Transmitter diversity effect in TDMA/TDD mobile radio transmission

Transmitter diversity, which employs a single transmit/receive antenna at the portable stations and two transmit/receive antennas at the base station, is experimentally investigated for a TDMA/TDD (time division duplex) mobile radio system. Experimental results show that transmitter diversity can significantly improve the BER (bit error rate) performance of the portable station, due to AWGN (additive white gaussian noise), delay spread, and CCI (cochannel interference) in Rayleigh fading environments.<>     

Effects on correlation between two mobile radio base-station antennas

The correlation between signals received by two mobile radio base-station antennas is investigated to determine spacing requirements for space diversity. Measurements of the fading of UHF signals received by two base-station horn antennas oriented at different angles with respect to the incoming mobile radio signal were made for different antenna spacings. The experimental results are compared with an analytical expression derived in this paper; they agree fairly well. A further experiment was made after removing the possible local scatterers surrounding the base station. Comparing these two experimental results, we find that the following are true. 1) Propagation in the direction of a line connecting the two base-station antennas is the critical case and requires a large separation of 70 wavelengths. As soon as the incoming wave is 10° away from the in-line axis, the spacing requirement drops to 30 wavelengths. 2) Local scatterers at the base station tend to decrease the correlation between signals received at the two antennas. We conclude that an upper limit to the spacing of antennas used for diversity can be obtained and that it is within the achievable range.     

Effects on Correlation Between Two Mobile Radio Base-Station Antennas

The correlation between signals received by two mobile radio base-station antennas is investigated to determine spacing requirements for space diversity. Measurements of the fading of UHF signals received by two base-station horn antennas oriented at different angles with respect to the incoming mobile radio signal were made for different antenna spacings. The experimental results are compared with an analytical expression derived in this paper; they agree fairly well. A further experiment was made after removing the possible local scatterers surrounding the base station. Comparing these two experimental results, we find that the following are true. 1) Propagation in the direction of a line connecting the two base-station antennas is the critical case and requires a large separation of 70 wavelengths. As soon as the incoming wave is 10° away from the inline axis, the spacing requirement drops to 30 wavelengths. 2) Local scatterers at the base station tend to decrease the correlation between signals received at the two antennas. We conclude that an upper limit to the spacing of antennas used for diversity can be obtained and that it is within the achievable range.     

Multipath tolerant ranging and data transfer techniques for air-to-ground and ground-to-air links

This work addresses data acquisition problems associated with an improved air traffic control system. A mathematical theory of scattering which describes the multipath process is developed. From this model, the optimum signal structure required for ranging and communicating data through the multipath propagation medium is derived. It is shown that the Reed-Solomon Codes have near optimum properties for ranging and data communications. A baseline data transmission system for air traffic control is designed which will accommodate 4000 aircraft per second at the rate of 100 bits per second in a 60 by 120 mile area. The baseline system combines coding with frequency hopping as the best means of combating severe multipath interference. Several coding systems are compared and curves relating error rate to Eb/N0(signal-to-noise energy per bit) for a channel characterized by Rayleigh fading are presented. This paper concludes that a ground-to-air and air-to-ground high capacity data link at a reasonable cost will be feasible for an improved air traffic control system.     

Performance of feedback and switch space diversity 900 MHz FM mobile radio systems with Rayleigh fading

A theoretical and experimental comparison of performance has been made between two types of predetection switching space diversity mobile radio systems. This comparison was made at a frequency of 840 MHz using simulated Rayleigh fading for a vehicle speed of about 80 mi/h. The switch diversity system was a conventional receiver antenna switching technique with two simulated physically separated receiving antennas and a single transmitting antenna. The feedback diversity system used a single receiving antenna with two simulated physically separated transmitting antennas. The transmitting antennas were switched remotely from the receiver. The difference in the performance of the two systems was shown to be primarily due to time delay inherent in the remote antenna switching technique.     

Performance of Feedback and Switch Space Diversity 900 MHz FM Mobile Radio Systems with Rayleigh Fading

A theoretical and experimental comparison of performance has been made between two types of predetection switching space diversity mobile radio systems. This comparison was made at a frequency of 840 MHz using simulated Rayleigh fading for a vehicle speed of about 80 mi/h. The switch diversity system was a conventional receiver antenna switching technique with two simulated physically separated receiving antennas and a single transmitting antenna. The feedback diversity system used a single receiving antenna with two simulated physically separated transmitting antennas. The transmitting antennas were switched remotely from the receiver. The difference in the performance of the two systems was shown to be primarily due to time delay inherent in the remote antenna switching technique.     

The diversity gain of transmit diversity in wireless systems withRayleigh fading

In this paper, we study the ability of transmit diversity to provide diversity benefit to a receiver in a Rayleigh fading environment. With transmit diversity, multiple antennas transmit delayed versions of a signal to create frequency-selective fading at a single antenna at the receiver, which uses equalization to obtain diversity gain against fading. We use Monte Carlo simulation to study transmit diversity for the case of independent Rayleigh fading from each transmit antenna to the receive antenna and maximum likelihood sequence estimation for equalization at the receiver. Our results show that transmit diversity with M transmit antennas provides a diversity gain within 0.1 dB of that with M receive antennas for any number of antennas. Thus, we can obtain the same diversity benefit at the remotes and base stations using multiple base-station antennas only     

System Performance of Transmit Diversity Methods and a Two Fixed-Beam System in WCDMA

Downlink transmit diversity modes for WCDMA together with a two fixed-beam antenna array system are compared relative to the single antenna sectorized system in a radio network simulator. The transmit diversity methods investigated are: space-time transmit diversity and closed-loop mode I transmit diversity. Frequency selective (COST 259) and flat fading channels are considered and their impact to speech-only and data-only services is evaluated. A third service, which highlights the system performance of the various advanced antennas, is also investigated.The results in this investigation point out that the diversity gain in flat fading channels is substantial. In frequency-selective fading, the benefits of fixed beam systems is encouraging, whereas transmit diversity methods (especially Space-Time Transmit Diversity) is unsatisfactory.     

System Performance of Transmit Diversity Methods and a Two Fixed-Beam System in WCDMA

Downlink transmit diversity modes for WCDMA together with a two fixed-beam antenna array system are compared relative to the single antenna sectorized system in a radio network simulator. The transmit diversity methods investigated are: space-time transmit diversity and closed-loop mode I transmit diversity. Frequency selective (COST 259) and flat fading channels are considered and their impact to speech-only and data-only services is evaluated. A third service, which highlights the system performance of the various advanced antennas, is also investigated.The results in this investigation point out that the diversity gain in flat fading channels is substantial. In frequency-selective fading, the benefits of fixed beam systems is encouraging, whereas transmit diversity methods (especially Space-Time Transmit Diversity) is unsatisfactory.     

A Wireless Network for Wide-Band Indoor Communications

We propose and analyze a wide-band indoor communication system that uses radio as the transmission medium either on a stand-alone basis or to supplement a hard-wired network for those situations where complete portability is desired. One principal impairment to such a system is intersymbol interference caused by frequencyselective fading. A novel media-access scheme is proposed which permits the use of resource sharing, wherein a small pool of time slots is effectively shared among all users to provide added protection against channel impairments on an as-needed basis. Our results show that the use of resource sharing and diversity provide excellent protection against intersymbol interference caused by frequency-selective fading with negligible impact on throughput. Furthermore, resource sharing plus diversity can permit significantly higher data rates without large queueing delays. For example, a wireless network with a 10 Mbit/s data rate in a 10 MHz bandwidth using four antennas at the base station has a less than 10^-4outage probability at a 10^-4BER in buildings with less than 58 ns rms delay spread. A loading of 75 percent is permitted for a queueing delay of less than 20 packet transmission times all but 0.01 percent of the time.     

Antenna selection in a multisector packet radio system

To improve performance on the inbound (mobile-to-base-station) channel of a packet radio system consisting of a base station and a number of mobile users, the area around the base station is divided into M sectors. Signals originating from users in different sectors are received by different directional antennas at the base station. It is shown that, if the number of receivers at the base station is less than M, the selection of the antennas to be connected to the receivers becomes an issue. A number of antenna selection schemes are compared for three different channel models, assuming an ideal antenna pattern. It is found that the scheme that selects the antennas with the largest received signal powers is nearly optimum. The effects of a more practical nonideal antenna pattern are discussed     

Error probability performance for W-CDMA systems with multiple transmit and receive antennas in correlated Nakagami fading channels

The bit error rate (BER) performance of a two-dimensional (2-D) RAKE receiver, in combination with transmit diversity on the downlink of a wide-band CDMA (W-CDMA) system, is presented. The analyses assume correlated fading between receive antenna array elements, and an arbitrary number of independent but nonidentical resolvable multipaths combined by the RAKE receiver in the general Nakagami-m (1960) fading channel framework. The impact of the array configuration (e.g., the number of transmit antennas and receive antennas, the antenna element separation) and the operating environment parameters (such as the fading severity, angular spread and path delay profile) on the overall space-path diversity gain can be directly evaluated. In addition, the exact pairwise error probability of a convolutional coded system is obtained, and the coding gain of a space-path diversity receiver is quantified.     

Experiment on coherent adaptive antenna array for wideband DS-CDMAmobile radio

A four-element pilot symbol-assisted coherent adaptive antenna array diversity receiver for 4.096 Mchip/s wideband direct sequence code division multiple access mobile radio is implemented and its performance in a multipath fading environment is evaluated by a laboratory experiment using hardware fading simulators. The receiver comprises an adaptive antenna array using the normalised least mean square algorithm and Rake combiner. It is demonstrated that, for the three-user case, the required average signal-to-interference ratio obtaining average BER of 10^-3 can be reduced by ~8 dB compared to four-branch antenna diversity     

Performance Analysis of Multiuser Diversity in MIMO Systems with Antenna Selection

In this paper, a framework is presented to analyze the performance of multiuser diversity (MUD) in multiuser point-to-multipoint (PMP) MIMO systems with antenna selection. Based on this framework, the tight closed-form expressions of outage capacity and average symbol error rate are derived for the multiuser transmit antenna selection with maximal-ratio combining (TAS/MRC) system, by which we show how and with what characteristics antenna selection gains, MIMO antenna configurations and fading gains impact on the system performance, with an emphasis on the study of multiuser diversity influence. From both theoretical and simulation results, our study shows that in multiuser PMP TAS/MRC systems an diversity order equals to the product of the number of transmit antennas, number of receive antennas and number of users can be achieved; what's more, users plays a key role in the system performance and can be viewed as equivalent 'virtual" transmit antennas, which is the source of the multiuser diversity inherent exists in the multiuser system. This kind of diversity can be efficiently extracted in the design of multiantenna systems.     

Combined transmit and receive antenna diversity for WCDMA in multipath fading channels

We analyze the performance of various receiver antenna diversity combining schemes in combination with transmit diversity on the downlink of a WCDMA system. The analytical framework developed can handle an arbitrary number of transmit and receive antennas, as well as the study on the effect of dissimilar received signal strengths at the mobile handset on the average symbol error rate performance of a multitude of digital modulation schemes.     

On the available diversity gain from different dual-polarizedantennas

Dual-polarized antennas are traditionally characterized in terms of port-to-port isolation and co- and cross-polar radiation patterns. For base station antennas in a mobile communications system, the critical parameter is instead the far-field coupling between the two channels. In a mobile communication system, base station antennas with a nominal ±45° to vertical linear polarization are commonly used. Such antennas are difficult to design with constant polarization characteristics in azimuth. We calculate the antenna output power correlation and diversity gain under Rayleigh fading conditions and different values of the environment cross-polar discrimination. Two different antennas are compared: a dual-polarized aperture coupled patch and a slanted dipole configuration, both over an infinite groundplane. We show that the aperture coupled patch provides lower output correlation and higher diversity gain than the slanted dipoles in all investigated cases     

Effect of imperfect channel estimation on transmit diversity in CDMA systems

In this paper, the effect of imperfect channel estimation on the transmit diversity based on space-time block coding for the downlink of a direct-sequence code-division multiple-access system is studied. Two transmit antenna and one receiving antennas are employed. However, the results of this paper can be extended to the system with more receiving antennas. Each channel is modeled as frequency-selective Rayleigh fading and the pair of channels corresponding to two transmit antennas are mutually independent. Both spatial diversity gain and multipath diversity gain are obtained in the system. The system performance is evaluated in terms of bit-error rate under the perfect and imperfect channel estimation. A pilot-assisted channel-estimation scheme with one common spreading code sequence is exploited. It is shown that the inaccurate channel estimates suffering from multiple access and multipath interference significantly degrade the system performance and can be effectively improved by use of a simple low-pass filter. The investigation of the power ratio of pilot to data channels illustrates that the base station should dynamically adjust the transmit power of the pilot channel according to the varying system configurations in order to achieve the best performance.     

Optimum combining in digital mobile radio with cochannel interference

This paper studies optimum signal combining for space diversity reception in cellular mobile radio systems. With optimum combining, the signals received by the antennas are weighted and combined to maximize the output signal-to-interference-plus-noise ratio. Thus, with cochannel interference, space diversity is used not only to combat Rayleigh fading of the desired signal (as with maximal ratio combining) but also to reduce the power of interfering signals at the receiver. We use analytical and computer simulation techniques to determine the performance of optimum combining when the received desired and interfering signals are subject to Rayleigh fading. Results show that optimum combining is significantly better than maximal ratio combining even when the number of interferers is greater than the number of antennas. Results for typical cellular mobile radio systems show that optimum combining increases the output signal-to-interference ratio at the receiver by several decibels. Thus, systems can require fewer base station antennas and/or achieve increased channel capacity through greater frequency reuse. We also describe techniques for implementing optimum combining with least mean square (LMS) adaptive arrays.     

Performance of Alamouti Scheme with Transmit Antenna Selection for M-ray Signals

In this letter, we present a comprehensive performance analysis of orthogonal space-time block codes (STBCs) with transmit antenna selection under uncorrelated Rayleigh fading channels. Two best transmit antennas that maximize the instantaneous received signal-to-noise (SNR) are selected. Using the well-known moment generating function-based analysis approach, we derive the exact average symbol error rate (SER) for M-ary signals. Furthermore, we provide tight upper bounds on the SER for any number of transmit antennas and receive antennas. The tightness is verified by simulation results. It is shown that the diversity order, with antenna selection, is maintained as that of the full complexity systems