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75
A RateAdaptive MAC Protocol for MultiHop Wireless Networks
, 2001
"... Wireless local area networks (WLANs) have become increasingly popular due to the recent availability of affordable devices that are capable of communicating at high data rates. These high rates are possible, in part, through new modulation schemes that are optimized for the channel conditions bring ..."
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Cited by 323 (5 self)
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Wireless local area networks (WLANs) have become increasingly popular due to the recent availability of affordable devices that are capable of communicating at high data rates. These high rates are possible, in part, through new modulation schemes that are optimized for the channel conditions bringing about a dramatic increase in bandwidth efficiency. Since the choice of which modulation scheme to use depends on the current state of the transmission channel, newer wireless devices often support multiple modulation schemes, and hence multiple data rates, with mechanisms to switch between them. Users are given the option to either select an operational data rate manually or to let the device automatically choose the appropriate modulation scheme (data rate) to match the prevailing conditions. Automatic rate selection protocols have been studied for cellular networks but there have been relatively few proposals for WLANs. In this paper we present a rate adaptive MAC protocol called the ReceiverBased AutoRate (RBAR) protocol. The novelty of RBAR is that its rate adaptation mechanism is in the receiver instead of in the sender. This is in contrast to existing schemes in devices like the WaveLAN II [15]. We show that RBAR is better because it results in a more efficient channel quality estimation which is then reected in a higher overall throughput Our protocol is based on the RTS/CTS mechanism and consequently it can be incorporated into many medium access control protocols including the widely popular IEEE 802.11 protocol. Simulation results of an implementation of RBAR inside IEEE 802.11 show that RBAR performs consistently well.
VariableRate VariablePower MQAM for Fading Channels
 IEEE Trans. Commun
, 1997
"... We propose a variablerate and variablepower MQAM modulation scheme for highspeed data transmission over fading channels. We first review results for the Shannon capacity of fading channels with channel side information, where capacity is achieved using adaptive transmission techniques. We then de ..."
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Cited by 316 (30 self)
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We propose a variablerate and variablepower MQAM modulation scheme for highspeed data transmission over fading channels. We first review results for the Shannon capacity of fading channels with channel side information, where capacity is achieved using adaptive transmission techniques. We then derive the spectral efficiency of our proposed modulation. We show that there is a constant power gap between the spectral efficiency of our proposed technique and the channel capacity, and this gap is a simple function of the required biterror rate (BER). In addition, using just five or six different signal constellations, we achieve within 12 dB of the maximum efficiency using unrestricted constellation sets. We compute the rate at which the transmitter needs to update its power and rate as a function of the channel Doppler frequency for these constellation sets. We also obtain the exact efficiency loss for smaller constellation sets, which may be required if the transmitter adaptation rate is constrained by hardware limitations. Our modulation scheme exhibits a 510dB power gain relative to variablepower fixedrate transmission, and up to 20 dB of gain relative to nonadaptive transmission. We also determine the effect of channel estimation error and delay on the BER performance of our adaptive scheme. We conclude with a discussion of coding techniques and the relationship between our proposed modulation and Shannon capacity.
Multiuser OFDM with Adaptive Subcarrier, Bit, and Power Allocation
 IEEE Journal on Selected Areas of Communications
, 1999
"... Multiuser orthogonal frequency division multiplexing (OFDM) with adaptive multiuser subcarrier allocation and adaptive modulation is considered. Assuming knowledge of the instantaneous channel gains for all users, we propose a multiuser OFDM subcarrier, bit, and power allocation algorithm to minimiz ..."
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Cited by 187 (1 self)
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Multiuser orthogonal frequency division multiplexing (OFDM) with adaptive multiuser subcarrier allocation and adaptive modulation is considered. Assuming knowledge of the instantaneous channel gains for all users, we propose a multiuser OFDM subcarrier, bit, and power allocation algorithm to minimize the total transmit power. This is done by assigning each user a set of subcarriers and by determining the number of bits and the transmit power level for each subcarrier. We obtain the performance of our proposed algorithm in a multiuser frequency selective fading environment for various time delay spread values and various numbers of users. The results show that our proposed algorithm outperforms multiuser OFDM systems with static timedivision multiple access (TDMA) or frequencydivision multiple access (FDMA) techniques which employ fixed and predetermined timeslot or subcarrier allocation schemes. We have also quantified the improvement in terms of the overall required transmit power, the biterror rate (BER), or the area of coverage for a given outage probability.
Adaptive Coded Modulation for Fading Channels
 IEEE TRANS. COMMUN
, 1998
"... We apply coset codes to adaptive modulation in fading channels. Adaptive modulation is a powerful technique to improve the energy efficiency and increase the data rate over a fading channel. Coset codes are a natural choice to use with adaptive modulation since the channel coding and modulation desi ..."
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Cited by 130 (10 self)
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We apply coset codes to adaptive modulation in fading channels. Adaptive modulation is a powerful technique to improve the energy efficiency and increase the data rate over a fading channel. Coset codes are a natural choice to use with adaptive modulation since the channel coding and modulation designs are separable. Therefore, trellis and lattice codes designed for additive white Gaussian noise (AWGN) channels can be superimposed on adaptive modulation for fading channels, with the same approximate coding gains. We first describe the methodology for combining coset codes with a general class of adaptive modulation techniques. We then apply this methodology to a spectrally efficient adaptive Mary quadrature amplitude modulation (MQAM) to obtain trelliscoded adaptive MQAM. We present analytical and simulation results for this design which show an effective coding gain of 3 dB relative to uncoded adaptive MQAM for a simple fourstate trellis code, and an effective 3.6dB coding gain for an eightstate trellis code. More complex trellis codes are shown to achieve higher gains. We also compare the performance of trelliscoded adaptive MQAM to that of coded modulation with builtin time diversity and fixedrate modulation. The adaptive method exhibits a power savings of up to 20 dB.
Degrees of freedom in adaptive modulation: a unified view
 IEEE Trans. Commun
, 2001
"... Abstract—We examine adaptive modulation schemes for flatfading channels where the data rate, transmit power, and instantaneous BER are varied to maximize spectral efficiency, subject to an average power and BER constraint. Both continuousrate and discreterate adaptation are considered, as well as ..."
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Cited by 124 (4 self)
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Abstract—We examine adaptive modulation schemes for flatfading channels where the data rate, transmit power, and instantaneous BER are varied to maximize spectral efficiency, subject to an average power and BER constraint. Both continuousrate and discreterate adaptation are considered, as well as average and instantaneous BER constraints. We find the general form of power, BER, and data rate adaptation that maximizes spectral efficiency for a large class of modulation techniques and fading distributions. The optimal adaptation of these parameters is to increase the power and data rate and decrease the BER as the channel quality improves. Surprisingly, little spectral efficiency is lost when the power or rate is constrained to be constant. Hence, the spectral efficiency of adaptive modulation is relatively insensitive to which degrees of freedom are adapted. Index Terms—Adaptive modulation, communication systems, fading channels, spectral efficiency. I.
Capacity of Rayleigh Fading Channels under Different Adaptive Transmission and . . .
 IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY
, 1999
"... We study the Shannon capacity of adaptive transmission techniques in conjunction with diversity combining. This capacity provides an upper bound on spectral efficiency using these techniques. We obtain closedform solutions for the Rayleigh fading channel capacity under three adaptive policies: opti ..."
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Cited by 86 (7 self)
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We study the Shannon capacity of adaptive transmission techniques in conjunction with diversity combining. This capacity provides an upper bound on spectral efficiency using these techniques. We obtain closedform solutions for the Rayleigh fading channel capacity under three adaptive policies: optimal power and rate adaptation, constant power with optimal rate adaptation, and channel inversion with fixed rate. Optimal power and rate adaptation yields a small increase in capacity over just rate adaptation, and this increase diminishes as the average received carriertonoise ratio (CNR) or the number of diversity branches increases. Channel inversion suffers the largest capacity penalty relative to the optimal technique, however, the penalty diminishes with increased diversity. Although diversity yields large capacity gains for all the techniques, the gain is most pronounced with channel inversion. For example, the capacity using channel inversion with twobranch diversity exceeds that of a singlebranch system using optimal rate and power adaptation. Since channel inversion is the least complex scheme to implement, there is a tradeoff between complexity and capacity for the various adaptation methods and diversitycombining techniques.
Adaptive Modulation over Nakagami Fading Channels
, 1998
"... We first study the capacity of Nakagami multipath fading (NMF) channels with an average power constraint for three power and rate adaptation policies. We obtain closedform solutions for NMF channel capacity for each power and rate adaptation strategy. Results show that rate adaptation is the key to ..."
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Cited by 75 (5 self)
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We first study the capacity of Nakagami multipath fading (NMF) channels with an average power constraint for three power and rate adaptation policies. We obtain closedform solutions for NMF channel capacity for each power and rate adaptation strategy. Results show that rate adaptation is the key to increasing link spectral efficiency. We analyze therefore the performance of constantpower variablerate MQAM schemes over NMF channels. We obtain closedform expressions for the outage probability, spectral efficiency and average biterrorrate (BER) assuming perfect channel estimation and negligible time delay. We also analyze the impact of time delay on the BER of adaptive MQAM. Keywords Link Spectral Efficiency, Adaptive Modulation Techniques, and Nakagami Fading. I. Introduction The radio spectrum available for wireless services is extremely scarce, while demand for these services is growing at a rapid pace [1]. Hence spectral efficiency is of primary concern in the design of fut...
Giannakis, “Crosslayer combining of adaptive modulation and coding with truncated ARQ over wireless links
 IEEE Trans. Wireless Commun
, 2004
"... Abstract—We developed a crosslayer design which combines adaptive modulation and coding at the physical layer with a truncated automatic repeat request protocol at the data link layer, in order to maximize spectral efficiency under prescribed delay and error performance constraints. We derive the a ..."
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Cited by 56 (10 self)
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Abstract—We developed a crosslayer design which combines adaptive modulation and coding at the physical layer with a truncated automatic repeat request protocol at the data link layer, in order to maximize spectral efficiency under prescribed delay and error performance constraints. We derive the achieved spectral efficiency in closedform for transmissions over Nakagami block fading channels. Numerical results reveal that retransmissions at the data link layer relieve stringent error control requirements at the physical layer, and thereby enable considerable spectral efficiency gain. This gain is comparable with that offered by diversity, provided that the maximum number of transmissions per packet equals the diversity order. Diminishing returns on spectral efficiency, that result when increasing the maximum number of retransmissions, suggest that a small number of retransmissions offers a desirable delaythroughput tradeoff, in practice. Index Terms—Adaptive modulation and coding (AMC), automatic repeat request (ARQ) protocol, crosslayer design, quality of service (QoS), wireless networks. I.
A gametheoretic approach to energyefficient power control in multicarrier CDMA systems
 IEEE Journal on Selected Areas in Communications (JSAC
, 2006
"... Abstract—A gametheoretic model for studying power control in multicarrier codedivision multipleaccess systems is proposed. Power control is modeled as a noncooperative game in which each user decides how much power to transmit over each carrier to maximize its own utility. The utility function co ..."
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Cited by 42 (8 self)
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Abstract—A gametheoretic model for studying power control in multicarrier codedivision multipleaccess systems is proposed. Power control is modeled as a noncooperative game in which each user decides how much power to transmit over each carrier to maximize its own utility. The utility function considered here measures the number of reliable bits transmitted over all the carriers per joule of energy consumed and is particularly suitable for networks where energy efficiency is important. The multidimensional nature of users ’ strategies and the nonquasiconcavity of the utility function make the multicarrier problem much more challenging than the singlecarrier or throughputbasedutility case. It is shown that, for all linear receivers including the matched filter, the decorrelator, and the minimummeansquareerror detector, a user’s utility is maximized when the user transmits only on its “best ” carrier. This is the carrier that requires the least amount of power to achieve a particular target signaltointerferenceplusnoise ratio at the output of the receiver. The existence and uniqueness of Nash equilibrium for the proposed power control game are studied. In particular, conditions are given that must be satisfied by the channel gains for a Nash equilibrium to exist, and the distribution of the users among the carriers at equilibrium is characterized. In addition, an iterative and distributed algorithm for reaching the equilibrium (when it exists) is presented. It is shown that the proposed approach results in significant improvements in the total utility achieved at equilibrium compared with a singlecarrier system and also to a multicarrier system in which each user maximizes its utility over each carrier independently. Index Terms—Energy efficiency, game theory, multicarrier codedivision multipleaccess (CDMA), multiuser detection, Nash equilibrium, power control, utility function. I.
Effect of Channel Estimation Error on MQAM BER Performance in . . .
 IEEE TRANSACTIONS ON COMMUNICATIONS
, 1999
"... We determine the biterror rate (BER) of multilevel quadrature amplitude modulation (MQAM) in flat Rayleigh fading with imperfect channel estimates. Despite its high spectral efficiency, MQAM is not commonly used over fading channels because of the channel amplitude and phase variation. Since the ..."
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Cited by 33 (0 self)
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We determine the biterror rate (BER) of multilevel quadrature amplitude modulation (MQAM) in flat Rayleigh fading with imperfect channel estimates. Despite its high spectral efficiency, MQAM is not commonly used over fading channels because of the channel amplitude and phase variation. Since the decision regions of the demodulator depend on the channel fading, estimation error of the channel variation can severely degrade the demodulator performance. Among the various fading estimation techniques, pilot symbol assisted modulation (PSAM) proves to be an effective choice. We first characterize the distribution of the amplitude and phase estimates using PSAM. We then use this distribution to obtain the BER of MQAM as a function of the PSAM and channel parameters. By using a change of variables, our exact BER expression has a particularly simple form that involves just a few finiterange integrals. This approach can be used to compute the BER for any value of M. We compute the BER for 16QAM and 64QAM numerically and verify our analytical results by computer simulation. We show that for these modulations, amplitude estimation error leads to a 1dB degradation in average signaltonoise ratio and combined amplitudephase estimation error leads to 2.5dB degradation for the parameters we consider.