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 trans-
`mit 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
`MMM receive antennas to provide a diversity order of 2MMM . 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, maximal-
`ratio combining, Rayleigh fading, smart antennas, space block
`coding, space–time coding, transmit diversity, wireless commu-
`nications.
`
`I. INTRODUCTION
`
`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 mul-
`tipath 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 spec-
`trum 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 ª
`
`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 ser-
`vices (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, subur-
`ban, 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 re-
`main 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 wire-
`less 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 signal-
`to-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.
`
`199 IEEE
`
`Authorized licensed use limited to: Fish & Richardson PC. Downloaded on May 16,2022 at 13:16:50 UTC from IEEE Xplore. Restrictions apply.
`
`1
`
`SAMSUNG 1023
`
`

`

`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 applica-
`tions 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 implemen-
`tations 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
`
`Noise and interference are added at the two receivers. The
`resulting received baseband signals are
`
`(1)
`
`represent complex noise and interference.
`and
`where
`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)
`
`(2)
`
`(3)
`
`where
`signals
`
`is the squared Euclidean distance between
`calculated by the following expression:
`
`and
`
`(4)
`
`The receiver combining scheme for two-branch MRRC is as
`follows:
`
`Expanding (3) and using (4) and (5) we get
`
`choose
`
`iff
`
`(5)
`
`(6)
`
`Authorized licensed use limited to: Fish & Richardson PC. Downloaded on May 16,2022 at 13:16:50 UTC from IEEE Xplore. Restrictions apply.
`
`2
`
`

`

`ALAMOUTI: SIMPLE TRANSMIT DIVERSITY TECHNIQUE FOR WIRELESS COMMUNICATIONS
`
`1453
`
`Fig. 1. Two-branch MRRC.
`
`or equivalently
`
`choose
`
`iff
`
`For PSK signals (equal energy constellations)
`
`(7)
`
`(8)
`
`is the energy of the signal. Therefore, for PSK
`where
`signals, the decision rule in (7) may be simplified to
`
`choose
`
`iff
`
`(9)
`
`,
`The maximal-ratio combiner may then construct the signal
`as shown in Fig. 1, so that the maximum likelihood detector
`may produce
`, which is a maximum likelihood estimate of
`.
`
`III. THE NEW TRANSMIT DIVERSITY SCHEME
`
`A. Two-Branch Transmit Diversity with One Receiver
`Fig. 2 shows the baseband representation of the new two-
`branch transmit diversity scheme.
`The scheme uses two transmit antennas and one receive
`antenna and may be defined by the following three functions:
`• the encoding and transmission sequence of information
`symbols at the transmitter;
`• the combining scheme at the receiver;
`• the decision rule for maximum likelihood detection.
`
`Fig. 2. The new two-branch transmit diversity scheme with one receiver.
`
`1) The Encoding and Transmission Sequence: At a given
`symbol period,
`two signals are simultaneously transmitted
`from the two antennas. The signal transmitted from antenna
`and from antenna one by
`. During the
`zero is denoted by
`) is transmitted from antenna
`next symbol period signal (
`is transmitted from antenna one where
`zero, and signal
`is the complex conjugate operation. This sequence is shown
`in Table I.
`the encoding is done in space and time
`In Table I,
`(space–time coding). The encoding, however, may also be
`done in space and frequency. Instead of two adjacent symbol
`periods, two adjacent carriers may be used (space–frequency
`coding).
`
`Authorized licensed use limited to: Fish & Richardson PC. Downloaded on May 16,2022 at 13:16:50 UTC from IEEE Xplore. Restrictions apply.
`
`3
`
`

`

`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
`
`time may be modeled by a complex
`The channel at
`multiplicative distortion
`for transmit antenna zero and
`for transmit antenna one. Assuming that fading is
`constant across two consecutive symbols, we can write
`
`where
`is the symbol duration. The received signals can then
`be expressed as
`
`(10)
`
`(11)
`
`where
`and
`are the received signals at time
`and
`are complex random variables representing
`and
`and
`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)
`
`Fig. 3. The new two-branch transmit diversity scheme with two receivers.
`
`TABLE II
`THE DEFINITION OF CHANNELS BETWEEN THE TRANSMIT AND RECEIVE ANTENNAS
`
`It is important to note that this combining scheme is different
`from the MRRC in (5). Substituting (10) and (11) into (12)
`we get
`
`TABLE III
`THE NOTATION FOR THE RECEIVED SIGNALS AT THE TWO RECEIVE ANTENNAS
`
`(13)
`
`com-
`3) The Maximum Likelihood Decision Rule: These
`bined 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 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
`
`are complex random variables representing
`, and
`,
`,
`receiver thermal noise and interference. The combiner in Fig. 3
`builds the following two signals that are sent to the maximum
`
`(14)
`
`Authorized licensed use limited to: Fish & Richardson PC. Downloaded on May 16,2022 at 13:16:50 UTC from IEEE Xplore. Restrictions apply.
`
`4
`
`

`

`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:
`
`Substituting the appropriate equations we have
`
`(15)
`
`(16)
`
`These combined signals are then sent to the maximum like-
`lihood decoder which for signal
`uses the decision criteria
`expressed in (17) or (18) for PSK signals.
`
`Choose
`
`iff
`
`Choose
`
`iff
`
`Similarly, for
`iff
`
`(17)
`
`(18)
`
`using the decision rule is to choose signal
`
`or, for PSK signals,
`
`choose
`
`iff
`
`(19)
`
`(20)
`
`The combined signals in (16) are equivalent to that of four-
`branch 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, includ-
`ing 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: Fish & Richardson PC. Downloaded on May 16,2022 at 13:16:50 UTC from IEEE Xplore. Restrictions apply.
`
`5
`
`

`

`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.
`these performance curves are simple
`As stated before,
`reference illustrations. The important conclusion is that the
`new scheme provides similar performance to MRRC, regard-
`less 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 implementa-
`tion 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 dom-
`inant source of estimation errors for narrowband systems,
`however, is time variance of the channel. The channel esti-
`mation 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
`the primary requirement for
`For all practical purposes,
`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: Fish & Richardson PC. Downloaded on May 16,2022 at 13:16:50 UTC from IEEE Xplore. Restrictions apply.
`
`6
`
`

`

`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 scatter-
`ers, 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)]
`
`The combiner shown in Fig. 2 builds the following two
`combined signals according to (12):
`
`(21)
`
`(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.
`
`2 The 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: Fish & Richardson PC. Downloaded on May 16,2022 at 13:16:50 UTC from IEEE Xplore. Restrictions apply.
`
`7
`
`

`

`1458
`
`IEEE JOURNAL ON SELECT AREAS IN COMMUNICATIONS, VOL. 16, NO. 8, OCTOBER 1998
`
`[2] A. Wittneben, “Base station modulation diversity for digital SIMUL-
`CAST,” in Proc. 1991 IEEE Vehicular Technology Conf. (VTC 41st),
`May 1991, pp. 848–853.
`[3] A. Wittneben, “A new bandwidth efficient transmit antenna modula-
`tion 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 ra