Добірка наукової літератури з теми "Wideband Rate Adaptation"

We propose a concept system termed distributed base station (DBS) which enables distributed transmit beamforming at large carrier wavelengths to achieve significant range extension and/or increased downlink data rate, providing a low-cost infrastructure for applications such as rural broadband. We consider a frequency division duplexed (FDD) system using feedback from the receiver to achieve the required phase coherence. At a given range, N cooperating transmitters can achieve N2-fold increase in received power compared to that for a single transmitters, and feedback-based algorithms with near-ideal performance have been prototyped. In this paper, however, we identify and address key technical issues in translating such power gains into range extension via a DBS. First, to combat the drop in per-node SNR with extended range, we design a feedback-based adaptation strategy that is suitably robust to noise. Second, to utilize available system bandwidth, we extend narrowband adaptation algorithms to wideband channels through interpolation over OFDM subcarriers. Third, we observe that the feedback channel may become a bottleneck unless sophisticated distributed reception strategies are employed, but show that acceptable performance can still be obtained with standard uplink reception if channel time variations are slow enough. We quantify system performance compactly via outage capacity analyses.

碩士
國立清華大學
電機工程學系
89
In this thesis, we investigate real-time interference measurement techniques for the 3rd generation wireless communication Wideband CDMA networks. The WCDMA system has versatile services with different data rates corresponding to different channel qualities. Obviously, a real-time channel quality measurement technique is the key to the success of effectively providing the wireless multimedia services in the WCDMA system. The basic idea of interference measurement comes from solving the signal-to-interference ratio (SIR) estimation problem. Several SIR estimation methods have been proposed for TDMA and CDMA cellular systems. We extend a low complexity method called interference projection, which was originally proposed for TDMA system, to CDMA system under a frequency-selective fading channel environment. However, the method requires different training sequences for different users. Since there is no obvious so-called training sequence in CDMA system, a possible solution is to use different patterns of pilot bits to replace the training sequences. Nevertheless, there are only very few pilot patterns in the WCDMA system, this solution may not be feasible to support a large number of users. Besides, using pilot bits for measuring the interference is difficult to achieve the goal of real-time measurement because signals are processed at the symbol level. To make it more practically and meet the demand of in-service real-time measuring, we proposed an innovative chip-based interference measurement technique. The key concept of this proposed method is to use the different subscribers’different spreading codes to replace the training sequences in the interference projection method. There are two major advantages for this solution. First, all the users’spreading codes are different and known only to the receiver for the user of interest. Second, since the signals are processed at chip-level, it is possible for the measurement to converge within a certain number of chip durations thereby achieving the goal of real-time measurement. We evaluate the chip-based interference measurement technique in the frequency-selective fading channel for the Wideband CDMA system. For both uplink and downlink cases, we apply our algorithms under different combination of spreading and scrambling codes. It will be demonstrated that the proposed methods can obtain the interference measurement within 0.1 msec with an average percentage error less than 5%. Therefore, we can easily achieve the requirement of estimating the SIR for every 0.667 msec time slot specified in the Wideband CDMA system. As a result, by employing the proposed chip-based interference measurement, many radio resource management algorithms, such as rate adaptation, power control, handover, and so on, can be enhanced to the slot-by-slot based adaptation algorithms to take advantage of better knowledge of channel quality. Hence, it can be expected that the proposed chip-based interference measurement technique will be possible to play a key role to fulfill the dream of high speed wireless multimedia communications in the future.

Current and next generation wireless cellular systems strive to maximize spectral efficiency and meet the increasing demand for higher data rates despite being severely constrained by the limited availability of spectrum. Rate adaptation and scheduling are two key enabling techniques to achieve these challenging goals. In the former, the modulation and coding scheme (MCS) are adapted as a function of the instantaneous channel conditions, while in the latter, the base station (BS) adapts the user to which it transmits to. While appealing, several practical challenges arise in implementing rate adaptation and scheduling in current generation cellular systems that employ orthogonal frequency division multiplexing (OFDM). First, the symbols of a transmitted packet can experience different signal-to-noise ratios (SNRs) in a frequency-selective channel. Thus, rate adaptation in these systems is driven by the vector of SNRs experienced by the packets and is referred to as wideband rate adaptation. It is considerably more involved than conventional narrowband rate adaptation, which is based on a single SNR. Second, the channel state information (CSI) at the BS, which it obtains through feedback from the users, is outdated due to the delay between the time of channel estimation and the time of data transmission. This can lead to incorrect adaptations and a significant reduction in system throughput. Third, the CSI feedback in the uplink is limited in order to conserve uplink radio resources. The lack of enough CSI at the BS due to this can also degrade the throughput. We develop a new analytical framework to characterize the throughput of wideband rate adaptation. The framework employs the exponential effective SNR mapping (EESM), which simplifies the rate adaptation problem by mapping the vector of SNRs into a single equivalent flat-fading SNR. However, even rate adaption using EESM is challenging owing to its non-linear form and its dependence on the MCS and has not been analysed before. Our framework leads to a new upper bound and an accurate approximation for the cell throughput. We then extend the framework to also incorporate feedback delays and co-channel interference from neighbouring cells. We observe that the decrease in throughput with feedback delays is more gradual for wideband rate adaptation compared to narrowband rate adaptation. Further, it depends on the scheduler employed by the BS. We then propose novel BS-side estimation techniques to mitigate the loss in throughput due to the reduced feedback from the users. We develop a new minimum mean square error estimator-based approach and subsequently a new throughput-optimal adaptation policy for the practically relevant best-M reduced feedback scheme. These approaches exploit the correlation between the subchannels, and the structure of the information fed back to improve the effectiveness of rate adaptation and scheduling without requiring any additional feedback. The throughput-optimal policy leads to a new benchmark for the achievable cell throughput in systems that employ reduced feedback schemes.

Recent years have witnessed the rapid progress in handheld devices. This has resulted in a growing number of mobile phones or PDAs that have a built-in camera to record still pictures or live videos. Encouraged by the success of second generation cellular wireless networks, researchers are now pushing the 3G standard to support a seamless integration of multimedia data services. One of the main products is WCDMA (Holma & Toskala, 2001), short for wideband code division multiple access. WCDMA networks have 80 million subscribers in 46 countries at the time of this writing. WCDMA can be viewed as a successor of the 2G CDMA system. In fact, many WCDMA technologies can be traced back to the 2G CDMA system. However, WCDMA air interface is specifically designed with envision to support real time multimedia services. To name some highlights, WCDMA: • Supports both packet-switched and circuitswitched data services. Mobile best-effort data services, such as Web surfing and file downloads, are available through packet service. • Has more bandwidth allocated for downlink and uplink than the 2G systems. It uses a 5 MHz wide radio signal and a chip rate of 3.84 mcps, which is about three times higher than CDMA2000. • Support a downlink data rate of 384 kbps for wide area coverage and up to 2 Mbps for hot-spot areas, which is sufficient for most existing packet-data applications. WCDMA Release 5 (Erricson, 2004) adopts HSDPA (High-speed downlink packet access), which increases peak data rates to 14 Mbps in the downlink. To achieve high data rate, WCDMA uses several new radio interface technologies, including (1) shared channel transmission, (2) higher-order modulation, (3) fast link adaptation, (4) fast scheduling, and (5) hybrid automatic-repeat-request (HARQ). These technologies have been successfully used in the downlink HSDPA, and will be used in upcoming improved uplink radio interface in the future. The rest of this article will explain the key components of the radio interface in WCDMA.