed to have better quality and the receiver to the trar overage more pow and to both the transmitter and the receiver.Moreover.in som main affordable for wide ead market acceptance Inevitably applications there may not be a link to feed back the channel nformation the new pocket communicators must remain relatively simple Other e cuive tech nique ately,how the omy of scale allow r are time and frequency y may be pectrum.However,time interleaving results in large delays hen the channel is slowly varying.Equivalently,spread spe menon which makes reliable wire hniques are ective w path fac nen the coner challengewhen compared to fiber.coaxial cable.line-of-sight the channel microwave or even satellite transmissions e quality or In most scattering environments, antenna diversity is a reducing the and,hence,a ed te coding schemes.reducing the eective(BER) eiver and from 10-2 to 10- rform combining or selection and switching in order to mprove the quality of the received signal.The major problem ty and radio fre RF)chains (o circuits)makes the remote units larger and more expensive As a result.diversity techniques have almost exclusively been 3-8716 00©19EEE aded on Aoril 21.2020 at 07:05:52 UTC from IEEE Xplore.Restrictions appl

IEEE JOURNAL ON SELECT AREAS IN COMMUNICATIONS, VOL. 16, NO. 8, OCTOBER 1998 1451 A Simple Transmit Diversity Technique for Wireless Communications Siavash M. Alamouti Abstract— This paper presents a simple two-branch transmit diversity scheme. Using two transmit antennas and one receive antenna the scheme provides the same diversity order as maximal-ratio receiver combining (MRRC) with one transmit antenna, and two receive antennas. It is also shown that the scheme may easily be generalized to two transmit antennas and M receive antennas to provide a diversity order of 2M. The new scheme does not require any bandwidth expansion any feedback from the receiver to the transmitter and its computation complexity is similar to MRRC. Index Terms—Antenna array processing, baseband processing, diversity, estimation and detection, fade mitigation, maximalratio combining, Rayleigh fading, smart antennas, space block coding, space–time coding, transmit diversity, wireless communications. I. INTRODUCTION THE NEXT-generation wireless systems are required to have high voice quality as compared to current cellular mobile radio standards and provide high bit rate data services (up to 2 Mbits/s). At the same time, the remote units are supposed to be small lightweight pocket communicators. Furthermore, they are to operate reliably in different types of environments: macro, micro, and picocellular; urban, suburban, and rural; indoor and outdoor. In other words, the next generation systems are supposed to have better quality and coverage, be more power and bandwidth efficient, and be deployed in diverse environments. Yet the services must remain affordable for widespread market acceptance. Inevitably, the new pocket communicators must remain relatively simple. Fortunately, however, the economy of scale may allow more complex base stations. In fact, it appears that base station complexity may be the only plausible trade space for achieving the requirements of next generation wireless systems. The fundamental phenomenon which makes reliable wireless transmission difficult is time-varying multipath fading [1]. It is this phenomenon which makes tetherless transmission a challenge when compared to fiber, coaxial cable, line-of-sight microwave or even satellite transmissions. Increasing the quality or reducing the effective error rate in a multipath fading channel is extremely difficult. In additive white Gaussian noise (AWGN), using typical modulation and coding schemes, reducing the effective bit error rate (BER) from 10 to 10 may require only 1- or 2-dB higher signalto-noise ratio (SNR). Achieving the same in a multipath fading Manuscript received September 1, 1997; revised February 1, 1998. The author was with AT&T Wireless Services, Redmond, WA, USA. He is currently with Cadence Design Systems, Alta Business Unit, Bellevue, WA 98005-3016 USA (e-mail: siavash@cadence.com). Publisher Item Identifier S 0733-8716(98)07885-8. environment, however, may require up to 10 dB improvement in SNR. The improvement in SNR may not be achieved by higher transmit power or additional bandwidth, as it is contrary to the requirements of next generation systems. It is therefore crucial to effectively combat or reduce the effect of fading at both the remote units and the base stations, without additional power or any sacrifice in bandwidth. Theoretically, the most effective technique to mitigate multipath fading in a wireless channel is transmitter power control. If channel conditions as experienced by the receiver on one side of the link are known at the transmitter on the other side, the transmitter can predistort the signal in order to overcome the effect of the channel at the receiver. There are two fundamental problems with this approach. The major problem is the required transmitter dynamic range. For the transmitter to overcome a certain level of fading, it must increase its power by that same level, which in most cases is not practical because of radiation power limitations and the size and cost of the amplifiers. The second problem is that the transmitter does not have any knowledge of the channel experienced by the receiver except in systems where the uplink (remote to base) and downlink (base to remote) transmissions are carried over the same frequency. Hence, the channel information has to be fed back from the receiver to the transmitter, which results in throughput degradation and considerable added complexity to both the transmitter and the receiver. Moreover, in some applications there may not be a link to feed back the channel information. Other effective techniques are time and frequency diversity. Time interleaving, together with error correction coding, can provide diversity improvement. The same holds for spread spectrum. However, time interleaving results in large delays when the channel is slowly varying. Equivalently, spread spectrum techniques are ineffective when the coherence bandwidth of the channel is larger than the spreading bandwidth or, equivalently, where there is relatively small delay spread in the channel. In most scattering environments, antenna diversity is a practical, effective and, hence, a widely applied technique for reducing the effect of multipath fading [1]. The classical approach is to use multiple antennas at the receiver and perform combining or selection and switching in order to improve the quality of the received signal. The major problem with using the receive diversity approach is the cost, size, and power of the remote units. The use of multiple antennas and radio frequency (RF) chains (or selection and switching circuits) makes the remote units larger and more expensive. As a result, diversity techniques have almost exclusively been 0733–8716/98$10.00  199 IEEE Authorized licensed use limited to: XIDIAN UNIVERSITY. Downloaded on April 21,2020 at 07:05:52 UTC from IEEE Xplore. Restrictions apply

1452 IEEE JOURNAL ON SELECT AREAS IN COMMUNICATIONS.VOL 16.NO.&OCTOBER 1998 as it effectively reduces the effect of fading at the r remote units using multiple transmit antennas at the base stations. Section II,the classical maximal ratio receive diversit ons rath remo this reasor bining n I th d simple mathen it d one antenna and one transmit chain may he added to a has mesith one and with two receive ant as are discussed station toimprove the reception qualityof all the remote nt In Section IV,the bit-error performance of the new scheme in that ase station's coverage area vith coherent D nary phase eceive co etical in Recently,some interesting approaches for transmit diversity tations of the pror posed scheme and the classical MRRC.These differences are discussed in detail in Section V Witt and Winters 5 for a single base station in which copeso the same symbol are transmitted through multiple antennas at different time likeli (MMSE) ualize is the used to resove multipath distortion and obtain diversity gain approach is space-time trellis coding airlink,and the receive chain may be modeled by a complex Another s are oded a d of a magnitud respons and are decoded using a maximum likelihood decoder This nna and the receive antenna zero is den scheme is very effective,as it combines the benefits of forward between the transmit antenna and the receive antenna one is denoted by h where ing and dive fo tiall ho = as a function of bandwidth efficiency (bits/s/HZ)and the h1=1cJ所 () Therefore,for some applications it ma prac diversity scheme which receiver on one ri=h1s0 +n 2 the opp The where no and n rep esen e an at the for thes scheme mav easily he generalized to two trar mit anten and order of 2M.Thisis signals is to choose signalif and only if (i) from the re d(ro.hosi)+d(ri.hisi)s d(ro.host) +d(r.h). 付≠k (3) across multiple antennas,not in time or frequency. where (x,y)is the squared Euclidean distance between he new can improve the ero signals and y calculated by the following expression: ms The de ow the P红，=(x-x*- (④ of higher level modulation schemes to in the effective data rate.or smaller reuse ors in a multice environment scheme for tw-branch MRC to inc system c. also be us other words.the new scheme is effective in all of the applica =hi(h tions here multipath fading +no)+hi(hiso+n) =(a6+ai)so+hono+hin1. her ve w the Expanding (3)and using(4)and(5)we get of exis quaity and to be a superb candidate for next-generation wireless systems. choose s iff (a后+a)s2-0-64 ≤(a哈+a跳P-0s-6跳， ∀i≠k(6) 12020t0 E-ERUTO IEEr

1452 IEEE JOURNAL ON SELECT AREAS IN COMMUNICATIONS, VOL. 16, NO. 8, OCTOBER 1998 applied to base stations to improve their reception quality. A base station often serves hundreds to thousands of remote units. It is therefore more economical to add equipment to base stations rather than the remote units. For this reason, transmit diversity schemes are very attractive. For instance, one antenna and one transmit chain may be added to a base station to improve the reception quality of all the remote units in that base station’s coverage area.1 The alternative is to add more antennas and receivers to all the remote units. The first solution is definitely more economical. Recently, some interesting approaches for transmit diversity have been suggested. A delay diversity scheme was proposed by Wittneben [2], [3] for base station simulcasting and later, independently, a similar scheme was suggested by Seshadri and Winters [4], [5] for a single base station in which copies of the same symbol are transmitted through multiple antennas at different times, hence creating an artificial multipath distortion. A maximum likelihood sequence estimator (MLSE) or a minimum mean squared error (MMSE) equalizer is then used to resolve multipath distortion and obtain diversity gain. Another interesting approach is space–time trellis coding, introduced in [6], where symbols are encoded according to the antennas through which they are simultaneously transmitted and are decoded using a maximum likelihood decoder. This scheme is very effective, as it combines the benefits of forward error correction (FEC) coding and diversity transmission to provide considerable performance gains. The cost for this scheme is additional processing, which increases exponentially as a function of bandwidth efficiency (bits/s/Hz) and the required diversity order. Therefore, for some applications it may not be practical or cost-effective. The technique proposed in this paper is a simple transmit diversity scheme which improves the signal quality at the receiver on one side of the link by simple processing across two transmit antennas on the opposite side. The obtained diversity order is equal to applying maximal-ratio receiver combining (MRRC) with two antennas at the receiver. The scheme may easily be generalized to two transmit antennas and receive antennas to provide a diversity order of . This is done without any feedback from the receiver to the transmitter and with small computation complexity. The scheme requires no bandwidth expansion, as redundancy is applied in space across multiple antennas, not in time or frequency. The new transmit diversity scheme can improve the error performance, data rate, or capacity of wireless communications systems. The decreased sensitivity to fading may allow the use of higher level modulation schemes to increase the effective data rate, or smaller reuse factors in a multicell environment to increase system capacity. The scheme may also be used to increase the range or the coverage area of wireless systems. In other words, the new scheme is effective in all of the applications where system capacity is limited by multipath fading and, hence, may be a simple and cost-effective way to address the market demands for quality and efficiency without a complete redesign of existing systems. Furthermore, the scheme seems to be a superb candidate for next-generation wireless systems, 1 In fact, many cellular base stations already have two receive antennas for receive diversity. The same antennas may be used for transmit diversity. as it effectively reduces the effect of fading at the remote units using multiple transmit antennas at the base stations. In Section II, the classical maximal ratio receive diversity combining is discussed and simple mathematical descriptions are given. In Section III, the new two-branch transmit diversity schemes with one and with two receive antennas are discussed. In Section IV, the bit-error performance of the new scheme with coherent binary phase-shift keying (BPSK) modulation is presented and is compared with MRRC. There are cost and performance differences between the practical implementations of the proposed scheme and the classical MRRC. These differences are discussed in detail in Section V. II. CLASSICAL MAXIMAL-RATIO RECEIVE COMBINING (MRRC) SCHEME Fig. 1 shows the baseband representation of the classical two-branch MRRC. At a given time, a signal is sent from the transmitter. The channel including the effects of the transmit chain, the airlink, and the receive chain may be modeled by a complex multiplicative distortion composed of a magnitude response and a phase response. The channel between the transmit antenna and the receive antenna zero is denoted by and between the transmit antenna and the receive antenna one is denoted by where (1) Noise and interference are added at the two receivers. The resulting received baseband signals are (2) where and represent complex noise and interference. Assuming and are Gaussian distributed, the maximum likelihood decision rule at the receiver for these received signals is to choose signal if and only if (iff) (3) where is the squared Euclidean distance between signals and calculated by the following expression: (4) The receiver combining scheme for two-branch MRRC is as follows: (5) Expanding (3) and using (4) and (5) we get choose iff (6) Authorized licensed use limited to: XIDIAN UNIVERSITY. 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1454 IEEE JOURNAL ON SELECT AREAS IN COMMUNICATIONS, VOL. 16, NO. 8, OCTOBER 1998 TABLE I THE ENCODING AND TRANSMISSION SEQUENCE FOR THE TWO-BRANCH TRANSMIT DIVERSITY SCHEME The channel at time may be modeled by a complex multiplicative distortion for transmit antenna zero and for transmit antenna one. Assuming that fading is constant across two consecutive symbols, we can write (10) where is the symbol duration. The received signals can then be expressed as (11) where and are the received signals at time and and and are complex random variables representing receiver noise and interference. 2) The Combining Scheme: The combiner shown in Fig. 2 builds the following two combined signals that are sent to the maximum likelihood detector: (12) It is important to note that this combining scheme is different from the MRRC in (5). Substituting (10) and (11) into (12) we get (13) 3) The Maximum Likelihood Decision Rule: These combined signals are then sent to the maximum likelihood detector which, for each of the signals and , uses the decision rule expressed in (7) or (9) for PSK signals. The resulting combined signals in (13) are equivalent to that obtained from two-branch MRRC in (5). The only difference is phase rotations on the noise components which do not degrade the effective SNR. Therefore, the resulting diversity order from the new two-branch transmit diversity scheme with one receiver is equal to that of two-branch MRRC. B. Two-Branch Transmit Diversity with Receivers There may be applications where a higher order of diversity is needed and multiple receive antennas at the remote units are feasible. In such cases, it is possible to provide a diversity order of 2 with two transmit and receive antennas. For illustration, we discuss the special case of two transmit and two receive antennas in detail. The generalization to receive antennas is trivial. Fig. 3. The new two-branch transmit diversity scheme with two receivers. TABLE II THE DEFINITION OF CHANNELS BETWEEN THE TRANSMIT AND RECEIVE ANTENNAS TABLE III THE NOTATION FOR THE RECEIVED SIGNALS AT THE TWO RECEIVE ANTENNAS Fig. 3 shows the baseband representation of the new scheme with two transmit and two receive antennas. The encoding and transmission sequence of the information symbols for this configuration is identical to the case of a single receiver, shown in Table I. Table II defines the channels between the transmit and receive antennas, and Table III defines the notation for the received signal at the two receive antennas. Where (14) , , , and are complex random variables representing receiver thermal noise and interference. The combiner in Fig. 3 builds the following two signals that are sent to the maximum Authorized licensed use limited to: XIDIAN UNIVERSITY. Downloaded on April 21,2020 at 07:05:52 UTC from IEEE Xplore. Restrictions apply

ALAMOUTI: SIMPLE TRANSMIT DIVERSITY TECHNIQUE FOR WIRELESS COMMUNICATIONS 1455 Fig. 4. The BER performance comparison of coherent BPSK with MRRC and two-branch transmit diversity in Rayleigh fading. likelihood detector: (15) Substituting the appropriate equations we have (16) These combined signals are then sent to the maximum likelihood decoder which for signal uses the decision criteria expressed in (17) or (18) for PSK signals. Choose iff (17) Choose iff (18) Similarly, for using the decision rule is to choose signal iff (19) or, for PSK signals, choose iff (20) The combined signals in (16) are equivalent to that of fourbranch MRRC, not shown in the paper. Therefore, the resulting diversity order from the new two-branch transmit diversity scheme with two receivers is equal to that of the four-branch MRRC scheme. It is interesting to note that the combined signals from the two receive antennas are the simple addition of the combined signals from each receive antenna, i.e., the combining scheme is identical to the case with a single receive antenna. We may hence conclude that, using two transmit and receive antennas, we can use the combiner for each receive antenna and then simply add the combined signals from all the receive antennas to obtain the same diversity order as -branch MRRC. In other words, using two antennas at the transmitter, the scheme doubles the diversity order of systems with one transmit and multiple receive antennas. An interesting configuration may be to employ two antennas at each side of the link, with a transmitter and receiver chain connected to each antenna to obtain a diversity order of four at both sides of the link. IV. ERROR PERFORMANCE SIMULATIONS The diversity gain is a function of many parameters, including the modulation scheme and FEC coding. Fig. 4 shows the BER performance of uncoded coherent BPSK for MRRC and the new transmit diversity scheme in Rayleigh fading. It is assumed that the total transmit power from the two antennas for the new scheme is the same as the transmit power from the single transmit antenna for MRRC. It is also assumed that the amplitudes of fading from each transmit antenna to each receive antenna are mutually uncorrelated Rayleigh distributed and that the average signal powers at each receive antenna from each transmit antenna are the same. Further, we assume that the receiver has perfect knowledge of the channel. Although the assumptions in the simulations may seem highly unrealistic, they provide reference performance curves for comparison with known techniques. An important issue is Authorized licensed use limited to: XIDIAN UNIVERSITY. Downloaded on April 21,2020 at 07:05:52 UTC from IEEE Xplore. Restrictions apply

1456 IEEE JOURNAL ON SELECT AREAS IN COMMUNICATIONS,VOL 16,NO.S,OCTORER 199 mor B.Sensitivity to Channel Estimation Errors Throughout this paper,it is a t the rec iver ha wo trar single ver is he derived b oand extraction 7 in Section V-A,the 3-dB penalty is incurred because the [8].Known symbols are transmitted periodically from the simulations assume that each transmit antenna radiates half transmitter to the receiver.The receiver extracts the sa nple an estimate of the channe diat There are many factors that may degrade the performance of transmit antenna for MRRC,however,the performance would pilot insertion and extraction techniques,such as mismatched nterpolation c The dom was drawn against the average trans antenna pert the en and overlap with the MRRC curves Neverthe ss.even with mation eror is minimized when the pilot insertion frequen tor the dB receive antenna at a the mel is er the diversit not the e she due to channel estimation errors.For receive antennas at a BER of is about 24 dB,which is 3 dB I cha worse than MRRC with one transmit antenna and four receive els are avallable.Wit stated before signal.The channel estimation task is therefore different.To estimate the new scheme provides similar performance to MRRC.regard- from one transmit antenna to the sof the employed cod ng and modulatio schemes.M it ant To e all th with MRRC The ults from th the pilots must alternate between the antennas (or orthogona publications may be used to predict the performance of the antennas).In new scheme with these coding and modulation techniques. M times a s many s mea this re t twice as man pilots as in the two-branch receiver combining scheme are needed V.IMPLEMENTATION ISSUES C.The Delay Effects With N branch transmit diversity,if the transformed copies antenna and 2M receive antennas.From practical implementa f the signals are trar at m a for the two-branch diversity schem the del periods.For a multicarrier system,however,if the copies are A.Power Requirements The new scheme requires the simultaneous transmission of two different symbols out of two antennas.If the system D.Antenna Configurations radiation pov order to nav the same to correlation)and that they have almost equal average power power to chea er,sma less thar )Since the wireless medium is ienive for For insta there have been many measurements and experimental results intermodulation distortion effects)to employ half-power ndicating that t if two receive antennas are used to provide er than a di to p Ipower am on receiver.they must b d on the sizinglinearity.etc)then the total radiated power may be doubled and no performance penalty is incurred improvement at the remote units it is sufficient to separate the Aoril 21,2020 at 07:05:52 UTC from IEEE

1456 IEEE JOURNAL ON SELECT AREAS IN COMMUNICATIONS, VOL. 16, NO. 8, OCTOBER 1998 whether the new scheme is any more sensitive to real-world sources of degradation. This issue is addressed in Section V. As shown in Fig. 4, the performance of the new scheme with two transmitters and a single receiver is 3 dB worse than two-branch MRRC. As explained in more detail later in Section V-A, the 3-dB penalty is incurred because the simulations assume that each transmit antenna radiates half the energy in order to ensure the same total radiated power as with one transmit antenna. If each transmit antenna in the new scheme was to radiate the same energy as the single transmit antenna for MRRC, however, the performance would be identical. In other words, if the BER was drawn against the average SNR per transmit antenna, then the performance curves for the new scheme would shift 3 dB to the left and overlap with the MRRC curves. Nevertheless, even with the equal total radiated power assumption, the diversity gain for the new scheme with one receive antenna at a BER of 10 is about 15 dB. Similarly, assuming equal total radiated power, the diversity gain of the new scheme with two receive antennas at a BER of 10 is about 24 dB, which is 3 dB worse than MRRC with one transmit antenna and four receive antennas. As stated before, these performance curves are simple reference illustrations. The important conclusion is that the new scheme provides similar performance to MRRC, regardless of the employed coding and modulation schemes. Many publications have reported the performance of various coding and modulation schemes with MRRC. The results from these publications may be used to predict the performance of the new scheme with these coding and modulation techniques. V. IMPLEMENTATION ISSUES So far in this report, we have shown, mathematically, that the new transmit diversity scheme with two transmit and receive antennas is equivalent to MRRC with one transmit antenna and receive antennas. From practical implementation aspects, however, the two systems may differ. This section discusses some of the observed difference between the two schemes. A. Power Requirements The new scheme requires the simultaneous transmission of two different symbols out of two antennas. If the system is radiation power limited, in order to have the same total radiated power from two transmit antennas the energy allocated to each symbol should be halved. This results in a 3-dB penalty in the error performance. However, the 3-dB reduction of power in each transmit chain translates to cheaper, smaller, or less linear power amplifiers. A 3-dB reduction in amplifiers power handling is very significant and may be desirable in some cases. It is often less expensive (or more desirable from intermodulation distortion effects) to employ two half-power amplifiers rather than a single full power amplifier. Moreover, if the limitation is only due to RF power handling (amplifier sizing, linearity, etc.), then the total radiated power may be doubled and no performance penalty is incurred. B. Sensitivity to Channel Estimation Errors Throughout this paper, it is assumed that the receiver has perfect knowledge of the channel. The channel information may be derived by pilot symbol insertion and extraction [7], [8]. Known symbols are transmitted periodically from the transmitter to the receiver. The receiver extracts the samples and interpolates them to construct an estimate of the channel for every data symbol transmitted. There are many factors that may degrade the performance of pilot insertion and extraction techniques, such as mismatched interpolation coefficients and quantization effects. The dominant source of estimation errors for narrowband systems, however, is time variance of the channel. The channel estimation error is minimized when the pilot insertion frequency is greater or equal to the channel Nyquist sampling rate, which is two times the maximum Doppler frequency. Therefore, as long as the channel is sampled at a sufficient rate, there is little degradation due to channel estimation errors. For receive diversity combining schemes with antennas, at a given time, independent samples of the channels are available. With transmitters and a single receiver, however, the estimates of the channels must be derived from a single received signal. The channel estimation task is therefore different. To estimate the channel from one transmit antenna to the receive antenna the pilot symbols must be transmitted only from the corresponding transmit antenna. To estimate all the channels, the pilots must alternate between the antennas (or orthogonal pilot symbols have to be transmitted from the antennas). In either case, times as many pilots are needed. This means that for the two-branch transmit diversity schemes discussed in this report, twice as many pilots as in the two-branch receiver combining scheme are needed. C. The Delay Effects With branch transmit diversity, if the transformed copies of the signals are transmitted at distinct intervals from all the antennas, the decoding delay is symbol periods. That is, for the two-branch diversity scheme, the delay is two symbol periods. For a multicarrier system, however, if the copies are sent at the same time and on different carrier frequencies, then the decoding delay is only one symbol period. D. Antenna Configurations For all practical purposes, the primary requirement for diversity improvement is that the signals transmitted from the different antennas be sufficiently uncorrelated (less than 0.7 correlation) and that they have almost equal average power (less than 3-dB difference). Since the wireless medium is reciprocal, the guidelines for transmit antenna configurations are the same as receive antenna configurations. For instance, there have been many measurements and experimental results indicating that if two receive antennas are used to provide diversity at the base station receiver, they must be on the order of ten wavelengths apart to provide sufficient decorrelation. Similarly, measurements show that to get the same diversity improvement at the remote units it is sufficient to separate the Authorized licensed use limited to: XIDIAN UNIVERSITY. Downloaded on April 21,2020 at 07:05:52 UTC from IEEE Xplore. Restrictions apply

ALAMOUT:SIMPLE TRANSMIT DIVERSITY TECHNIQUE FOR WIRELESS COMMUNICATIONS 1457 antennas at the remote station by about three wavelengths.2 F.Impact on Interference This is due to the difference in the nature of the scattering s the simulta of the remo ers.while thebase sttion is ofe placed attue transmitted from each antenna,it appears that the number of we have twice the number with no nearby scatterers. erence tha of power has side of theink overall interference is Gaussian distributed.Depending on the ant should the transmit antennas be to provide diversity at the application,if this assumption holds,the new scheme results the same istribution and powe sing techniqu link This is hecause the nro effectively used,however,the scheme may have impact on the system design other words,to provide he use or n the dversity schemes station e must have the order oft aths of for interference mitigation has been studied for space-time separation between the two transmit antennas.Equivalently trellis codes [9].Similar efforts are under way to extend these techniques to the new transmit diversity scheme. by to provide c VI.CONCLUSIONS AND DISCUSSIONS It is worth noting that this property allows the use of existing A new ersity scheme has been presented It receive diversity antennas at the base stations for transmit diversity.Also.whe the vides the same diversity orde tw o antennas may be usec scheme p as MRRC with one transmit and two receive antennas.It is receive a E.Soft Failure One of the advar is to provide diversity improvement at all the remote units in sch antennas at al Should one of the receive chains fail,and the other receive nals of two receive is on the he re ver is similar to MRRC.When compared with MRRC,if the total to as soft failure.Fortunately,the new transmit diversity adiated powe e min the same,the transmt scheme provides the same soft failure.To illustrate this,we of that t ver is doubled then its .the Otherwise.if the total radiated be described as see (1 performance is identical to MRRC.Moreover,assuming equal radiated power,thes scheme requires tw ro =hoso +no for s r1=-hosi+n (21) also requires twice the number of pilot symbols for channel The combiner shown in Fig.2 builds the following two estimation when pilot insertion and extraction is used combined signals according to(12): ACKNOWLEDGMENT 3o =horo ho(hoso +no)=aoso+hono 31 =-hori =-ho(-hosi+ni)=agst-honi. (22) Tarokh,N.Seshadri,A icat and r the These combined signals are the same as if there was no diversity.Therefore,the diversity gain is lost but the signal Services for the investigation of the antenna pattem generated may still be detected.For the scheme with two transmit an by this scheme(not included in the paper but important for validation purposes);and T.Alberty of Bosch Telecom for his REFERENCES heights New York

ALAMOUTI: SIMPLE TRANSMIT DIVERSITY TECHNIQUE FOR WIRELESS COMMUNICATIONS 1457 antennas at the remote station by about three wavelengths.2 This is due to the difference in the nature of the scattering environment in the proximity of the remote and base stations. The remote stations are usually surrounded by nearby scatterers, while the base station is often placed at a higher altitude, with no nearby scatterers. Now assume that two transmit antennas are used at the base station to provide diversity at the remote station on the other side of the link. The important question is how far apart should the transmit antennas be to provide diversity at the remote receiver. The answer is that the separation requirements for receive diversity on one side of the link are identical to the requirements for transmit diversity on the other side of link. This is because the propagation medium between the transmitter and receiver in either direction are identical. In other words, to provide sufficient decorrelation between the signals transmitted from the two transmit antennas at the base station, we must have on the order of ten wavelengths of separation between the two transmit antennas. Equivalently, the transmit antennas at the remote units must be separated by about three wavelengths to provide diversity at the base station. It is worth noting that this property allows the use of existing receive diversity antennas at the base stations for transmit diversity. Also, where possible, two antennas may be used for both transmit and receive at the base and the remote units, to provide a diversity order of four at both sides of the link. E. Soft Failure One of the advantages of receive diversity combining schemes is the added reliability due to multiple receive chains. Should one of the receive chains fail, and the other receive chain is operational, then the performance loss is on the order of the diversity gain. In other words, the signal may still be detected, but with inferior quality. This is commonly referred to as soft failure. Fortunately, the new transmit diversity scheme provides the same soft failure. To illustrate this, we can assume that the transmit chain for antenna one in Fig. 2 is disabled, i.e., . Therefore, the received signals may be described as [see (11)] (21) The combiner shown in Fig. 2 builds the following two combined signals according to (12): (22) These combined signals are the same as if there was no diversity. Therefore, the diversity gain is lost but the signal may still be detected. For the scheme with two transmit and two receive antennas, both the transmit and receive chains are protected by this redundancy scheme. 2The separation required depends on many factors such as antenna heights and the scattering environment. The figures given apply mostly to macrocell urban and suburban environments with relatively large base station antenna heights. F. Impact on Interference The new scheme requires the simultaneous transmission of signals from two antennas. Although half the power is transmitted from each antenna, it appears that the number of potential interferers is doubled, i.e., we have twice the number of interferers, each with half the interference power. It is often assumed that in the presence of many interferers, the overall interference is Gaussian distributed. Depending on the application, if this assumption holds, the new scheme results in the same distribution and power of interference within the system. If interference has properties where interference cancellation schemes (array processing techniques) may be effectively used, however, the scheme may have impact on the system design. It is not clear whether the impact is positive or negative. The use of transmit diversity schemes (for fade mitigation) in conjunction with array processing techniques for interference mitigation has been studied for space-time trellis codes [9]. Similar efforts are under way to extend these techniques to the new transmit diversity scheme. VI. CONCLUSIONS AND DISCUSSIONS A new transmit diversity scheme has been presented. It is shown that, using two transmit antennas and one receive antenna, the new scheme provides the same diversity order as MRRC with one transmit and two receive antennas. It is further shown that the scheme may easily be generalized to two transmit antennas and receive antennas to provide a diversity order of . An obvious application of the scheme is to provide diversity improvement at all the remote units in a wireless system, using two transmit antennas at the base stations instead of two receive antennas at all the remote terminals. The scheme does not require any feedback from the receiver to the transmitter and its computation complexity is similar to MRRC. When compared with MRRC, if the total radiated power is to remain the same, the transmit diversity scheme has a 3-dB disadvantage because of the simultaneous transmission of two distinct symbols from two antennas. Otherwise, if the total radiated power is doubled, then its performance is identical to MRRC. Moreover, assuming equal radiated power, the scheme requires two half-power amplifiers compared to one full power amplifier for MRRC, which may be advantageous for system implementation. The new scheme also requires twice the number of pilot symbols for channel estimation when pilot insertion and extraction is used. ACKNOWLEDGMENT The author would like to thank V. Tarokh, N. Seshadri, A. Naguib, and R. Calderbank of AT&T Labs Research for their critical feedback and encouragement; T. Lo of AT&T Wireless Services for the investigation of the antenna pattern generated by this scheme (not included in the paper but important for validation purposes); and T. Alberty of Bosch Telecom for his insightful comments. REFERENCES [1] W. C. Jakes, Ed., Microwave Mobile Communications. New York: Wiley, 1974. Authorized licensed use limited to: XIDIAN UNIVERSITY. Downloaded on April 21,2020 at 07:05:52 UTC from IEEE Xplore. Restrictions apply

1458 IEEE JOURNAL ON SELECT AREAS IN COMMUNICATIONS, VOL. 16, NO. 8, OCTOBER 1998 [2] A. Wittneben, “Base station modulation diversity for digital SIMULCAST,” in Proc. 1991 IEEE Vehicular Technology Conf. (VTC 41st), May 1991, pp. 848–853. [3] A. Wittneben, “A new bandwidth efficient transmit antenna modulation diversity scheme for linear digital modulation,” in Proc. 1993 IEEE International Conf. Communications (ICC’93), May 1993, pp. 1630–1634. [4] N. Seshadri and J. H. Winters, “Two signaling schemes for improving the error performance of FDD transmission systems using transmitter antenna diversity,” in Proc. 1993 IEEE Vehicular Technology Conf. (VTC 43rd), May 1993, pp. 508–511. [5] J. H. Winters, “The diversity gain of transmit diversity in wireless systems with Rayleigh fading,” in Proc. 1994 ICC/SUPERCOMM, New Orleans, LA, May 1994, vol. 2, pp. 1121–1125. [6] V. Tarokh, N. Seshadri, and A. R. Calderbank, “Space-time codes for high data rate wireless communication: Performance criteria and code construction,” IEEE Trans. Inform. Theory, Mar. 1998. [7] J. K. Cavers, “An analysis of pilot symbol assisted modulation for Rayleigh fading channels,” IEEE Trans. Veh. Technol., vol. 40, pp. 686–693, 1991. [8] S. Sampei and T. Sunaga, “Rayleigh fading compensation method for 16 QAM in digital land mobile radio channels,” in Proc. IEEE Vehicular Technology Conf., San Francisco, CA, 1989, pp. 640–646. [9] V. Tarokh, A. Naguib, N. Seshadri, and A. R. Calderbank, “Spacetime codes for wireless communication: Combined array processing and space time coding,” IEEE Trans. Inform. Theory, Mar. 1998. Siavash M. Alamouti received the B.S. and the M.Sc. degrees in electrical engineering from the University of British Columbia, Vancouver, Canada, in 1989 and 1991, respectively. He has been involved in research and development activities in wireless communications since 1989. He is currently with the Alta Business Unit of Cadence Design Systems, Sunnyvale, CA, where he is a Senior Technical Leader involved in the specification and design of electronic design automation (EDA) tools for next-generation wireless communications systems. From 1995 to 1998, he was a Senior Scientist at the Strategic Technology Group of AT&T Wireless Services, Redmond, WA, where he was involved in air-link physical layer design of wireless systems. Prior to that, he was with Mobile Data Solutions, Inc. (MDSI), Richmond, Canada, and for three years was at MPR Teltech, Vancouver, Canada, where he was a Member of the Technical Staff involved in the design of physical and MAC layers for proprietary mobile data systems. His areas of interest include smart antenna techniques, coding and modulation, and physical and MAC layer design of narrowband and wideband wireless communication systems. Authorized licensed use limited to: XIDIAN UNIVERSITY. Downloaded on April 21,2020 at 07:05:52 UTC from IEEE Xplore. Restrictions apply