Wireless Communications

It is shown that, particularly for terrestrial cellular telephony, the interference-suppression feature of CDMA (code division multiple access) can result in a many-fold increase in capacity over analog and even over competing digital techniques. A single-cell system, such as a hubbed satellite network, is addressed, and the basic expression for capacity is developed. The corresponding expressions for a multiple-cell system are derived. and the distribution on the number of users supportable per cell is determined. It is concluded that properly augmented and power-controlled multiple-cell CDMA promises a quantum increase in current cellular capacity. >

On the capacity of a cellular CDMA system

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 time-division multiple access (TDMA) or frequency-division multiple access (FDMA) techniques which employ fixed and predetermined time-slot or subcarrier allocation schemes. We have also quantified the improvement in terms of the overall required transmit power, the bit-error rate (BER), or the area of coverage for a given outage probability.

/pdf/multiuser-ofdm-with-adaptive-subcarrier-bit-and-power-qfzvyfs4s3.pdf

Multiuser OFDM with adaptive subcarrier, bit, and power allocation

We propose a variable-rate and variable-power MQAM modulation scheme for high-speed 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 bit-error rate (BER). In addition, using just five or six different signal constellations, we achieve within 1-2 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 5-10-dB power gain relative to variable-power fixed-rate 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.

Variable-rate variable-power MQAM for fading channels

We obtain the Shannon capacity of a fading channel with channel side information at the transmitter and receiver, and at the receiver alone. The optimal power adaptation in the former case is "water-pouring" in time, analogous to water-pouring in frequency for time-invariant frequency-selective fading channels. Inverting the channel results in a large capacity penalty in severe fading.

/pdf/capacity-of-fading-channels-with-channel-side-information-xigg3wsr52.pdf

Capacity of fading channels with channel side information

We propose a variable-power and variable-rate coded MQAM modulation technique for high-speed data transmission on fading channels. Coding gain is obtained by superimposing trellis codes designed for AWGN channels on the adaptive modulation, and we obtain the same coding gains as these codes exhibit in AWGN. We present analytical and simulation results which show a 3dB coding gain relative to uncoded adaptive modulation for a simple 4-state trellis code, and a 4 dB coding gain for an 8-state trellis code. More complex trellis codes achieve higher gains.

/pdf/adaptive-coded-modulation-for-fading-channels-53qcx31pb6.pdf

Adaptive coded modulation for fading channels

http://ieeexplore.ieee.org/iel3/3677/10813/00501424.pdf

Soon-Ghee Chua

Papers

Variable-rate variable-power MQAM for fading channels

Adaptive coded modulation for fading channels

Variable- Power MQAM for )Fading Channels