Dear community friends, today I want to share with you information about multiple access and duplex technologies.
What is Multiple Access Technology?
In a cellular system, communication objects are distinguished by channels. One channel accommodates only one user for communication. Many users who communicate at the same time are distinguished by channels. This is called multiple access. Because the mobile communication system is a system with multiple channels working at the same time and has the characteristics of broadcasting and large-area radio wave coverage, the signal transmitted by one user in the network can be received by other users, Therefore, how to identify the signals sent to the user's address from the advertised signals becomes the primary problem for establishing a connection. In a radio wave coverage area of a wireless communications environment, establishing a wireless channel connection between users is a problem of a multiple access manner. The method to solve the multiple access problem is called multiple access technology.
What are the types of multiple access technologies?
Multiple access technologies are classified into four types.
FDMA
In communications, a technique in which a frequency band is divided into several channels, and multiple users with different addresses can use different carriers (channels) at the same time to achieve multiple access connections. In the FDMA system, the user is assigned a channel, that is, a pair of spectrums. One spectrum is used as the forward channel, that is, the channel from the base station to the mobile station, and the other is used as the reverse channel, that is, the channel from the mobile station to the base station.
TDMA
Carriers with the same frequency are divided into several timeslots in the time domain so that multiple users with different addresses can use different timeslots to implement a multiple-access connection communication manner.
CDMA
CDMA is a wireless communication technology for multiple access. Different code sequences formed by the spread spectrum technology are used for multiple users with different addresses to implement multiple-access connections.
OFDMA
OFDMA uses the OFDM technology to differentiate users. Because data is transmitted in parallel on different subcarriers, OFDMA is a frequency division multiplexing access method. In OFDMA, each user shares a fast Fourier transform space in the uplink, and the base station allocates subcarriers to the user so that different rates can be allocated according to different requirements of the user.
Concepts of Multiplexing and Multiple Access
From FDM/FDMA to OFDM/OFDMA
In LTE, OFDMA is used in the uplink and SC-FDMA is used in the downlink.
Downlink Multiple Access Technology OFDMA
Advantages
The spectrum allocation mode is flexible and adapts to the bandwidth range from 1.4 MHz to 20 MHz. Because OFDM subcarriers are orthogonally multiplexed, no guard band is required, and spectrum utilization is high.
Proper configuration of cyclic prefixes can effectively overcome the ISI caused by multipath interference in the radio environment, ensure the orthogonality of UEs in a cell, and improve the coverage at the cell edge.
This feature supports frequency-based link adaptation and scheduling to combat channel frequency selective fading, obtain multi-user diversity gains, and improve system performance.
The subcarrier bandwidth is approximately 10 kHz. Each subcarrier undergoes a flat fading of the spectrum, which makes equalization of the receiver easy to achieve.
OFDM can be easily combined with MIMO.
Disadvantage
High time-domain and frequency-domain synchronization requirements, small subcarrier spacing, system sensitivity to frequency offset, and inconsistent crystal oscillators at the transmit end and receive end also lead to ICI. Inaccurate frequency offset estimation results in poor signal detection performance.
In mobile scenarios, the frequency offset caused by the Doppler shift also causes ICI. Therefore, you need to set appropriate frequency synchronization parameters.
The peak-to-average power of OFDM is higher than that of PAPR. Therefore, OFDM has high requirements on the linearity and dynamic range of the PA.
Features of the Uplink Multiple Access Technology SC-FDMA
Due to the limitation of the terminal battery capacity and cost, the modulation technology with a relatively low PAPR needs to be used in the uplink to improve the efficiency of the power amplifier.
The LTE uplink uses SC-FDMA（single carrier frequency division multiple access）to flexibly implement dynamic frequency band allocation. The modulation of SC-FDMA is implemented using the DFT-S-OFDM （discrete Fourier transform spread spectrum OFDM）technology.
DFT-S-OFDM is similar to OFDM. Each user occupies a part of the system bandwidth. The occupied bandwidth depends on user requirements and system scheduling results.
Compared with the traditional single-carrier technology, in DFT-S-OFDM, different users occupy orthogonal subcarriers, and no guard band is required between users, resulting in higher frequency utilization.
Differences Between OFDMA and SC-FDMA
Compared with OFDMA, SC-FDMA has the following advantages:
Low PAPR design, convenient for UE power amplifier.
Compared with conventional single-carrier frequency reuse, frequency reuse between UEs may be completely orthogonal to ensure spectral efficiency.
Through processes such as DFT transform and orthogonal subcarrier mapping, user multiplexing can be conveniently implemented.
That's all I want to share about multi-access and duplex technology, and you're welcome to read and comment on it.
I'll share information about the LTE frame structure in the next post.
