Performance of an Experimental Angle-Diversity Troposcatter System

Angle-diversity measurements on a 179-mi experimental tropospheric scatter radio link have been made from August 1970 through March 1971. The diversity configuration consisted of three elevation paths and one azimuthal path. Statistics on correlation coefficients, signal strengths, and outage performance are presented and interpreted. The choice of antenna pointing angle is considered and found to be lower for angle-diversity systems than for conventional systems. The presence of a long-term correlation advantage for angle diversity is noted and in a comparison of angle and conventional diversity the simpler angle diversity systems are shown to be as effective in providing outage protection as the conventional systems.     

Mobile radio slotted ALOHA with capture and diversity

In this paper, the slotted ALOHA protocol in a mobile radio environment, in the presence of Ricean fading, twofold antenna diversity and multiple reception capability, is considered. The capture probabilities and the average throughput are computed, and the issue of stability is addressed. It is shown that diversity can improve the system performance, although only to a certain extent. Some extensions are also considered: in particular, the use of a higher degree of diversity is investigated.A shorter version of this paper was presented atIEEE INFOCOM'95 This work has been partially supported by MURST, Italy.     

Maximum-likelihood diversity combining in partial-band noise

Maximum-likelihood diversity combining is investigated for an FFH/MFSK spread spectrum system in partial-band noise (PBN). The structure of maximum-likelihood diversity reception in PBN plus white Gaussian noise is derived. It is shown that signal-to-noise ratio and the noise variance at each hop have to be known to implement this optimum diversity combiner. Several suboptimum diversity combining schemes are also considered. The performance of the optimum combining scheme is evaluated. It is shown that adaptive gain control diversity combining actually achieves the optimum performance when interference is not very weak     

Performance Improvements Provided by Route Diversity in Multihop Wireless Networks

In multihop wireless networks, the variability of channels results in some paths providing better performance than other paths. Although it is well known that some paths are better than others, a significant number of routing protocols do not focus on utilizing optimal paths. However, cooperative diversity, which is an area of recent interest, provides techniques for efficiently exploiting path and channel diversity. This paper examines the potential performance improvements offered by path diversity. Three settings are examined, namely, where the path loss and channel correlation are neglected, where path loss is considered, but channel correlation is neglected, and where path loss and channel correlation are both accounted for. It is shown that, by exploiting path diversity, dramatic improvements in the considered route metric may be achieved. Furthermore, in some settings, if the link statistics are held constant, then when path diversity is exploited, the route metric improves with path length. This implies that, if links statistics are fixed and if sufficient path diversity exists, then paths with more hops tend to support higher bit rates than paths with fewer hops. It is shown that such behavior occurs when a particular map has a nonzero fixed point.     

Performance and Optimization of Switched Diversity Systems for the Detection of Signals with Rayleigh Fading

The performance and optimization of switched diversity systems are considered. First, the one-dimensional distribution and probability density functions of the envelope of the received signal are obtained for three different switching strategies. This information is used to obtain the average probability of bit error for the case of non-coherent detection of binary FSK signals with Rayleigh fading envelopes and additive white Gaussian noise. The optimization of two of these switching strategies is then considered, and it is shown that by proper selection of switching thresholds, the average probability of bit error during detection can be minimized. It is also shown that these optimized switching strategies yield a significant improvement in performance over non-diversity systems and can approach the performance of more complex receivers such as maximal ratio combining. Computer simulations of switched diversity systems using a practical field model are used to verify the analysis.     

Diversity Analysis of Relay Selection Schemes for Two-Way Wireless Relay Networks

One-way wireless relay networks have recently received a lot of attention due to their ability to provide spatial diversity in fading wireless environment. Moreover, performing single-relay selection is a very attractive method due to its cost effective implementation and superior performance. However, one-way relay networks with the half-duplex signalling suffer from a spectral efficiency loss. To overcome such a drawback, two-way wireless relay networks have been proposed and these are also the networks considered in this paper. The paper analyzes the diversity orders of various relay selection schemes, including the best-relay selection, best-worse-channel selection, and maximum-harmonic-mean selection. The analysis is done for the amplify-and-forward protocol and under the two-step and three-step transmission procedures. In particular, it is shown that full diversity orders of R and R + 1 can be achieved in a R-relay wireless network with the two-step and three-step procedures, respectively. Numerical and simulation results are provided to verify our analysis.     

Exploiting Multipath Diversity in Multiple Antenna OFDM Systems With Spatially Correlated Channels

Multiple antennas are useful in orthogonal frequency division multiplexing (OFDM) systems for providing transmit and receive diversity to overcome fading. Typically, these designs require considerable separation between the antennas. Spatial correlation is introduced when antennas are not well separated, and it often leads to performance degradation in a flat fading environment. However, in frequency selective fading channels with rich multipath diversity, OFDM receivers can overcome this performance degradation due to antenna correlation. This is due to transformation of a highly spatially correlated channel impulse response to a less spatially correlated channel frequency response inherently by an OFDM system in the presence of rich multipath diversity. We illustrate this for a simple receive diversity OFDM system and hence introduce the concept of space sampling at the receiver where antennas are placed relatively close to each other. The minimum separation required between the antennas under such circumstances is derived analytically, and it is shown that even with a separation of only$0.44lambda$, the required spatial correlation in the channel frequency response becomes sufficiently low. Simulated performance results with such spacing for various multiple antenna OFDM systems corroborate the analytical results.     

Switched Dual Diversity Reception of M-ary DPSK Signals on Nakagami Fading Channels

The paper deals with dual diversity reception of M-ary differential phase-shift keying modulated signals in the presence of additive white Gaussian noise and Nakagami-distributed slow and nonselective fading. The performance of a switched diversity system is analysed and compared to that of the predetection selection diversity combining scheme. The general case of correlated diversity branches is considered, without restrictions on the fading severity parameter. Average symbol error rate formulas are analytically derived in terms of integral expressions that can be easily computed via numerical integration routines. Moreover, the numerical evaluation of the optimum switching threshold is carried out and the influence of the fading severity parameter, the branch correlation, and the cardinality of the symbol alphabet is analysed. Finally, three fixed switching threshold strategies that allow to obtain a satisfactory diversity gain are considered.     

Hybrid diversity combining techniques for DS‐CDMA over a multipath fading channel

The combined effect of antenna diversity and spread spectrum (SS) diversity for a direct sequence code division multiple access (DS‐CDMA) system using differential binary phase‐shift keying (DBPSK) modulation is evaluated. Both maximal ratio combining (MRC) and selection diversity (SD) are considered and the bit error rate (BER) performance for four following diversity schemes are compared: (1) SS diversity with MRC and antenna diversity with SD; (2) SS diversity with SD and antenna diversity with MRC; (3) MRC of both SS diversity and antenna diversity; (4) SD of both SS diversity and antenna diversity. It is shown that antenna diversity with MRC and SS diversity with selection has a performance very close to that of antenna diversity and SS diversity both with MRC. This revised version was published online in July 2006 with corrections to the Cover Date.     

Outage probabilities of diversity cellular systems with cochannelinterference in Nakagami fading

Analytical, closed-form expressions for cellular outage probabilities in generalized Nakagami fading are derived for three practical diversity combining schemes. The outage is defined as the probability that the signal-to-interference power ratio (SIR) is less than a power protection ratio. The analysis considers L-branch equal gain (EG), selection (SC), and switched (SW) diversity combining schemes. The analyses are not limited to a single interferer, but rather assume the presence of multiple independent cochannel interferers. Previous results have used some approximations to study the performance of the EG combiner. A precise method is used to analyze the performance of an L-branch EG combiner. Selection diversity combining using the total power algorithm, the desired power algorithm, and the signal-to-interference power algorithm is analyzed. The effects of diversity on the reuse factor and on the spectrum efficiency of cellular mobile radio systems are considered in detail. The results for the Rayleigh fading channel are obtained and presented as a special case of the generalized Nakagami fading model     

Improved Group multiuser detection with multiple receive antennas in the presence of improper multi-access interference

We consider the transmission of K binary phase-shift keying signals over M diversity channels through a rich scattering environment. Improved group detection rules are proposed to mitigate the effect of the improper multiaccess interference, and an enhancement of the system receive diversity order is shown to be obtained. For example, it is demonstrated that a further gain of |g~|/2 per group is obtained compared with conventional group detection, where |g~| is the interfering group size. In addition, when there is only one antenna per group, the considered receiver with the improved detection rules is equivalent to the decorrelator. In this case, it is shown that (K-1) interferers can be ed out, and that an M-(K-1/2) receive diversity order is achieved, compared with only M-/sup 2/K+1 with the conventional detection rules. Numerical results will further put in evidence such gains.     

Power control and diversity in feedback communications over a fading channel

Pilot-symbol-assisted modulation system using feedback minimum mean-square error (MMSE) power control in subject to an unavoidable feedback delay, and in conjunction with diversity, is considered over a slow Rayleigh fading channel. Feedback MMSE power control is defined as a power-control function, with feedback MMSE predictions of the current channel fading gains as input that minimizes the system-error probability. The use of feedback requires causality, and an MMSE predictor has to be employed for the purpose of power control. Previously, in the literature, the predictor was used also in detection. The pilot-symbol system with MMSE power control is shown to achieve a clear performance improvement by employing a smoother, instead of the predictor, in detection. Furthermore, the performance loss caused by a feedback delay of 10%-20% from the channel coherence time appeared to be minor with reasonable bit-error rate levels. Finally, additional performance improvement using low-order diversity was shown to be considerable.     

On the use of repetition coding with binary digital modulations onmobile channels

Repetition, diversity, and single-error-correcting codes are examined for use with binary modulation techniques over a mobile communication fading channel. Both bit-error performance and bandwidth efficiency are considered for phase-shift keyed, differential phase-shift keyed, and frequency-shift keyed modulations. The error-correction capability of repetition coding is considered, and optimum repetition is found. M-tuple diversity with maximum-ratio-combining is then compared with repetition coding both with a single-error-correcting code and without coding. It is found that a minimum of a 10 dB signal-to-noise power ratio is required for an improved performance with repetition code or diversity. The repetition code is about 3 dB inferior to optimal diversity. An optimized design of moderate complexity and bandwidth expansion can be reached with twofold diversity and three repetitions as an inner layer of coding concatenated with an outer single-error-correcting code     

Performance analysis for transmit antenna diversity with/withoutchannel information

Transmit antenna diversity (TAD) for the downlink channel has been investigated to improve the performance of wireless communications using multiple transmit antennas. We present a performance analysis for TAD with/without channel information. For performance analysis, we use the pairwise error probability and cutoff rate. For the closed-loop TAD in frequency-division duplex (FDD) mode, the downlink channel information should be transmitted to the base station from the mobile station. The feedback channel information may be imperfect because of the feedback delay, quantization error, and feedback error. The impact of imperfect channel information has been considered to analyze the performance of the TAD. We also present a performance analysis for a TAD technique that does not use the channel information, called space-time transmit diversity (STTD). Looking at the cutoff rates for the TAD techniques, the trellis-coded modulation (TCM) has been considered for better performance. Simulation results are shown to see the impact of the TCM on TAD systems     

Novel diversity receivers in the presence of Gaussian channel estimation errors

Novel diversity receivers that operate in the presence of Gaussian channel estimation errors are proposed for L independent and identically distributed fading channels. Previous work concerned with channel estimation errors has mainly examined the performance of maximal ratio combining (MRC) with estimation errors. It is shown here that MRC is not optimal when estimation errors occur. Moreover, it is shown that better diversity receivers that operate in the presence of Gaussian channel estimation errors can be obtained by using knowledge of the channel estimate statistics. Numerical results show that the derived new diversity receivers can perform as much as 2.0 dB in signal-to-noise ratio better than the conventional MRC receiver in some cases.     

A New Soft‐Fusion Approach for Multiple‐Receiver Wireless Communication Systems

In this paper, a new soft‐fusion approach for multiple‐receiver wireless communication systems is proposed. In the proposed approach, each individual receiver provides the central receiver with a confidence level rather than a binary decision. The confidence levels associated with the local receiver are modeled by means of soft‐membership functions. The proposed approach can be applied to wireless digital communication systems, such as amplitude shift keying, frequency shift keying, phase shift keying, multi‐carrier code division multiple access, and multiple inputs multiple outputs sensor networks. The performance of the proposed approach is evaluated and compared to the performance of the optimal diversity, majority voting, optimal partial decision, and selection diversity in case of binary noncoherent frequency shift keying on a Rayleigh faded additive white Gaussian noise channel. It is shown that the proposed approach achieves considerable performance improvement over optimal partial decision, majority voting, and selection diversity. It is also shown that the proposed approach achieves a performance comparable to the optimal diversity scheme.     

Performance comparison of transmit diversity and beamforming for the downlink of DS-CDMA system

A theoretical analysis has been made to compare the capacity performances of transmit diversity and beamforming in the downlink of direct-sequence code-division multiple-access (DS-CDMA) system. The performances of selection transmit diversity (STD), beamforming (BF), and beam selection transmit diversity (BSTD), which is a hybrid scheme of STD and BF, have been studied at system level with capacity as the performance index. Both fading and interference are jointly considered in multipath propagation environment. It was found that the comparison result strongly relates to the availability of path diversity. The system capacity is maximized by the BF in the presence of path diversity, otherwise it is the BSTD that achieves the best overall capacity performance.     

Performance of robust metrics with convolutional coding anddiversity in FHSS systems under partial-band noise jamming

The performance of robust metrics (metrics that can be computed from the outputs of the matched filters only) with convolutional coding and diversity under worst-case partial-band noise jamming is analyzed. Both binary and dual-k convolutional codes employing these metrics with diversity are compared via Union-Chernoff bounds. The performances of metrics considered in the literature that assume perfect side-information are given for comparison purposes. It is found that there exist very good robust metrics that provide performance comparable to metrics using perfect side-information. Among the robust metrics considered, the self-normalized metric offers the best performance and achieves performance practically identical to that of the square-law-combining metric with perfect side-information for M=8     

Diversity and coding for FH/MFSK systems with fading and jamming.II. Selective diversity

For pt.1 see ibid., vol.COM-35, p.1329-41 (1987). A performance evaluation is presented for selective diversity with feedback for frequency-hopping M-ary frequency-shift-keyed systems operating over Rayleigh faded channels in the presence of partial-band noise and partial-band tone jamming. The behavior of uncoded and coded systems is studied. For coded systems, the performance is evaluated for hard-decision receivers without channel state information and soft-decision receivers with perfect jammer state information. The results demonstrate that the performance of uncoded FH/MFSK with selective diversity is unacceptable. However, this diversity technique can offer definite improvements for coded FH/MFSK systems. Specifically, the effectiveness of selective diversity signaling depends on the provision of a feedback channel between the transmitter and receiver to provide the transmitter with the fading gains of the independently faded channels. To obtain an improvement from the selective diversity signaling scheme described here, there must be multiple independently faded channels between the transmitter and receiver. If not, the performance of the selective diversity signaling scheme will be identical to the performance of FH/MFSK without diversity     

Modulation and Equalisation Considerations for High Performance Radio LANs

The European Telecommunications Standards Institute (ETSI) has recently defined a European standard for a high performance radio LAN (known as HIPERLAN). To operate as wired LAN replacements, these systems will operate at 5.2 GHz and support instantaneous bit rates of just under 24 Mb/s. To counteract the time dispersive nature of the indoor radio channel, the use of adaptive equalisation is suggested. In this paper a number of possible modulation and equalisation techniques are presented and, in particular, the Bit Error Rate (BER) performance of quasi-coherent GMSK combined with Decision Feedback Equalisation is explored through computer simulation. The trade off between symbol spaced and fractionally spaced equalisation is considered together with the importance of feedfoward and feedback synchronisation to the channel's power delay profile. The paper also includes a comparison of the RLS and LMS based training algorithms and compares the modem developed under the ESPRIT III LAURA project with that specified in HIPERLAN.The application of dual antenna diversity is investigated and its impact on the number of received error free data packets obtained as a function of signal leval and rms delay spread. The use of such diversity is shown to greatly improve the BER performance of a HIPERLAN modem. The problem of frequency offset is considered and modifications are proposed to the HIPERLAN frame structure to improve the receiver's tolerance to such errors. Important practical issues such as frame and symbol synchronisation, frequency offset correction and hardware implementation are discussed from both the LAURA and HIPERLAN viewpoint.