平流层艇载CDMA系统的嵌入式多波束天线方案

该文改进了原先提出的微/宏波束天线覆盖方案,采用更为复杂但更贴近实际的模型,尤其是改进了艇载天线波束方向图和信道干扰的计算方法。该方案应用于平流层通信CDMA系统,通过在宏波束覆盖范围中嵌入微波束,解决平流层通信系统中由于用户分布失衡而引起的热点问题。针对不同的用户分布和热点位置,文中给出了调整微波束实现系统容量最大化的优化方法。仿真结果表明在用户分布失衡、热点位置变化等情况下使用本文的嵌入式波束覆盖方案仍然可以保持高而稳定的系统容量。     

平流层CDMA通信的波束分集接入方案

根据平流层通信应用多波束天线的特点,提出了一种适合于平流层CDMA通信的波束分集接入方案,应用此方案,在平流层平台上可以实现各波束接收信号的最大比合并,得到高系统容量和集成的软切换性能。     

基于混合光电传输的无人机中继通信系统性能分析

针对基于光电混合架构的无人机中继通信系统下行链路多用户传输场景,提出了一种基于统计信道状态信息的波束成形方案,并进一步分析了系统的中断概率。在该混合架构中,基站—无人机链路和无人机—用户链路采用自由空间光技术和射频传输完成通信过程。在已知RF链路统计信道状态信息的情况下,首先提出了一种波束成形方案以抑制用户间干扰,实现所有用户同时通信,提升了系统的频谱效率。然后,在自由空间光链路和射频链路分别服从Gamma-Gamma分布和Nakagami-m分布的条件下,推导出了系统中断概率的闭合表达式。进一步,得到了高信噪比条件下中断概率的渐进表达式,获得了系统的分集度。最后,仿真结果验证了理论分析的正确性以及所提方案的优越性。     

超宽带太赫兹通信中天线结构设计及其波束色散影响分析

为克服超宽带太赫兹通信中的波束色散,当前已设计了多种基于实时延(TTD)的天线结构,但其功耗大、复杂度高。为解决这一问题,该文提出一种低功耗、低复杂度的基于串行等间距延时器的稀疏射频链天线结构。通过联合优化延时器时延和移相器相位,可以改变子载波波束方向,实现多载波波束扩展与波束聚拢,从而为不同分布场景的用户服务。具体而言,在所提天线结构下,提出一种优化延时器时延和移相器相位的方案将多载波波束扩散到不同方向,实现波束扩展,以服务分布在不同方向上的用户。然后,通过调节时延和相位使所有载波波束对准同一方向,实现波束聚拢,以服务分布在同一方向上的用户。仿真结果表明了所设计天线结构和所提优化方案的有效性。     

面向无人机小区覆盖的毫米波波束优化设计

为了提高无人机毫米波波束覆盖区域内的通信容量,改善覆盖区域内用户通信质量,本文结合均匀平面阵列天线,提出采用一种基于目标覆盖区域量化的波束优化设计方法。该方法首先将目标区域进行量化,根据理想波束增益得到理想混合预编码向量;其次,以实际预编码向量与理想预编码向量之间的均方误差最小化为目标获得最优波束,进一步利用改进的正交匹配跟踪算法去拟合产生适合实际系统的波束成形矢量。仿真表明该波束设计方法在收敛速度和收敛精度上均优于传统方法,并可以获得更好的波束形状和波束增益,从而提高无人机目标覆盖区域内用户的通信质量。      

MIMO系统中的随机波束成形技术

P.Viswanath等首次提出"波束成形"的概念。Jaehak Chung在其论文中指出:MIMO系统中使用随机波束成形技术,只要向基站反馈有效的信噪比就可以同时获得下行链路多用户分集增益,空间复用增益和阵列增益。另外,功率控制技术和注水原理可以提高相干信道的信道容量。慢衰落信道中,蜂窝中有很多用户时,利用他们提出的方法获得的吞吐量逼近采用相干波束成形技术获得的吞吐量。M.Sharif and Babak Hassibi在其文章中提出了如下的方案:基站构建M个随机波束,并和SINR最大的用户通信。M固定,用户数n增加,上述方案的总容量可表示为M·lg(lg(n)),这与已知精确信道状态信息情况下获得的总容量一致。本文比较了上述文章中的方案,并分析出每个方案的优缺点。     

物理层安全的随机波束成型传输算法

针对传统信息论物理层安全研究中,对合法用户信道质量要求较高的问题,文中提出随机波束成型的物理层安全传输方法。该方法通过在主信道特征向量的垂直方向构造人工随机波束成型信道,使得合法用户与窃听方信道传输特征的差异性转化为信道质量的差异,最终得到正的保密容量。为保证算法的完备性,文章解决了三方面问题:1）给出随机波束成型的空间构造方法;2）随机波束成型方法的保密容量和最佳信源分布;3）人工随机信道的构造方法。最后针对采用随机波束成型算法的发送分集系统,通过设计、仿真其噪声发生和功率分配方案,进一步说明即使在合法用户信道质量差的情况下,随机波束成型传输算法也可获得保密传输容量。      

一种基于MOSA的低轨卫星星座多波束分配策略

针对LEO宽带卫星星座中多卫星同时覆盖某一区域情况下的多波束资源分配问题,采用地面固定波束体制,研究了多卫星多波束的分配策略。以地面用户接入容量需求和端到端时延最小化为优化目标,通过对多目标模拟退火算法进行改进优化,建立了一种可同时满足用户容量与时延要求的多卫星多波束的分配方法。仿真结果表明,算法能够根据用户优先级,有效提升星座波束的通信与覆盖性能,通信时延降低了4.1%～20.6%。     

毫米波通信低复杂度波束选择和用户调度算法

针对毫米波混合波束成形系统中用户调度方案复杂度过高的问题,提出两种低复杂度的波束选择和用户调度联合优化算法。混合波束成形架构使得用户调度问题面临着新的挑战,变成了模拟波束选择和用户调度的联合优化问题。考虑发送端无法获得完美信道状态信息的实用场景,采用基于固定码本的波束训练方案获取等效信道状态信息,引入调用指示函数将联合优化问题建模成非凸组合优化规划,分别以粒子群优化和贪婪算法为核心,提出两种低复杂度的次优解决方法。仿真结果表明,相较穷举搜索,所提算法能在性能和复杂度之间取得很好的折中。      

相控阵短波发信系统相位校正方案设计

相控阵短波发信系统通过波束合成手段能够实现通信的实时性和按需可控,并且提高通信的稳定性和抗毁性,满足各种条件下的通信要求。提出了相控阵短波发信系统相位校正方案,并且将整个校正过程分为相位控制和交换两个模块。通过对相位控制模块中AD采样、下变频和相位计算的具体设计以及交换模块中DSP和FPGA的具体设计实现了系统的相位校正。最后对仿真数据和实测数据进行比较,验证了本相位校正方案的可行性。     

MIMO系统信道容量研究

针对单用户MIMO系统信道的容量特性展开研究。首先详细推导了无衰落信道下信道容量表达式,然后重点分析了瑞利衰落信道下,接收端已知信道状态信息,发射端已知信道状态分布时的容量特性。最后分别针对瑞利衰落信道下,采用发射分集、接收分集以及BLAST传输结构的系统容量进行仿真。仿真结果表明:给定发射功率,独立的瑞利衰落信道条件下,MIMO系统容量随最小天线数目的增加而线性增加,极大地提高了系统容量。     

多服务多用户OFDM系统资源分配算法

提出了一种正交频分复用（OFDM）系统的多服务多用户资源分配算法。与传统多用户OFDM系统资源分配算法只考虑一种类型的服务不同,提出的算法同时考虑了数据业务和语音业务,分析了两种业务不同的资源分配需求,对语音业务采用了时分多址（TDMA）的方式,并且加入了对每种数据业务最低传输速率的要求。仿真结果表明,新算法能够在满足语音用户固定传输容量的基础上,提升数据用户总的传输容量,同时保证每一个数据用户的最低传输容量的要求。     

Multiple-access capacity in mobile user satellite systems

The channel capacity of a satellite direct sequence CDMA system is analyzed including the effects of faded user interference, overlapping antenna beams, imperfect equalization of the antenna pattern across an antenna cell, and diversity reception. Simplified models are used to describe the impact of these effects on the channel capacity of single and multiple cofrequency CDMA systems. In a comparison of the uplink and downlink paths, the uplink of the CDMA system is shown to limit the channel capacity because the downlink can utilize code-orthogonality and coherent demodulation. In a multiple system comparison between band-shared CDMA and band-segmented FDMA/TDMA technologies, FDMA/TDMA is shown to provide about the same capacity for uniformly distributed traffic conditions over many cells and dramatically better capacity when traffic is concentrated in one cell. Due to the peak nature of telephony, this result supports the use of band-segmented systems in mobile user satellite systems     

On the optimum number of beams for fixed beam smart antennas in UMTS FDD

Fixed-beam smart antennas are a simple but effective method to boost the downlink capacity of UMTS FDD. In this paper we compare the two possible strategies. Using a four element uniform linear antenna array, we find the optimum number of fixed beams per 120/spl deg/ sector dependent on the direction of departure (DoD) spread at the base station. For the first method we find the optimum number of beams to be four for low DoD spreads and two or three for large DoD spreads. For the second method the optimum number of beams per sector is seven for small DoD spreads and goes down to four or five beams per sector for large DoD spreads depending on base station spacing. By extensive system level simulations, we show, for 1 km inter base station distance, a capacity gain of more than 160% over a conventional 3-sectored reference system by both fixed beam methods.     

Simulation and analysis of software defined cognitive frequency hopping multi beam satellite system

Traditional multi-beam satellites cannot adapt to the changing traffic because of the fixed bandwidth and cannot adjust dynamically for the traffic load.In the future,the multi-beam broadband satellite system must have great flexibility and can be dynamically adjusted according to the change of the traffic flow.Beam hopping for multi-beam broadband satellite network system has been proposed to improve the traditional multi-beam broadband satellite network system performance,whose principle is to assign different beams to different time slots,rather than the allocation of bandwidth,so that in each time slot,the entire available bandwidth is allocated to each beam,and the time window is periodically applied to the beam selection system ,in the full band hopping mode,the band can be selected optimally in the duration of each beam to meet user transmission bandwidth and delay requirements.With the development of cognitive radio technology,cognitive beam hopping for multi-beam satellite system can further improve the efficiency of spectrum utilization,to meet the growing shortage of spectrum resources,to achieve large-capacity high-bandwidth broadband satellite network system,and to support the rising user service capacity and the transmission efficiency of user services under different load conditions.In addition,software definition technology is also used in satellite networks,which could achieve effective management of satellite resources to improve the utilization of satellite resources.In this paper,the principle of software-defined cognitive hopping multi-beam broadband satellite network system is discussed.The spectrum utilization efficiency and throughput are analyzed,which can be used as a reference for broadband satellite network system design.     

On the Capacity of Randomly Used Shared Channels

The channel capacity of a communication channel made available to a user on a shared basis with other users is here considered. An overall system bandwidth B is divided into segments and parceled out to subscribers in an overlapping manner. Viewing emissions by the sharing user as Gaussian noise, the channel capacity is treated as a random variable, and its probability function, mean, and variance are studied. For a fixed power to each user, it is found that the average capacity of a user is maximum when the number of multiple assignments of a frequency band is approximately the inverse of the probability of use. The increase in channel capacity over exclusive assignment is also determined. The probability function of the capacity is worked out for some specific case to show its dependence on the frequency assignment policy, and a general procedure for determining the probability function is given.     

A Low Complexity Resource Allocation Method for OFDMA System Based on Channel Gain

In orthogonal frequency division with multiple access (OFDMA) systems dynamic radio resource allocation improves overall performance by exploiting the multiuser diversity gains. A key issue in OFDMA is the allocation of the OFDM subcarriers and power among users sharing the channel. This paper proposes a new rate adaptive resource allocation scheme in the OFDMA downlink transmission system. Our proposed algorithm is based on the users’ sensitivity to the subcarrier allocation which means how frequency selective is the channel from the user’s perspective. As a result of frequency selectivity of the channel, different subchannels of the same user experience different levels of fade. However, how different they undergo fading could be measured by difference between maximum and minimum channel gain of that user. Our proposed method is based on difference between maximum channel gain and minimum channel gain for each user and uniform distribution of power among subcarriers. Simulation results show that the proposed algorithm achieves higher capacity over fixed TDMA method, and reported suboptimal methods with acceptable rate proportionality.     

Multibeam cellular communication systems with dynamic channel assignment across multiple sectors

In cellular communication systems, directional multibeam antennas at cell sites can be used to reduce cochannel interference, increase frequency reuse and improve system capacity. When combined with dynamic channel assignment (DCA), additional improvement is possible. We propose a multibeam scheme using dynamic channel assignment across multiple sectors. A cell is divided into several sectors, each of which is covered by several directional beams. Specific channels are allocated to each sector as in fixed channel assignment (FCA). A channel of a sector is dynamically assigned to a wireless user who communicates through one of the several beams of the sector. The assignment is made so that constraints on the allowable cochannel interference are satisfied. Limitations due to cochannel interference are analyzed. A tractable analytical model for the proposed scheme is developed using multidimensional birth–death processes. Theoretical traffic performance characteristics such as call blocking probability, forced termination probability, handoff activity, carried traffic and channel rearrangement rate are determined. With the proposed scheme, call blocking probability can be reduced significantly for a fixed offered traffic. Alternatively, system capacity can be increased while blocking probability is maintained below the required level. Smaller forced termination probability is obtainable in comparison with corresponding FCA schemes.     

Rate scheduling in multiple antenna downlink wireless systems

We consider scheduling strategies for multiantenna and multibeam cellular wireless systems for high-speed packet data services on the downlink. We establish a fundamental connection between the stability region of the queuing system and the set of feasible transmission rates, which provides the basis for the scheduling algorithm proposed in this paper. Transmission using adaptive steerable beams and fixed sector beams are considered and average delay versus throughput results are obtained through simulations for the proposed scheduling scheme in each case. While in single antenna systems multiuser diversity gains are achieved by the scheduling algorithms that transmit to a single user in each scheduling interval, our results show that with multiple antennas, transmitting to a carefully chosen subset of users has superior performance. The multiantenna scheduling problem is closely related to the problem of coordinated scheduling for transmission through multiple base stations, where a user can receive signals from several base stations simultaneously. We consider the special case when three single-antenna base stations are allowed to cooperate and transmit to the users in the triangular region between the base stations and propose scheduling strategies that demonstrate significant gains.     

On the capacity of MIMO broadcast channels with partial side information

In multiple-antenna broadcast channels, unlike point-to-point multiple-antenna channels, the multiuser capacity depends heavily on whether the transmitter knows the channel coefficients to each user. For instance, in a Gaussian broadcast channel with M transmit antennas and n single-antenna users, the sum rate capacity scales like Mloglogn for large n if perfect channel state information (CSI) is available at the transmitter, yet only logarithmically with M if it is not. In systems with large n, obtaining full CSI from all users may not be feasible. Since lack of CSI does not lead to multiuser gains, it is therefore of interest to investigate transmission schemes that employ only partial CSI. We propose a scheme that constructs M random beams and that transmits information to the users with the highest signal-to-noise-plus-interference ratios (SINRs), which can be made available to the transmitter with very little feedback. For fixed M and n increasing, the throughput of our scheme scales as MloglognN, where N is the number of receive antennas of each user. This is precisely the same scaling obtained with perfect CSI using dirty paper coding. We furthermore show that a linear increase in throughput with M can be obtained provided that M does not not grow faster than logn. We also study the fairness of our scheduling in a heterogeneous network and show that, when M is large enough, the system becomes interference dominated and the probability of transmitting to any user converges to 1/n, irrespective of its path loss. In fact, using M=/spl alpha/logn transmit antennas emerges as a desirable operating point, both in terms of providing linear scaling of the throughput with M as well as in guaranteeing fairness.