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具有较高数据传输率的MMDS系统能够提供更多的节目,信道带宽6MHz,奈奎斯特滤波器滚降系数约为1/5。因此,符号传输速率被限制在6/(1+1/5_)=5Msym/sec。采用增加每码元传送比特数的方法。信道容量能进一步提高。64QAM赋予每码元6比特将产生30Mbps的传输速率。256QAM赋予每码元8比特将产生40Mbps的数据率,传输速率提高33％。 但是,每符号携带比特数的增加就要求更高的信噪比(SNR)以及更加严格的线性特性。信号星座的几何特性揭示出:当增加每符号的传送比特数时,理想符号与判定边界间的间距将减小。对于给定的BER值,256QAM所要求的SNR值比64QAM所要求的SNR值高6dB。与64QAM相比较,在256QAM调制时,高斯噪声,本振相位噪声,混频器和放大器的调幅-调幅(AM-AM),调幅-调相(AM-PM)非线性更容易引起误码。 分析比特差错率(BER)与sNR之间的关系常受到信道引入的加性高斯白噪声的限制。在本文中,我们避开理论计算而探索256QAM MMDS发射机的实用误码性能。针对一实际的256QAMMMDS发射机,我们给出测试结果和分析,以阐明数据率,SNR,线性特性之间的综合关系。
MMDS systems with higher data rates offer more programs with channel bandwidth of 6MHz and Nyquist filter roll-off factor of about 1/5. Therefore, the symbol transmission rate is limited to 6 / (1 + 1/5 _) = 5Msym / sec. A method of increasing the number of bits transmitted per symbol is used. Channel capacity can be further improved. 64QAM giving 6 bits per symbol will result in a transmission rate of 30 Mbps. 256QAM gives 8 bit per symbol a data rate of 40Mbps and a 33% increase in transfer rate. However, the increased number of bits per symbol requires higher signal-to-noise ratio (SNR) and more stringent linearity. The geometry of the signal constellation reveals that as the number of bits per symbol is increased, the spacing between the ideal symbol and the decision boundary will decrease. For a given BER value, the required SNR value for 256QAM is 6dB higher than the SNR value required for 64QAM. Compared with 64QAM, Gaussian noise, LO phase noise, AM-AM and AM-PM nonlinearity of mixers and amplifiers are more likely to cause bit errors in 256QAM modulation. Analyzing the relationship between the bit error rate (BER) and sNR is often limited by the additive Gaussian white noise introduced by the channel. In this article, we explore the practical error performance of 256QAM MMDS transmitters, bypassing theoretical calculations. For a practical 256QAMMMDS transmitter, we present the test results and analysis to clarify the overall relationship between data rates, SNR, and linearity.