Literature review to Compare “Adaptive Beamforming” and any “multiple antenna methods” employing “adaptive modulation” or not employing.

1. Adaptive Beamforming + Adaptive Modulation

Title / Dynamic Spatial Subchannel Allocation with Adaptive Beamforming for MIMO/OFDM Systems
Author(s) / Ya-Han Pan, Khaled ben Letaief, Zhigang Cao
Publication (place & year if book) / IEEE Transactions on Wireless Communications
olume no, Month & Year, pages / Vol.3, No.6, November 2004
Problem being addressed / Challenge to provide high data rates with high QoS over hostile mobile environments with limited spectrum and ISI.
Purpose / Propose a new technique
Contributions /
  1. Propose a dynamic spatial subchannel allocation with adaptive beamforming for broadband OFDM wireless transmission system
  2. Considered the performance of the proposed system combine with adaptive modulation

System / technology used / Broadband OFDM, single user, downlink system, with M transmitting antennas and K receiving antennas, QPSK modulation, 128 subcarriers, 256 bits per OFDM symbol
Adaptive Modulation /
  • Adaptive beamforming based OFDM system at both BS and MS can be regard as an OFDM system with single transmission antenna at both MS and BS. So, can apply all adaptive modulation algorithms developed for OFDM systems with single transmission antenna.
  • Used fixed throughput based adaptive algo since most applications of broadband wireless systems such as speech and real-time multimedia in which the bit rate are generally fixed by compression algorithm, the transmission rate is constant
  • Maximize the maximum SNR on each subcarrier obtained by adaptive beamforming under the constraints of power and overall bit rate
  • Use Lagrange Multiplier to obtain optimal bit distribution and corresponding power distribution
  • Advantage: much lower computational complexity compared to other adaptive algo while maintaining a much better bit error rate
  • Under various configurations for a spectral efficiency of 2b/OFDM symbol with QAM
  • Each subcarrier in an OFDM block is assigned to carry 2, 4, and 6 b or 0 b for each modulated symbol, depending on the quality of the corresponding spatial subchannel.

Adaptive Beamforming /
  • Adaptively select the eigenvectors associated with the relatively large spatial subchannel eigenvalues to generate the beamforming weights at the mobile and basestations and then dynamically assigns the corresponding best spatial subchannels to transmit the OFDM block symbols.
  • The eigenvectors associated with the N largest eigenvalues (out of the 2N already identified largest eigenvalues) are adaptively selected to generate the beamforming weights at the mobile and base stations. The corresponding best spatial subchannels are always dynamically assigned for data transmission
  • Applying Eigenvalue Decomposition Method (EVD) on ith subcarrier channel matrix, ni parallel spatial subchannel can be generated at this frequency subcarrier
  • Each received spatial subchannel signal is effectively the transmitted symbol weighted by an equivalent channel gain and corrupted by white noise
  • Beamforming vector chosen to maximize the instantaneous SNR at the receiver array output on spatial subchannel l
  • The optimum transmit beamforming vector is the lth eigenvector corresponding to the lth largest eigenvalue of the ith subcarrier channel matrix
  • Conventional adaptive antenna-arryays-based OFDM system only deals with the spatial subchannels related to largest eigenvalues and neglect other spatial subhannels. It turns out that it is possible that the largest eigenvalues of some subcarriers are smaller than, say, the second or third largest eigenvalues of other subcarriers. Used statistical method to justify this
  • Only consider the two spatial subchannels corresponding to the largest and second largest eigenvalue on each subcarrier since the probability of third to be larger is low. Select the N best parallel spatial subchannels from N X 2 spatial subchannels corresponding to the largest and 2nd largest eigenvalues to transmit N parallel symbols
  • Knowledge of eigenvectors is need at the transmitter. Need to feedback the weight vectors or indexes if there is a look up table as the conventional approach. The only difference, it has double look-up table size.
  • Slight increase in system complexity

Mathematical techniques used /methodology / L-ray model multipath fading channel,
Assumptions / Perfect channel information is known at both base and mobile station
Key Results /
  1. proposed system can achieve better performance than an adaptive antenna-arrays-based OFDM system without dynamic spatial subchannel allocation over multipath fading channels
  2. proposed system is far less susceptible to feedback delay in rapid time-varying channels and a little more sensitive to channel estimation errors than conventional adaptive antenna-arrays-based OFDM systems.

Conclusions / The proposed system could be prove to be an effective scheme for providing significant increases in system’s capacity and bandwidth efficicency as well as improvement in QoS
Critique / Issue: DCA
  • Itulize adaptive beamforming in OFDM channel allocation system, shown improvement in result when combined with adaptive modulation

Title / Comparison of Adaptive Beamforming and Adaptive OSTBC with Outdated Channel Feedback
Author(s) / Youngwook Ko and Cihan Tepedelenlioglu, Arizona State University
Publication (place & year if book) / IEEE Conference Record of the Thirty-Eighth Asilomar Conference on Signals, Systems and Computers, 2004.
Volume no, Month & Year, pages / Volume 1, 7-10 Nov. 2004 Page(s):1232 - 1236 Vol.1
Problem being addressed / Feedback delay
Purpose /
  • compare the impact of outdated channel feedback on the achievable performance for adaptive systems using OSTBC and BF
  • determine which adaptive scheme is a better choice to exploit multiple antennas.

Contributions / illustrate the effects of feedback delay on the performance of A–BF and A–OSTBC
System/ technology used / MIMO flat Rayleigh fading channels, 2 & 4 transmit antennas, 1 receive antenna, constellation sizes M={0,4,16,64,256}, Desired BER: 10-4
Adaptive modulation /
  • optimize the switching thresholds to maximize the average throughput subject to an average BER constraint using Lagrange Multiplier
  • Average BER are fixed
  • Constant power, rate-adaptive system
  • M-QAM constellation points are chosen based on the estimated total received SNR at the receiver
  • Gray bit mapping

Adaptive BF /
  • b = a unit transmit beamforming vector
  • s = information-bearing symbol with normalized power chosen from one of M-QAM constellation points
  • The beamforming vector, b is chosen as the eigenvector corresponding to the largest eigenvalue of the complex Wishart matrix
  • Applying maximal ratio combining at the receiver
  • Receiver computes the estimate of the total received SNRand select the constellation Mj
  • For next symbol transmission, both chosen beamforming vector b and Mj constellation is feedback to the transmitter.

Mathematical techniques used /methodology /
  • optimize the switching thresholds to maximize the average throughput subject to an average BER constraint
  • Based on the closed–form expressions for average BER and throughput, we compare the average throughput for A–BF and A–OSTBC employing adaptive M–QAM when their average BERs are fixed.

Assumptions /
  • Perfect channel knowledge available at the receiver

Key results /
  • A–BF scheme outperforms the A–OSTBC scheme with a small feedback delay
  • A–OSTBC scheme achieves higher average throughput due to the reduced diversity advantage of A–BF when the feedback delay is large.

Conclusions
Critique / Issue: Outdated Channel Feedback
  • Combining the two techniques, comparing with STBC taking into account outdated channel feedback

Title / COMPARISON OF MULTIUSER DIVERSITY USING STBC AND TRANSMIT BEAMFORMING WITH OUTDATED FEEDBACK
Author(s) / Qian Ma and Cihan Tepedelenlio˘glu
Publication (place & year if book) / IEEE Conference
Volume no, Month & Year, pages / 2005
Problem being addressed / Channel feedback delay
Purpose / compare the spectral efficiency for space-time block coding (STBC) and transmit beamforming (BF) with outdated channel
feedback.
Contributions
System/ technology used / Multiuser diversity employing adaptive modulation
  • Scheduling approach: BS transmit to only one user who has the best instantaneous SNR, optimal in the sense of maximizing the throughput
  • Target BER= 10-4
  • Multiuser system with user K=50 users

Adaptive modulation /
  • Derive optimal thresholds to maximize spectral efficiency subject to an average BER constraint
  • Constant power, variable rate M-QAM
  • Discrete finite set of Gray-coded square M-QAM constellations with sizes in M={0, 4, 16, 64, 256}
  • Define a generic threshold to maintain the instantaneous BER below the target BER for all channel realization when there is no feedback delay

Mathematical techniques used /methodology /
  • Derive the optimal thresholds to maximize spectral efficiency subject to an average BER constraint, and illustrate the impact of feedback delay on the achievable multiuser diversity gain with either STBC or BF.
  • consider the optimal channel assigning strategy where the channel is assigned to the user with the greatest instantaneous SNR
  • With the objective of maximizing spectral efficiency under an average BER constraint with respect to the switching thresholds, we consider a robust constant power, variable rate M-QAM scheme that is less sensitive to feedback delay. Based on the closed-form expressions for average BER and average rate, we compare the spectral efficiency for STBC and transmit BF with outdated channel feedback in adaptive modulation systems achieving multiuser diversity.

Assumptions / Perfect channel estimation at the receiver, receiver feedback its own instantaneous received SNR to the BS over error free feedback path
Key results /
  • with perfect channel feedback, BF is the optimal, it outperforms STBC,
  • more transmit antennas yield higher spectral efficiency for BF. However, this becomes inverted using STBC, due to the effect of channel-hardening [1], which is different from the result for single-user systems, where no channel hardening effect occurs
  • With a small feedback delay, the BF scheme outperforms the STBC scheme. However, due to the reduced diversity order of BF, the STBC scheme achieves higher spectral efficiency as feedback delay becomes large enough, same phenomenon is also observed for single user systems
  • as the feedback delay becomes larger, the adaptive M-QAM system using the optimal thresholds converges to a non-adaptive M-QAM system with constellation _ _ , which is most often selected to satisfy the BER requirement
  • multiuser diversity brings less sensitivity to feedback delay, more users result in higher spectral efficiency for either STBC or BF

Conclusions /
  • BF scheme is more complex since the beamformer as well as the SNR estimate are fed back while only the SNR estimate is required for STBC
  • multiuser diversity brings less sensitivity to feedback delay. In a practical multiuser diversity system, more transmit antennas bring higher spectral efficiency for BF. But this becomes inverted using STBC, due to the effect of channel-hardening. With a small feedback delay, the BF scheme outperforms the STBC scheme, which is reversed at large delays.

Critique / Issue: Outdated channel feedback
Same technique as above, but focusing on multiuser diversity
Title / Comparative Study of Adaptive Beam-Steering and Adaptive Modulation-Assisted Dynamic Channel Allocation Algorithms
Author(s) / Jonathan Blogh, Peter. Cherriman, and Lajos Hanzo, Senior Member, IEEE
Publication (place & year if book) / IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY,
Volume no, Month & Year, pages / VOL. 50, NO. 2, MARCH 2001, page 398-415
Problem being addressed / Co-channel interference
Purpose / Comparative study on a range of dynamic channel allocation (DCA) algorithms, namely, distributed control and locally distributed control assisted DCA arrangements.
Contributions /
  • discussed various DCA techniques and compared their performance to FCA both with and without shadow fading and under both uniform and nonuniform traffic distributions
  • provide a performance comparison in terms of the carried teletraffic, call blocking probability, call dropping probability, probability of low-quality access, and outage probability for a range of DCA techniques under identical conditions, where all users seamlessly roam across the simulation area rather than simply assigning them to arbitrary, uncorrelated, but essentially stationary random positions.
  • demonstrated that the proposed burst-by-burstadaptive modulation scheme results in substantial teletraffic performance improvements.

System/ technology used /
  • locally optimized least interference algorithm (LOLIA), locally optimized most interference algorithm (LOMIA)
  • reallocation SINR threshold: average SINR required by the specific transceiver to maintain a 5% transmission FER.
  • Outage SINR: below which the system declares the radio channel has degraded to such a level as to cause a service outage.( Call terminated). Average SINR required to maintain a 10% FER .

Adaptive Modulation type /
  • Adaptive mod + power control: aims to minimize the probability that the SINR will drop below both the reallocation and outage SINR for given modulation mode
  • attempt to reduce the average transmission power, by determining the most appropriate combination of transmit power and modulation mode for the next transmit burst.
  • Not only combats the effect of a poor quality channel but also attempts to maximize the throughput while maintaining a given target FER
  • Trade-off between mean FER and data throughput, govern by the switching threshold
  • The switching algo attempt o minimize the transmission power while maintaining a high throughput with a less than 5% target FER
  • Target SINR : BPSK (21dB), 4-QAM (27dB, 16-QAM(32dB)

Adaptive beamforming /
  • employ (sample matrix inversion) SMI algorithm to calculate receive antenna array weights
  • assume up and downlink channels were identical, enabling the same antenna pattern to be used in both transmit and receive mode
  • 4 antenna elements, linear array, λ/2 element spacing
  • angle of arrival at the BS determined using geometrically based single bouncei elliptical model (GBSBEM)

Assumptions
Key results /
  • The dynamic nature and the small “exclusion zone” of the LOLIA using n=7 resulted in its superior performance under both uniform and nonuniform traffic conditions, and also in a slowfading propagation environment
  • The LOMIA with n=19 offered similar performance under nonuniform traffic conditions and under the uniform traffic distribution in the lenient scenario.
  • LOMIA did not perform well in the shadowing environment, with the LOLIA offering a significantly higher network capacity in the shadowing environment than the FCA algorithm or the LOMIA.
  • The benefits of using adaptive antenna arrays have been demonstrated, with improved network quality for an increased number of users, for both the FCA algorithm and the LOLIA.
  • These benefits were then extended through the use of adaptive modulation to further improve call quality while simultaneously increasing the data throughput and the number of users adequately supported by the network.

Conclusions
Further research /
  • A range of interesting further research problems arise when these algorithms are combined with statistical multiplexing algorithms or when burst-by-burst adaptive modems and slot classification algorithms are invoked, where more robust modem modes can be employed in strongly interfered channels while supporting more bandwidth-efficient modem modes in uninterfered slots.
  • Exploiting the spatial dimension with the aid of adaptive antennas is also a promising research area.
ISSUE: DCA
  • Using adaptive beam steering to improve the network quality and extended by using adaptive modulation to improve call quality

Title / On the Networking Performance of UTRA-like TDD and FDD CDMA Systems Using Adaptive Modulation and Adaptive Beamforming
Author(s) / Song Ni, Jonathan S. Blogh, Lajos Hanzo
Publication (place & year if book) / IEEE Semiannual Cehicular Technology Conference, VTC 2003
Volume no, Month & Year, pages / Volume 1, 22-25 April 2003 page 606-610
Problem being addressed / MS-MS interference (intra and intercell), BS-BS interference
Purpose
Contributions /
  • Characterize the capacity of an adaptive modulation assisted, beam steering aided TDD/CDMA system.
  • Studies the achievable network performance by simulation and compares it to that of FDD/UTRA system

System / technology used / Non-shadowed, log-normal shadowed, Sample Matrix Inversion (SMI)
Adaptive Modulation
Adaptive beamforming /
  • SMI algorithm
  • 8 bit BPSK reference signals to identify desired user
  • calculated only the receiver’s antenna weights ( used by base station in uplink)
  • antenna array weights were recalculated for every power control step

Mathematical techniques used /methodology / Performance metrics used are call dropping probability, Grade of Service,
Assumptions / The base station is equipped with the Minimum Mean Squared Error Block Decision Feedback Equaliser based MUD
Key Results / The capacity is poorer than UTRA-like FDD/CDMA because it suffers from the effects of the extra inter-cell interference.
Conclusions / The employment of adaptive arrays in conjunction with AQAM limited the detrimental effects of co-channel interference and resulted in performance improvements both in terms of achievable call quality and the system’s capacity
Critique / ISSUE: Performance of combining 2 methods (network layer analysis)

2. MIMO + Adaptive modulation

Title / Adaptive Modulation for Multiple Antenna Channels
Author(s) / June Chul Roh and Bhaskar D. Rao
Publication (place & year if book) / Conference Record of the Thirty-Sixth Asilomar Conference on Signals, Systems and Computers, 2002
Volume no, Month & Year, pages / Volume 1, 3-6 Nov. 2002 Page(s):526 - 530 vol.1
Problem being addressed
Purpose
Contributions /
  • derive anefficient bit allocation algorithmwhich maximizes the transmission rate for a given transmit power. The algorithm is generally a greedy algorithm; however, we derive a sufficient condition for the bit allocation algorithm to be globally optimal, which is found to be satisfied in all digital modulation schemes

System / technology used / multiple transmit and multiple receive antenna system
Mathematical techniques used /methodology
Assumptions / Instantaneons channel information known to both the receiver and
the transmitter
Key Results /
  • average transmission rate of the adaptive uncoded M-aryQAM is about 9 dB away from the channel capacity when thetarget symbol error probability is set to 10-5

Conclusions / a transmit beamforming scheme, which needs less channel information, gives a comparable performance when the power constraint is low