在确定一个信道中信息的有效和无误转让时，调制和信道编码是主要考虑因素。它们也对另一个系统获得的干扰和另一个系统产生的干扰存在潜在的冲击。每符号有大量比特的调制会占相对较小的带宽，但它将需要较高的放大器线性和接收方高的 C\\N比率。它也是更容易比每符号比特较少的调制受到干扰。高索引调制需要显著比低索引调制的更多利润。在为一个微型地球站系统选择最适当的调制和信道编码时，实现易用性也是主要因素，因为微型地球站是成本很敏感的。面对这些的权衡决策，微型地球站系统中最常见的调制形式是 QPSK 和当频谱效率是不太重要时的BPSK。在一个拥有理想 RRC 筛选器和无通道编码的理想QPSK 系统，Eb/N0 10.6 分贝的值将提供一个10-6误码率，对应于一个接收器的整体 C/N 比13.6dB， 忽略任何执行边距。如果通道应用编码 ，C/N要求可以大大减少。 （注释：四相相移键控信号简称“QPSK”。它分为绝对相移和相对相移两种。由于绝对相移方式存在相位模糊问题，所以在实际中主要采用相对移相方式QDPSK。它具有一系列独特的优点，目前已经广泛应用于无线通信中，成为现代通信中一种十分重要的调制解调方式。 BPSK全称 ： Binary Phase Shift Keying。把模拟信号转换成数据值的转换方式之一。是利用偏离相位的复数波浪组合来表现信息键控移相方式的一种。BPSK使用了基准的正弦波和相位反转的波浪，使一方为0，另一方为1，从而可以同时传送接受2值(1比特)的信息。）
信道编码
信道编码可以采取块代码或卷积码的形式。卷积编码是一个编码和解码过程是应用于序位组，而不是一次一个位的过程，正如块代码一般。在编码的比特数序列，k被称为卷积码约束长度。在解码过程中，k位用于评估每一位传播码的价值。由于编码过程应用到信号之前传输，用于检测和纠正位错误，故被称为前向纠错（FEC）的代码。同样，块纠错码应用到通道前传输。卷积和块码可以用在一个通道上。一个例子是通道，首先有内部卷积码应用到比特序列，然后有一个外部交错的代码，如Reed -Solomon码应用。Reed - Solomon码结合良好的错误检测和高码率的能力。这种连接的形式编码被广泛应用于许多通讯系统，由于反交错编码将抵制突发错误，同时卷积前向纠错编码将阻止单个位错误。直播卫星电视是一个这样的例子编码方法，音乐CD的录制是另一个。
对于那些小的交通流的VSAT系统，多余的处理延迟可以大大增加终端到终端的链路延迟。拥有强大的版块处理能力对于地球同步轨道和卫星低轨/中轨系统是非常重要的。源于信号相互交错的处理延迟增加了一个固定数额的开销，以及要求在两端缓冲传输链路。出于这个原因，Reed-Solomon外代码添加到的信号通常不会有低于256 kbit / s的信息速率，即使较低的Eb / No值对给定的误码率性能如此重要。那些对于即时响应时间和多媒体互动性无要求的链接对一个给定的Eb / No需要最佳的误码率性能（大部分互联网链接典型），Reed-Solomon码是一种非常实际的方式来减少对于一个给定的链接和BER功率要求。
干扰问题
有相似特征的系统间干扰通常是激烈辩论的主题，特别是当一个新的系，旨在接近现行制度，在轨道上分离或天线梁方向运作。试图确保无害干扰是由各自系统引起的操作者之间的相互影响被称为协调。协调过程是国际电联和国家频率管理当局的严格监管项目。协调演习的关键在于确定辐射的干扰站方向的干扰力量。所收到的计算干扰功率有四个要素：
干扰站的发射放大器输出功率； 干扰站的发射天线增益； 干扰站传输天线的接收增益； 两个站之间的路径损耗。
Modulation, Coding, and Interference Issues
VAST Systems
Modulation and channel coding are key considerations in determining the efficient and error-free transfer of information over a communications channel. They also have an impaction on the potential for interference to another system and from another system. A modulation that has a large number of bits per symbol will occupy a relatively small bandwidth but it will require relatively high amplifier linearity and a high C\\N ratio in the receiver. It is also more susceptible to interference than modulations with fewer bits per symbol. High-index modulations require significantly more margin than low-index modulations. In choosing the most appropriate modulation and channel coding for a VSAT system, ease of implementation is also a major factor since VSATs are very cost-sensitive. Faced with these trade-off decisions, the most common forms of modulation used in VSAT systems are QPSK and, when spectrum efficiency is less important, BPSK. In an ideal QPSK
system with ideal RRC filters and no channel coding, a value of Eb/N0 of 10.6 dB will provide a BER of 10-6, corresponding to a receiver overall C/N ratio of 13.6dB, ignoring any implementation margin. The C/N requirement can be significantly reduced if channel coding is applied.
Channel coding
Channel coding can take the form of a block code or a convolutional code. Convolutional coding is a process where the encoding and decoding process is applied to a group of bits in sequence rather than a bit at a time, as in a block code. The number of bits in the encoding sequence, k, is called the constraint length of the convolutional code. In the decoding process, k bits are used to evaluate the value of each bit transmitted. Since the encoding process is applied to the signal prior to transmission and is used to detect and correct for bit errors, it is called a forward error correcting (FEC) code. In a like manner, a block FEC code is applied to the channel prior to transmission. Convolutional and block codes can be used together on a channel. One example is channel that first has an inner convolution code applied to the bit sequence and then has an outer interleaved code such as Reed-Solomon code applied. Reed-Solomon codes combine good error detection capability with high code rates. This form of concatenated coding is used extensively in many communications systems, since the interleaved coding will counter burst errors while the convolutional FEC coding will counter individual bit
errors. Direct broadcast satellite television is one example of such a coding approach, and the recording of music on CDs is another. For VSAT systems that have small traffic streams, excess processing delay can add significantly to the end-to-end link delay. This is very important for GEO systems and for LEO/MEO systems with satellites that have large onboard processing capabilities. The processing delay due to first interleaving a signal and then de-interleaving it adds a fixed amount of overhead, as well as requiring buffering at both ends of the transmission link. For this reason, Reed-Solomon outer codes are not normally added to signals that have information rates below about 256 kbit/s, even though the lower Eb/No value for a given BER performance is so significant. For links that have no real requirement for instant response times and multimedia interactivity, but require the best BER performance for a given Eb/No (typical of most Internet links), Reed-Solomon codes are a very practical way of reducing the power requirements for a given link and BER specification.
Interference
Interference between systems operating with similar characteristics is usually the subject of intense debate, particularly when a new system seeks to operate close to an existing system, in terms of orbital separation or antenna beam directions. The interaction between operators seeking to ensure that no harmful interference is caused by, or
to, their respective systems are called coordination. The coordination process is the subject of extensive regulation by the ITU and national frequency management authorities. The key aspect in such coordination exercises lies in determining the power radiated by the interfering station in the direction of the interfered-with station. The calculation of the received interference power will have four elements:
The output power of interfering station’s transmit amplifier
The transmit gain of the interfering station’ antenna in the direction of the interfered-with station
The receive gain of the interfered-with station’s antenna in the direction of the interfering transmissions The path loss between the two stations
参考文献：[美]Timothy Pratt, Charles Bosition, Jeremy Allnutt.（英文版）卫星通信（第二版） [M]. 北京：电子工业出版社，2003, (1): 355-364.
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