Clive Tang

Multiple users adaptive modulation schemes for MC-CDMA

Multi-carrier CDMA (MC-CDMA) unites CDMA and OFDM together and potentially offers the advantages of both technologies. Under a frequency-selective fading channel, the subchannel frequency functions have different amplitude and phase. While the frequency domain spreading process gains some advantage from the frequency diversity available, it does not make the most of the different sub-channel conditions. Instead, the subcarriers should be adaptively loaded according to their states. This would improve the performance in a way similar to what adaptive bit loading offers in OFDM. The paper investigates this topic and proposes a configuration that not only permits multiple users to employ adaptive modulation, but also leads to an equivalent sub-carrier concept that enables a group of sub-carriers to be represented by an equivalent sub-carrier of a conventional OFDM modem, thereby allowing various powerful bit/power loading schemes, originally developed for OFDM, to be directly deployed to MC-CDMA.

An intelligent learning scheme for adaptive modulation

Over the years adaptive modulation has been shown to improve data throughput and system capacity in a wireless system. The basic idea is to adapt the modulation and/or coding scheme to the fading channel quality, using different schemes for different channel conditions. Therefore one primary issue is to determine the switching thresholds between the modulation schemes. Typically these thresholds are fixed in a system. This paper proposes a novel adaptive learning scheme that is capable of dynamically adjusting the thresholds so as to maximize the throughput, while maintaining a target frame error rate at the same time. A key feature of the proposed self-learning scheme is that no dedicated training signal is required, instead it utilizes the long-term average throughput and frame error rate to continuously update the thresholds as the data is being transmitted.

An adaptive learning approach to adaptive OFDM

Orthogonal frequency division multiplex (OFDM) technique has been successfully deployed in indoor wireless LAN and outdoor broadcasting applications. It is well-known that under a frequency-selective fading channel the resultant sub-channel frequency functions are frequency-variant and may also be time-variant, i.e. the channel magnitude may be highly fluctuating across the sub-carriers and varies from symbol to symbol. Hence adaptive modulation may be appropriately applied to improve the error performance and data throughput in an OFDM modem by assigning different modulation and coding schemes to different sub-carriers. One fundamental issue in deploying adaptive modulation is to determine what modulation and coding scheme to use. Virtually all past investigations were either based on heuristic methods or analytical means. This paper proposes a novel adaptive learning control approach that is capable of dynamically adjusting the switching thresholds to improve or maximize the data throughput.

A decentralized adaptive learning scheme for adaptive OFDM

Orthogonal Frequency Division Multiplex (OFDM) technique has been successfully deployed in indoor wireless LAN and outdoor broadcasting applications. It is well-known that under a frequency-selective fading channel the resultant sub-channel frequency functions are frequency-variant and may also be time-variant. Hence adaptive modulation may be appropriately applied to improve the error performance and data throughput in an OFDM modem by assigning different modulation and coding schemes to different sub-carriers. One fundamental issue in deploying adaptive modulation is to determine what modulation and coding scheme to use according to the sub-channel SNR. Virtually all past investigations were either based on heuristic methods or analytical means. This paper proposes a novel decentralized adaptive learning scheme that is capable of dynamically adjusting the switching thresholds to improve or maximize the data throughput.

Performance of adaptive learning schemes for adaptive modulation

Adaptive modulation has emerged as a prime technique in wireless systems to improve system capacity and spectral efficiency. Its basic idea is to adapt the modulation scheme to the fading channel quality, using different schemes for different channel conditions. Therefore, a primary issue is to determine the thresholds for switching between the modulation schemes. Typically, these thresholds are fixed. Recently, a novel adaptive learning approach capable of dynamically adjusting the thresholds to maximize a performance criterion, for example the data throughput, was introduced (see Tang, C., 2001 Int. Conf. on Third Generation Wireless and Beyond, p.30-5, 2001; IEEE 54th Vehicular Technology Conf., vol.1, p.144-8, 2001). The paper presents further simulation results for this new method.