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Protect 5G Spectrum Investment Through Synchronization and TDD

This blog post discusses how time and phase synchronization are essential for mobile operators to make the most efficient use of the 5G spectrum.


The Essential Role of Synchronization in 5G Spectrum Efficiency


This blog post discusses how time and phase synchronization are essential for mobile operators to make the most efficient use of the 5G spectrum. Spectrum licenses are sold at a very high cost and governments are using them to raise substantial state revenues. Mobile operators spend billions of dollars to buy the 5G spectrum. The four main mobile operators in the United States—AT&T, Verizon, T-Mobile and DISH Network—purchased nearly $100 billion worth of 5G mid-band spectrum.


For the previous generation of mobile networks, synchronization was essential to facilitate call handovers between base stations as users move from one cell to another. In 5G, synchronization plays an even bigger role because it enables the capacity that users can enjoy. 5G networks contain more bandwidth than 4G networks; in 4G, a 3-Gbyte movie can be downloaded in about 30 minutes while in 5G, it takes as little as 35 seconds. 4G operates mainly in Frequency-Division Duplex (FDD) mode. It uses a paired frequency band to allow simultaneous transmission for uplink and downlink with a guard band to keep them apart. FDD requires a fixed, symmetric ratio of 50/50 downlink/uplink, which is sub-optimal in terms of spectrum utilization.


5G introduced the Time-Division Duplex (TDD) mode to provide more flexibility and to increase capacity. Only one band is needed for both uplink and downlink with a small guard period (time gap) when switching from downlink transmission to uplink transmission. The transmission is multiplexed in time slots that can be used for uplink and downlink. They are allocated in a very flexible manner to dynamically adapt to the downlink/uplink traffic.



The available spectrum can be utilized efficiently to dimension the downlink and uplink capacity according to the traffic characteristics. TDD is also cheaper than FDD to implement since it does not need a diplexer (filter) to isolate transmission and reception. Finally, TDD is much better suited for 5G critical features such as massive Multiple Input Multiple Output (mMIMO), beamforming antennas and the pre-coding techniques that are required for increasing spectral efficiency and reliable coverage. Beamforming refers to directing the beam towards a specific direction while suppressing the signals from other directions to gain better coverage and capacity. mMIMO increases capacity to up to 256 transmit antennas and multiple receive antennas.


Time and phase synchronization is required for TDD to avoid cross-slot interference between the downlink and uplink. The base stations need to use the same downlink and uplink time slot assignment reference. Without synchronization, it is possible to mitigate a little bit of interference but with stricter RF emission requirements, isolation distance and more importantly a larger guard band at the expense of spectrum efficiency. This also requires more expensive equipment and higher operation costs related to site coordination and isolation.


In summary, time and phase synchronization are critical functions of 5G networks because they enable TDD to optimize spectrum utilization. It allocates more bandwidth to users by optimizing the spectrum efficiency. The GSM Association (GSMA) recommends that all TDD networks, whether LTE or 5G NR, operate in the same frequency range and within the same area to be synchronized. It also urges regulators to prioritize TDD synchronization to cope with interference issues among national and international networks. Read the GSMA’s guidelines for the coexistence of TDD networks.


For more information about our timing and synchronization products and solutions, please visit our clock and timing web page


Karim Traore, Mar 28, 2024

Tags/Keywords: Communications

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