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GNSS Signals and Range Determination
Published in Basudeb Bhatta, Global Navigation Satellite Systems, 2021
In FDMA, specific frequencies are allocated to transmitters in order to be identified by the receiver; whereas, in TDMA, individual time slots allow identification; and in CDMA, different codes are assigned to each transmitter (Bellavista and Corradi 2007). Some systems also use a combination of these. The main advantage of CDMA is that the whole bandwidth allocated can be used by every receiver. GPS, Galileo, and BeiDou systems use CDMA, but GLONASS uses FDMA. However, GLONASS is being transformed from FDMA to CDMA. This switchover started in 2014 (with the launch of a GLONASS-M satellite) and is expected to be completed by 2040. Russia has a plan to launch the first GLONASS-K2 satellite by the year 2022. The GLONASS-K2 satellite will allow Russia to evaluate CDMA signals that are planned to broadcast on L1, L2, and L3 frequencies. However, GLONASS will also continue transmitting FDMA signals for the ‘unlimited future’ to provide backward capability.
Force-System Resultants and Equilibrium
Published in Richard C. Dorf, The Engineering Handbook, 2018
CDMA is significantly different than FDMA and TDMA. CDMA systems rely on spread spectrum techniques, where all users can transmit in the same frequency band and at the same time. However, each transmitter-receiver pair uses a specific spreading waveform which is modulated by the information. The spreading waveforms of different users have very low cross correlation. Thus, while TDMA and FDMA provided ideally orthogonal multiple access schemes, CDMA is inherently interference limited. While it is theoretically possible to create an orthogonal CDMA system, it would require synchronous reception of all signals on both the uplink and downlink. This is clearly possible on the downlink, but extremely difficult on the uplink. Further, when there are multipath delays which are greater than a chip duration, maintaining orthogonal reception upon synchronization is not possible. CDMA, while inferior
Third-Generation Cellular Communications: An Air Interface Overview
Published in Jerry D. Gibson, Mobile Communications Handbook, 2017
Although digital cellular communications was a great advancement, in the 1980s many researchers started to examine code division multiple access (CDMA) as a method to increase capacity given the limited bandwidth available for transmission. Up until this point, CDMA was a technology primarily used for military applications. CDMA is based on spread-spectrum communications, which involves the transformation of a user's narrowband information signal to a much wider bandwidth for transmission. By modulating the information signal with a high-frequency binary code, the transmitted signal can be recovered even if it is steeped in background noise. CDMA is achieved by assigning each user a unique code.
Non-ionizing radiation as possible carcinogen
Published in International Journal of Environmental Health Research, 2022
Shiwangi Gupta, Radhey Shyam Sharma, Rajeev Singh
Several research groups are also working on different wireless technology and its effect on human health. Code division multiple access (CDMA) is a communication method used by different radio technologies that allow multi users to share same frequency band. The transmission power of CDMA is limited to a maximum of 6–7milliWatts (Garg 2000). In early 1990s, 2 G was developed for digital mobile communication systems (GSM or Global System for Mobile Communication). The transmission power intensity of mobile phone is 2 watts in GSM850/900 and 1 watt in GSM1800/1900 (ICNIRP 2009). The third generation (3 G) mobile network technology used to access the internet and the fourth generation (4 G) technology offers high-speed internet access, videos and higher data rates. 5 G or fifth generation of mobile technology provides complete wireless communication with no limitations and 5 G has high speed of transmission rate (Prabhakara et al. 2017). Different specifications of 3 G, GSM and CDMA mobile phone technologies have been shown in Table 4. In connection to the above, Wi-fi can also create a small wireless computer system in a place where they are often called wireless local area networks (WLANs).
Comparative analysis of quaternion modulation system with OFDM systems
Published in International Journal of Electronics Letters, 2021
Anam Zahra, Qasim Umar Khan, Shahzad Amin Sheikh
Every communication system tries to achieve a lower bit error rate (BER) ensuring minimum data loss. With the increasing use of wireless communication technology, the demand for improved BER services over both wired and wireless links is also increased. New techniques are required to transfer data with better BER which existing techniques cannot support (Saxena & Prasad, 2003; Zou & Wu, 1995). In communication system, BER can be achieved at a certain limit at the expense of bandwidth or PAPR (Gangwar & Bhardwaj, 2012). There are various applications to achieve lower BER is more essential as compared to bandwidth such as military communication, point-to-point communication and point-to-multipoint communication (Gallagher, 1979). In the past, various techniques have been implemented in military communication systems to provide protection against noise interference. Spread spectrum is a modulation technique, extensively used in military (Zou & Wu, 1995). It resists interference and makes it hard for enemies to intercept, whereas in spread spectrum modulation technique, the bandwidth of transmitted signal is much greater than the bandwidth of original message. One technique to improve BER is CDMA; it has been used in military applications (Gangwar & Bhardwaj, 2012; Saxena & Prasad, 2003). CDMA is a multiple access technique used with spread spectrum, which allows numerous signals to occupy a single transmission channel in 2G and 3G wireless communications.
Measurement, Analysis, and Understanding of the Error Vector Magnitude (EVM) of Navigation Signals
Published in IETE Journal of Research, 2018
Md. Tosicul Wara, Raghavendra M. R., Kodandaram M., Bhuvaneshwari M. S.
The navigation data (of lower bit rate) is first multiplied with the high speed CDMA code and the multiplied data is then modulated onto the high frequency radio frequency (RF) carrier. The multiplication of data with high speed CDMA code results in spreading of the spectrum and thereby provides the better performance (against noise/jamming, interference, and multipath fading) and better security to the service. For a given navigation satellite, the navigation data, the CDMA code, and the RF carrier all are synchronized in phase. Also, all the satellites in the constellation are synchronized.