China
Africa
Explore chapters and articles related to this topic
Link Adaptation Mechanisms in WirelessMAN
Published in Yan Zhang, Hsiao-Hwa Chen, Mobile Wimax, 2007
Gurkan Gür, Fatih Alagöz, Tuna Tugcu
In BWA systems, channel conditions can vary significantly due to propagation anomalies. It is therefore desirable to adapt the modulation and coding scheme to the channel conditions. While voice networks are designed to deliver a fixed bit rate, data services can be delivered at a variable rate. Voice networks are engineered to deliver a certain required bit rate at the edge of the cell, which constitutes the worst case. Most users, however, have more favorable channel conditions. Therefore data networks can take advantage of adaptive modulation and coding (AMC) to improve the overall throughput. In a typical adaptive modulation scheme, a dynamic variation in the modulation order (constellation size) and forward error correction (FEC) code rate is possible. In practice, the receiver feeds back information on the channel, which is then used to control the adaptation. Adaptive modulation can be used in both uplinks and downlinks. The adaptation can be performed in various ways such as user-specific only, user- and time-specific, or quality of service (QoS) dependent.
LoRa and LoRaWAN
Published in Mahbub Hassan, Wireless and Mobile Networking, 2022
A striking difference between LoRa and the conventional wireless networks is that the symbol duration in LoRa is not fixed but is a function of the modulation order. The larger the modulation order, the longer the symbol duration, and vice versa. Both the modulation order and the symbol duration are controlled by the parameter called spreading factor (SF). For M-ary modulation, SF = log2(M) and Ts = 2SF/B seconds, where B is in Hz. This means that by increasing SF by 1 would not only double the modulation order (increase bits per symbol by 1), but also double the symbol duration. The relationship between SF, modulation order, and the symbol duration is illustrated in Fig. 4 for up-chirps.
Data Link Layer
Published in Mário Marques da Silva, Cable and Wireless Networks, 2018
Adaptive modulation and channel coding rate considers changes to the modulation and coding rate as a function of the link conditions. If a user experiences poor link conditions, his modulation order can be reduced (e.g., from 16QAM to QPSK), reducing the required SNR level to achieve an acceptable BER performance or, alternatively, decreasing the coding rate. The opposite can happen when a user has very good link conditions, increasing the modulation order and/or increasing the coding rate, to achieve a higher throughput.
Performance of a QAM/FSO communication system employing spatial diversity in weak and saturation turbulence channels
Published in Journal of Modern Optics, 2019
Wan Jiao, Hongzhan Liu, Jianjun Yin, Zhongchao Wei, Aiping Luo, Dongmei Deng
When the turbulence intensity is , the performance of the system employing I × J = 16,64,256 quadrature amplitude modulation signals is analyzed under different EGC schemes over the log-normal channel in Figure 5. With an increase in the quadrature amplitude modulation order, the system BER performance deteriorates under given SNR conditions, whereas the amount of transmitted information increases. In other words, the system can achieve a higher spectral efficiency. This trend indicates a trade-off between reliability and effectiveness in communication systems; thus, the value of the quadrature amplitude modulation format must be carefully chosen when compromising between the two aspects of spectrum efficiency and BER performance. When a relatively high spectral efficiency is not required, the 16 quadrature amplitude modulation is a better choice than higher quadrature amplitude modulation formats. From the previous analysis in Figure 3, the performance of QAM/FSO systems with the diversity scheme is superior to that of the SISO system.