Sarah Kate Wilson

Opportunistic Beamforming and Scheduling for OFDMA Systems

Orthogonal frequency-division multiple access (OFDMA) is an attractive technique for exploiting multiuser diversity in the downlink of a cellular system. This paper addresses three problems in multiuser diversity for OFDMA systems. First, we propose a way to significantly reduce the amount of channel state information (CSI) feedback without sacrificing performance too much, by selective and adaptive feedback. Second, we propose a way to increase the cell throughput and fairness by applying an opportunistic beamforming scheme to orthogonal frequency-division multiplexing. This beamforming scheme increases the frequency fading rate, which increases the multiuser diversity effect. Thirdly, we deal with the issue of fairness and quality-of-service (QoS) in opportunistic systems by proposing a modified proportional fair (PF) scheduler for OFDMA. Key features in the scheduler are that it incorporates QoS classes into the PF scheduler and that it has a tunable fairness level. Extensive simulation results are presented to evaluate the performance of the proposed schemes. The opportunistic beamforming scheme performed well in comparison with several other schemes. The modified PF scheduler was able to give users different QoS, based on their requirements, while still exploiting multiuser diversity

A simplified opportunistic feedback and scheduling scheme for OFDM

Opportunistic beamforming schedules users when they experience a high instantaneous signal-to-noise ratio. Multiple antennas at the transmitter can he used to induce temporal fading to ensure that all users fade at a rate fast enough to ensure fairness. Because feedback is required, the fading rate must be fast enough to ensure fairness among users, but slow enough so that the feedback information is not out of date. An OFDM system with opportunistic beamforming has the advantage that multiple users can be scheduled at the same time in a frequency-selective channel, thus allowing a slower fading rate. However, the overhead cost of feeding back every sub-carrier for every user is very high. We propose a simplified opportunistic feedback scheme that divides the OFDM symbol into clusters. Each user feeds back a figure-of-merit listing its strongest clusters. This scheme greatly reduces the feedback overhead, without sacrificing performance significantly. In addition, the scheme has an inherent on/off waterfilling property. We compare this simplified feedback scheme in a HIPERLAN/2 scenario to a feeding back of all subcarriers and also to a smart antenna system and show that, when there are many users, it outperforms the smart antenna system.

Comparison of Orthogonal Frequency-Division Multiplexing and Pulse-Amplitude Modulation in Indoor Optical Wireless Links

We evaluate the performance of three direct-detection orthogonal frequency-division multiplexing (OFDM) schemes in combating multipath distortion in indoor optical wireless links, comparing them to unipolar M-ary pulse-amplitude modulation (M-PAM) with minimum mean-square error decision-feedback equalization (MMSE-DFE). The three OFDM techniques are DC-clipped OFDM and asymmetrically clipped optical OFDM (ACO-OFDM) and PAM-modulated discrete multitone (PAM-DMT). We describe an iterative procedure to achieve optimal power allocation for DC-OFDM. For each modulation method, we quantify the received electrical SNR required at a given bit rate on a given channel, considering an ensemble of 170 indoor wireless channels. When using the same symbol rate for all modulation methods, M-PAM with MMSE-DFE has better performance than any OFDM format over a range of spectral efficiencies, with the advantage of (M-PAM) increasing at high spectral efficiency. ACO-OFDM and PAM-DMT have practically identical performance at any spectral efficiency. They are the best OFDM formats at low spectral efficiency, whereas DC-OFDM is best at high spectral efficiency. When ACO-OFDM or PAM-DMT are allowed to use twice the symbol rate of M-PAM, these OFDM formats have better performance than M-PAM. When channel state information is unavailable at the transmitter, however, M-PAM significantly outperforms all OFDM formats. When using the same symbol rate for all modulation methods, M-PAM requires approximately three times more computational complexity per processor than all OFDM formats and 63% faster analog-to-digital converters, assuming oversampling ratios of 1.23 and 2 for ACO-OFDM and M-PAM, respectively. When OFDM uses twice the symbol rate of M-PAM, OFDM requires 23% faster analog-to-digital converters than M-PAM but OFDM requires approximately 40% less computational complexity than M-PAM per processor.

Zipper: a duplex method for VDSL based on DMT

We present a new duplex scheme, called Zipper, for discrete multitone (DMT)-based very high bit-rate digital subscriber line (VDSL) systems on copper wires. This scheme divides the available bandwidth by assigning different subcarriers for the upstream and downstream directions. It has high flexibility to divide the capacity between the up and downstream, as well as good coexistence possibilities with other systems such as ADSL. Simulation results show the high bit-rate performance in different environments such as mixed ADSL and VDSL traffic under radio frequency interference and with different background noise sources.

Analysis of intertone and interblock interference in OFDM when the length of the cyclic prefix is shorter than the length of the impulse response of the channel

Orthogonal frequency division multiplexing (OFDM) is a modulation technique that resists the effects of a multipath channel when provided an ample cyclic prefix. No intersymbol interference is caused in channels whose impulse responses are shorter than the cyclic prefix length. However interference arises when this is not the case. This paper describes the degradation of the system under this scenario. In particular, simulations describing a suburban hilly environment are done for several cyclic prefix lengths.

A QoS-aware proportional fair scheduler for opportunistic OFDM

OFDM is an attractive technique to implement multiuser diversity for the downlink. This paper deals with the problem of combining the increased cell throughput of multiuser diversity schemes with a fair distribution of resources among the users. A proportional fair scheduler for opportunistic OFDM is proposed. Its key features are that it can accommodate several quality-of-service (QoS) classes, that it has a tunable fairness level, and that it can be integrated with an opportunistic beamformer to increase system fairness. Simulation results show that the cell throughput of the opportunistic scheme approaches that of a more complex smart antenna scheme for many users. For a densely populated system, the proposed scheme shows a graceful QoS degradation, possibly leading to a high average user satisfaction. The fairness parameter effectively tunes the scheduler between high cell throughput and high fairness.

Transmitter and receiver methods for improving asymmetrically-clipped optical OFDM

This paper introduces a new constellation mapping scheme for asymmetrically-clipped optical OFDM (ACO-OFDM) that puts more bits on the low-frequency subcarriers. The lowpass nature of the wireless optical channel motivates the use of this generic bit-loading scheme. This scheme has no knowledge of the channel at the receiver; it performs significantly better than the traditional constant constellation mapping on all subcarriers in a variety of channels. The average bit-error rate (BER) curves for the new modulation scheme have up to a 5 dB improvement for signal-to-noise ratios (SNRs) where the BER is in the order of 10-3. In addition to the generic low-frequency bit-loading, this paper also proposes clipping the ACO-OFDM signal at the receiver to mitigate the additive electronic noise.

Novel techniques of single-carrier frequency-domain equalization for optical wireless communications

We investigate the use of single carrier frequency domain equalization (SCFDE) over a diffuse optical wireless (DOW) communications. Recently orthogonal frequency division multiplexing (OFDM) has been applied to DOW communications. However, due to high peak-to-average power ratio (PAPR), the performance of OFDM can severely be affected by the nonlinear characteristics of light emitting diodes (LED). To avoid a PAPR problem, we present in this paper a modified form of SCFDE for DOW communications. We propose three different ways of using SCFDE with DOW communications and show that they exhibit lower PAPR and provide better bit-error rate (BER) performance in the presence of the LED nonlinearity.

On the use of a cyclic extension in OFDM

OFDM (orthogonal frequency division multiplexing) is an effective technique for achieving reliable high-bit-rate transmission over wireless channels. Generally, for wireless applications, a guard interval, most often in the form of a cyclic extension of the OFDM data symbol, is inserted between OFDM blocks to minimize interblock interference. We quantify the penalties incurred when a zero-padded guard interval is used instead of a cyclic extension. Specifically, the resulting interchannel interference is computed and the degradation in the bit error rate is quantified.

Digital Modulation Techniques for Optical Asymmetrically-Clipped OFDM

We present a new modulation scheme for asymmetrically-clipped optical OFDM (ACO-OFDM). We show that a generic bit-loading scheme that puts more bits on the low-frequency subcarriers is more efficient for wireless optical channels than the constant modulation scheme typically used in radio frequency (RF) OFDM schemes. We discuss the low- pass nature of the wireless optical channel that motivates the use of this generic bit-loading scheme. This scheme performs significantly better than the traditional constant modulation on all tones in a variety of channels. The average Bit-error rate (BER) curves for the new modulation scheme have up to a 5 dB improvement for Signal-to-Noise Ratios where the BER is in the order of 10 -3.