LTE frequency bands and duplex modes

LTE (4G) functions on a wide range of frequencies. These frequency ranges are called bands, and are standardised across the world.

These frequencies range from as low as 410 MHz up to almost 6 GHz in some cases, but not all of these frequency bands are supported on every device.

How does frequency link to speed?

In theory, it doesn't really, and it's a common misconception of how radio frequency (RF) systems work. Frequency on its own makes no difference to the speeds you can achieve on LTE.

The frequency is a carrier frequency, meaning that its only role is to carry the LTE signal. This signal is encoded into a channel with a set width (such as 10 MHz or 20 MHz).

In fact, the data carried on that signal can be moved to any other frequency as long as the available bandwidth is the same. In the example animation below, we can copy a 20 MHz bandwidth signal from 2100 MHz to 800 MHz without any loss in data.

The only way that frequency and speed are linked is in their ability to penetrate surfaces.

Penetrating power

In general, lower frequency (longer wavelength) waves can pass more easily through dense substances. For example, you can hear bass notes from music through walls more easily than you can hear vocals.

As a wave, such as radio waves, travel through materials, they lose some of their energy. While they travel through the air, energy loss is minimal, allowing waves to travel for kilometres in ideal circumstances. When they travel through denser materials, such as brick or concrete, they lose more energy.

FrequencyAmplitude

Frequency: 2000 Hz

Wavelength: 150 km

Amplitude: 40 m

Duplex modes

Duplex refers to the ability for a communication channel to transmit and receive data. LTE can be either full duplex (meaning that transmitting and receiving can happen simulataneously), or half duplex (meaning that transmitting and receiving can happen, but not at the same time).

Both LTE-TDD and LTE-FDD share the vast majority of their core infrastructure, allowing chipset and modem manufacturers to more easily support both modes with one single chip.

Time-division Duplex (TDD)

Time-division Duplex works by using one single frequency, and by switching between uploading and downloading over time. The proportion of time spent uploading or downloading can be changed on-the-fly to account for longer periods spent mainly downloading or uploading.

There needs to be a small period in between the transition from uplink to downlink and vice versa. This is called the guard period or guard interval. This is used to ensure one device's transmission is fully received before the other begins to transmit. This period must get longer as the devices grow further apart, resulting in significant throughput loss over long distances.

LTE-TDD is mainly used at higher frequencies, covering bands between 1850 MHz and 3800 MHz. The LTE-TDD spectrum allocation is normally cheaper than LTE-FDD and also has less traffic.

Simulation

RatioPeriod lengthDistance

Frequency

uplink–downlink ratio: 5:5

period length: 0.133s

guard period: 0.200s

UPLINK

2310 MHz

GUARD PERIOD

DOWNLINK

2310 MHz

Frequency-division Duplex (FDD)

Frequency-division duplex works by using two different frequencies, where each is dedicated to either upload or download only.

For FDD to work, networks need to use paired frequencies. For example, 800 MHz Band 20 uses an offset spacing of -41 MHz, meaning that downlink frequencies are found -41 MHz from uplink frequencies (e.g. if uplink is at 800 MHz, then downlink is at 769 MHz). As this spectrum must be paired and purchased in these sets, it's often much more expensive to set up and run than LTE-TDD due to licensing costs.

As the frequencies for uplink and downlink are locked for each network, it is hard for them to dynamically change their speeds. When using TDD, networks can dynamically set the amount of time that is spent either uploading or downloading, but with FDD the relationship is fixed to the spectrum allocations owned.

Simulation

Rx/Tx speedBandwidth

Frequency

Bandwidth: 10 MHz

guard band width: 110 MHz

duplex spacing: 120 MHz

LTE Band: B7

FrequencyDistance

2510 MHz

GUARD BAND

2630 MHz

Benefits of LTE-TDD and LTE-FDD

Both TDD and FDD have their own benefits, which is why most networks choose to deploy both across their network.

In the UK, for example, all 4 major networks (EE, Vodafone, Three and Virgin Media O2) use a combination of TDD and FDD as part of their network coverage. The vast majority of their networks use FDD bands, such as B1, B3, B7 and B20, however some networks are beginning to roll out TDD-based spectrum.

Vodafone are starting to roll out bands 32 and 38 within larger cities for extra coverage and throughput. As B7 leaves a large 120 MHz gap between 2570 MHz and 2620 MHz for duplex spacing, band 38 allows for TDD communication within this gap, providing extra throughput while not affecting the existing B7.

Here's a table comparing the benefits of each technology:

LTE-TDD	LTE-FDD
Allows for dynamically adjusting downlink slots For high demand, extra downlink slots can be added, increasing speeds at the cost of uplink throughput	Overall higher throughput Transmitting and receiving can happen at once, achieving max throughput of 2x TDD at same bandwidth In TDD, the guard period (dead air time) increases with distance, reducing efficiency
Less expensive No need to buy expensive diplexer Less spectrum needs to be licensed	Lower latency No need to wait for the next period to send data like TDD
	Less complex No complex time sychronisation requirements

While FDD is the most beneficial to consumers, the higher cost makes it unfavourable for networks looking to increase their network capacity in densely populated areas, which is why LTE-TDD is more common in new deployments in cities and large towns across the globe.

FDD is still preferred by networks choosing to deploy in new areas or to generally increase speeds and coverage as it can handle more devices connected to a single band due to it using two channels of equal bandwidth rather than one single channel.

List of all LTE bands

There are many different LTE bands, but not all bands are supported on all devices.

Chipset manufacturers often charge extra for every band manufacturers would like to support with the phone. To cut costs, most phones only choose to support around 20 bands, but this is usually enough to cover all commercial bands in the majority of countries.