自适应TDD MIMO-OFDM系统中一种补偿干扰不对称的改进方法

在时分双工(TDD)系统中,由于信道的对称性,发射端无需通过反馈即能得到信道状态信息,并可以用来显著提高多输入多输出(MIMO)系统的频谱利用率.但是,上下行的干扰结构根本不相关.如果上下行干扰差别很大,利用反向测量决定的调制编码参数可能导致较大的误帧率.提出一种发射端能动态跟踪接收端实际干扰值简单的闭环反馈干扰自适应新方法,并研究其在具有HARQ的自适应TDD MIMO-OFDM系统中的性能.仿真结果表明,干扰估计值能很好地跟踪实际值,系统能始终满足误帧率指标要求.     

MIMO-OFDM快衰落信道的稀疏自适应感知估计

在多输入多输出（MIMO）-正交频分复用（OFDM） 系统中,怎样在较高频谱利用率的情况下对快时变信道进行较为准确的估计是一个具有挑战性的课题。该文在利用压缩感知理论可提高系统频谱利用率的基础上,提出了一种适合于快时变环境下MIMO-OFDM 系统的稀疏自适应信道估计方法。该方法不再受到奈奎斯特采样频率条件约束,避免了传统导频辅助信道估计方法频谱利用率低的缺点。该文方法通过构建多天线群时频结构特征稀疏基,利用多天线间和群时变OFDM符号内信道冲激响应具有更强稀疏性的特点,对MIMO-OFDM快衰落信道进行稀疏变换。由于实际MIMO-OFDM快衰落信道往往处于频率选择性、时变性和多种干扰并存的复杂环境,受到干扰的信道参数对系统而言是未知,采用该方法克服了现有基于压缩感知理论的信道估计方法需要预先知道信道冲激响应稀疏度才能重构信道参数的不足,在信道稀疏度未知道的情况下,运用稀疏自适应的方法来对不同时频结构特征的信道参数进行估计。仿真结果表明所提估计方法具有对快时变信道参数估计的鲁棒性和较高频谱利用率,且均方误差小。      

Superposition-based adaptive modulated space time block coding for MIMO-OFDM systems

We propose a superposition-based adaptive modulated STBC (SPAM-STBC) for MIMO-OFDM systems to improve adaptive modulation optimization of space time block coding (STBC). When transmit antennas have the different channel conditions, the previous adaptive modulated STBC selects the same modulation based on averaging of the multiple channel gains. If the different modulation is selected to each transmit antenna, the STBC decoding problem occurs. In this letter, we select the optimal modulation corresponding to each channel condition by the superpositioned space time encoding and decoding. Simulation shows the proposed SPAM-STBC scheme outperforms both the fixed and adaptive modulated STBC schemes by the maximum 0.407 bits/sec/Hz in terms of spectral efficiency.     

Average external interference in cellular radio CDMA systems

An analytical and numerical evaluation by Gilhoussen et. al.(see IEEE Trans. Veh. Technol, vol.40, no.2, p.303-3121, 1991) of the capacity of a cellular radio CDMA system has been proposed. This evaluation has shown the interest of using spread spectrum techniques in the area of cellular radio systems. However, considering the computations made by Gilhoussen et. al. for the reverse link, we note the following: the calculations lead at a given moment to the evaluation of the average external interference for which a quantity is claimed only intuitively to be an upper bound. We construct a sequence of approximations for the average external interference, where the average external interference is the sequence limit. The claimed upper bound of Gilhoussen is also the first element of this sequence of approximations. We show by numerical means that the sequence is decreasing which justifies the claim of Gilhoussen     

自适应干扰抵消系统的双边带调幅干扰抵消特性

根据系统模型给出了自适应干扰抵消系统针对双边带调幅制干扰时的稳态权值表达式,推导出干扰抵消比计算式,分析了影响干扰抵消效果的因素及其作用规律,并给出了提高干扰抵消比的设计思路和方法。对于双边带调幅制信号,通过提高系统增益来提高干扰抵消比,将受到信号带宽和等效传输延迟距离的制约。信号带宽越宽或等效传输延迟距离越大,通过增大增益所能实现的干扰抵消比提高越有限。对于一定的信号带宽,可以通过增益和等效传输延迟距离的联合设计来提高干扰抵消比。仿真结果验证了理论分析的正确性和可行性。     

一种新的MIMO-OFDM系统自适应快时变信道估计算法

为了能够获得较精确的快时变信道估计,利用模糊模型拟合快时变信道,提出了一种新的信道估计算法。算法采用自适应技术进行导频子载波频域传输函数模型参数的识别,然后通过插值拟合全部信道的频域传输函数。仿真结果表明,在系统多普勒频移小于0.1的情况下,信道估计的MSE性能得到改善。     

On the performance of adaptive modulation in cellular systems

Adaptive modulation techniques have the potential to substantially increase the spectrum efficiency and to provide different levels of service to users, both of which are considered important for third-generation cellular systems. In this work, we propose a general framework to quantify the potential gains of such techniques. Specifically, we study the throughput performance gain that may be achieved by combining adaptive modulation and power control. Our results show that: (1) using adaptive modulation even without any power control provides a significant throughput advantage over using signal-to-interference-plus-noise ratio (SINR) balancing power control and (2) combining adaptive modulation and a suitable power control scheme leads to a significantly higher throughput as compared to no power control or using SINR-balancing power control. The first observation is especially important from an implementation point of view. Adjusting the modulation level without changing the transmission power requires far fewer measurements and feedback as compared to the SINR-balancing power control or the optimal power control. Hence, it is significantly easier to implement. Although presented in the context of adaptive modulation, the results also apply to other variable rate transmission techniques, e.g., rate adaptive coding schemes, coded modulation schemes, etc. This work provides valuable insight into the performance of variable rate transmission techniques in multi-user environments     

Power allocation and adaptive modulation strategy for MIMO-OFDM systems with imperfect CSI

The presented scheme named M-CAP （Maximum CAPacity） uses the CSI （Channel State Information） and its statistics to deduce an equivalent channel according to which the transmit power is allocated to the subchannels. And then modulation scheme is determined adaptively according to the power allocated to each subchannel. The advantage of the M-CAP scheme is that it combines power allocation and adaptive modulation while maintaining a large capacity. We demonstrate by computer simulations that the proposed M-CAP scheme can significantly improve system performance compared with the traditional schemes.     

MIMO-OFDM系统中一种高效的分子带自适应比特功率分配算法

自适应比特、功率分配(ABPA)可根据各子载波在频率选择性衰落信道中不同的瞬时增益,动态地分配数据比特和发射功率,从而达到优化系统性能,提高频率效率的目的。针对MIMO-OFDM系统中ABPA算法计算复杂度高的问题,本文提出了一种高效分子带ABPA算法,该算法采用新颖的分簇随机选取的方式进行子带划分及等效,将以子载波为分配单元的Fischer算法扩展为以子带为分配单元的ABPA算法,并对此算法进行改进,使其可在一次迭代过程中剔除多个比特数小于等于0的不可用簇。仿真结果表明,该算法能够在保持系统性能不变的前提下,提高筛选效率,同时可通过选择不同的簇宽度,进一步灵活地调整算法的复杂度,使其在性能与计算量之间取得良好的折衷。     

Narrow-band interference excision in spread-spectrum systems usingself-orthogonalizing transform-domain adaptive filters

Among many transform-domain interference excision techniques, transform-domain adaptive filtering has many advantages. It is based on a true optimization of some particular performance parameters such as the bit-error rate (BER). Moreover, it is insensitive to jammer frequency. However, transform-domain adaptive filtering also has the drawback of being incapable of tracking a rapidly changing interference because most adaptive algorithms require time to converge to the optimal solution. In this paper, a self-orthogonalizing transform-domain least mean square (SO-TRLMS) algorithm is used to speed up the convergence. Compared to a traditional transform-domain least mean square (TRLMS) algorithm, the SO-TRLMS algorithm can significantly improve the convergence rate of the LMS algorithm, thus making the transform-domain adaptive filtering technique more suitable for real-time processing. In order to show how the system performance is affected by various factors such as interference power and the transform used, this paper presents an analytical result for the BER performance that is applicable for arbitrary orthogonal linear transforms. Simulation results are also presented to demonstrate the validity of the analysis     

An overview of adaptive techniques for interference minimization in CDMA systems

This paper gives a brief review of several adaptive techniques for interference minimization in CDMA (code division multiple access) communication systems.Co-channel interference (CCI) in CDMA is the most dominant factor that limits the user capacity (definition). An overview of CCI cancellation and suppression schemes for sub-optimum but practical single user and multi-user demodulation in CDMA is presented.These techniques include adaptive temporal and spatial cancellers and adaptive digital filters. It has been demonstrated that adaptive receivers can effectively remove CCI and significantly increase the user capacity relative to the conventional matched filter receivers.     

Orthogonalization based adaptive interference suppression fordirect-sequence code-division multiple-access systems

Rapid converging adaptive interference suppression algorithms for direct sequence code-division multiple-access (DS/CDMA) systems are presented after showing the limitations of conventional adaptive algorithms. The rapid converging algorithms are based on an orthogonal transformation preprocessing, and are adaptive implementations of the linear minimum mean square error (MMSE) receiver for interference suppression. These algorithms do not require a priori knowledge on interfering signal parameters such as spreading sequences and relative signal power levels     

UHF television interference associated with cellular mobile telephone systems

An investigation is made into two types of interference which may occur between UHF television and 850 MHz cellular mobile telephone systems: 1) ITV--interference to the cellular mobile telephone receivers, created by a UHF TV transmitter; 2) TVI--interference to the UHF TV receiver, created by the cellular system mobile transmitters. In the case of ITV it has been determined that TV transmitters operating on TV channels 65 through 69 may produce third-order intermodulation, within a radius of about 0.5 mi, to certain telephone channels assigned to mobiles operating in a cellular system fringe area. A judicious choice of telephone channel assignments would mitigate this type of degradation. Alternatively, in the case of TVI, it is shown that within a minimum grade B television service area, it is possible for cellular mobile telephones to at times produce image interference to present-day UHF TV receivers tuned to channels 58 through 61. However, because of cellular telephone subscriber density and channel position (and loading) the probability of interference is expected to be low. An increase of TV receiver image rejection by 20 dB would essentially eliminate all likelihood of this interference.     

Single-antenna co-channel interference cancellation for TDMA cellular radio systems

Co-channel interference cancellation is particularly challenging in the downlink of cellular radio systems because usually only one receive antenna is available at the mobile terminal. This tutorial provides an overview of promising a single-antenna co-channel interference cancellation techniques. Focus is on the downlink of time-division multiple access systems. The results may, however, be extended to related applications, including interference suppression in multiple-input multiple-output systems.     

Training analysis of adaptive interference suppression fordirect-sequence code-division multiple-access systems

This paper studies the transient behavior of an adaptive near-far resistant receiver for direct-sequence (DS) code-division multiple-access (CDMA) known as the minimum mean-squared error (MMSE) receiver. This receiver structure is known to be near-far resistant and yet does not require the large amounts of side information that are typically required for other near-far resistant receivers. In fact, this receiver only requires code timing on the one desired signal. The MMSE receiver uses an adaptive filter which is operated in a manner similar to adaptive equalizers. Initially there is a training period where the filter locks onto the signal that is sending a known training sequence. After training, the system can then switch to a decision-directed mode and send actual data. This work examines the length of the training period needed as a function of the number of interfering users and the severity of the near-far problem. A standard least mean-square (LMS) algorithm is used to adapt the filter and so the trade-off between convergence and excess mean-squared error is studied. It is found that in almost all cases a step size near 1.0/(total input power) gives the best speed of convergence with a reasonable excess mean-squared error. Also, it is shown that the MMSE receiver can tolerate a 30-40 dB near-far problem without excessively long convergence time     

Intercarrier interference self-cancellation space-frequency codes for MIMO-OFDM

Space-frequency (SF) codes that exploit both spatial and frequency diversity can be designed using orthogonal frequency division multiplexing (OFDM). However, OFDM is sensitive to frequency offset (FO), which generates intercarrier interference (ICI) among subcarriers. We investigate the pair-wise error probability (PEP) performance of SF codes over quasistatic, frequency selective Rayleigh fading channels with FO. We prove that the conventional SF code design criteria remain valid. The negligible performance loss for small FOs (less than 1%), however, increases with FO and with signal to noise ratio (SNR). While diversity can be used to mitigate ICI, as FO increases, the PEP does not rapidly decay with SNR. Therefore, we propose a new class of SF codes called ICI self-cancellation SF (ISC-SF) codes to combat ICI effectively even with high FO (10%). ISC-SF codes are constructed from existing full diversity space-time codes. Importantly, our code design provide a satisfactory tradeoff among error correction ability, ICI reduction and spectral efficiency. Furthermore, we demonstrate that ISC-SF codes can also mitigate the ICI caused by phase noise and time varying channels. Simulation results affirm the theoretical analysis.     

Cochannel interference computation and asymptotic performanceanalysis in TDMA/FDMA systems with interference adaptive dynamic channelallocation

It has been shown through simulation results that the interference adaptive dynamic channel allocation (IA-DCA) scheme is a promising resource allocation strategy in time/frequency-division multiple-access (TDMA/FDMA) communication systems. The major obstacle in analyzing IA-DCA is the computation of cochannel interference without the constraint of conventional channel reuse factors. To overcome this difficulty, one needs a computationally efficient representation which can approximate the interference distribution accurately. For this purpose, a concept called channel reuse zone (CRZ) is introduced. Based on this new concept, both downlink and uplink cochannel interference are computed with two different propagation models, namely, a simplified deterministic model and a shadowing model. The results are then used to calculate the outage probability of the idealized, interference adaptive maximum parking (IAMP) scheme. Finally, as a significant contribution, an asymptotic performance bound for the two-way IA-DCA strategy is derived     

Frequency reuse scheme with reduced co-channel interference forfixed cellular systems

A new frequency reuse scheme which reduces co-channel interference in comparison with the conventional scheme is presented for use in fixed cellular or wireless local loop systems where the antennas of subscriber units are fixed and directional. The new scheme is evaluated in a three-sector cell plan employing frequency reuse numbers N=4 and N=7 as a function of the subscriber unit antenna directivity. It is shown that a substantial improvement is obtained with the new scheme compared to the conventional scheme if the subscriber unit antenna beam width is narrower than ~150 degrees. For the case of a 60 degree antenna beam width, for example. The improvement is >7 dB in the signal to co-channel interference ratio averaged along the cell boundary for both N=4 and N=7 cases     

分层小区系统自适应控制择层策略

本文提出了一种分层小区系统自适应控制择层策略,一个自适应门限和两个辅助门限被利用进行层选。同时,利用各小区逗留时间之间的相关性对呼叫业务在将要进入的小区内的归一化逗留时间进行预测估计,提供了一种估计移动台在小区中的逗留时间的新思路。最后通过仿真验证了该策略的性能,得出了具体结论。