What is Band 7 LTE

LTE transmission technology

LTE transmission technology is designed for a frequency range from 700 to 2,700 MHz (0.7 to 2.7 GHz). The transmission channels can vary flexibly between 1.25 and 20 MHz. This makes it easier to adapt to the different frequency ranges around the world. In order for international roaming to be possible, the end devices must be multi-frequency capable. That means they have to support multiple frequency ranges.

LTE defines a completely new radio interface. The transmission method is based on OFDM (with 64QAM) and SC-FDM. The radio interface is accessed in the downlink with OFDMA and in the uplink with SC-FDMA. MIMO (multiple antenna system) is also provided, which is already used in HSPA + and in WLANs according to IEEE 802.11n.

Frequency ranges in Germany

In Germany, three frequency bands are mainly used for LTE. 800, 1,800 and 2,600 MHz. In Germany, an LTE device should ideally be able to operate all three frequency ranges so that it works in all networks and thus with all network operators.

  • Band 20: 800 MHz, universal service with long range, penetration of buildings and coverage of large areas (land).
  • Band 3: 1,800 MHz, universal service support through LTE hotspots (city).
  • Band 7: 2,600 MHz, universal service support through LTE hotspots (city).

In many places the frequency range around 1,800 MHz is still used for GSM. As there are fewer and fewer pure GSM mobile radio devices, the demand for this technology is slowly declining. The space in the frequency spectrum could be allocated to LTE via spectrum refarming.

  • Band 28: 700 MHz, supplementing the basic service in rural areas (in future)
  • Band 22: 3,500 MHz, support of the universal service through hotspots.

There are still reserves in the range of 700, 1,500 and 3,500 MHz. In addition, there are several hundred MHz unlicensed spectrum in the 5 GHz band that can be bundled with LAA-LTE.

Originally analog UHF television channels (radio) were housed in the frequency range between 790 and 862 MHz (800 MHz). This frequency range became free through the conversion of terrestrial TV reception to DVB-T / DVB-T2 and the associated switch-off of analog TV transmission via radio. This frequency range is therefore also known as the digital dividend.
In addition, all network operators in the frequency range around 1,800 MHz have frequencies available that can be used for LTE.

While the frequencies around 2,600 MHz are mainly used in heavily frequented locations (hotspots) in large cities, the mobile network operators are obliged to cover the blank areas of broadband expansion (areas not covered) with the 800 MHz frequency range. Depending on needs and demands, it is to be expected that this frequency range will at some point be overcrowded and that frequencies around 2,600 MHz will also be used in rural areas.

However, the higher frequency range has a shorter range. Since the 800 MHz frequency range is slightly below the 900 MHz GSM band, the propagation conditions for the radio signals are similar. This means that the 800 MHz band offers the greatest range of all three frequency ranges and manages with fewer base stations in terms of network coverage. With LTE, however, the distance between the base station and the end device must not be more than 10 kilometers.

Worldwide roaming with LTE

tapeAreaUplinkDownlinkBandwidth (MHz)businessregion
12,100 MHz2,110-2,170 MHz1,920 - 1,980 MHz5, 10, 15, 20FDDEurope, Asia
21,900 MHz1,850 - 1,910 MHz1,930 - 1,990 MHz1.4, 3, 5, 10, 15, 20FDDAsia, USA
31,800 MHz1,710-1,785 MHz1,805 - 1,880 MHz1.4, 3, 5, 10, 15, 20FDDDE, Europe, Asia, USA
41,700 MHz1,710 - 1,755 MHz2,110-2,155 MHz1.4, 3, 5, 10, 15, 20FDDUnited States
5850 MHz824-849 MHz869 - 894 MHz1.4, 3, 5, 10FDDUSA, Israel
72,600 MHz2,500 - 2,570 MHz2,620-2,690 MHz5, 10, 15, 20FDDDE, Europe, Asia, Canada
8900 MHz880-915 MHz925-960 MHz1.4, 3, 5, 10FDDEurope, Japan
12700 MHz699-716 MHz729 - 746 MHz1.4, 3, 5, 10FDDUnited States
13700 MHz777 - 787 MHz746 - 756 MHz5, 10FDDUnited States
14700 MHz788 - 798 MHz758 - 768 MHz5, 10FDDUnited States
17700 MHz788 - 798 MHz734 - 746 MHz5, 10FDDUnited States
19850 MHz830-845 MHz875-890 MHz5, 10, 15FDDJapan
20800 MHz832 - 862 MHz791-821 MHz5, 10, 15, 20FDDDE, Europe
223,500 MHz3,410 - 3,490 MHz3,510 - 3,590 MHz5, 10, 15, 20FDDnot yet in use
251,900 MHz1,850 - 1,915 MHz1930-1995 MHz1.4, 3, 5, 10, 15, 20FDDUnited States
26850 MHz814-849 MHz859 - 894 MHz1.4, 3, 5, 10, 15FDDUnited States
28700 MHz  5, 10FDDDE

There are over 40 different frequency bands worldwide that are used for LTE. This apparently generous frequency availability poses problems for device manufacturers. The effort and costs increase with each individual frequency band that has to be supported in a mobile radio device. According to the current state of the art, not all LTE bands can be supported by every LTE mobile device, but only a few of them. This means that the manufacturers' mobile devices work on different frequencies depending on the region. It can happen that an LTE device works in one country and not in another because the frequency bands there are completely different and are not supported.
Unlike with GSM and UMTS, when buying LTE devices that you want to use abroad, you (still) have to pay attention to the supported frequencies.

The 1,800 MHz band is considered the main LTE frequency band. In many countries, the 1,800 MHz band is the broadest frequency block available for LTE. The 1,800 MHz band is ideally located between lower frequencies, which are used for area coverage, and higher frequencies, which are used to increase capacity. They fit very well with an infrastructure in metropolitan areas. Since most travelers travel to metropolitan areas, the likelihood is very high that global roaming is possible in the 1,800 MHz band.

So that worldwide roaming for LTE is possible, an LTE mobile device must support additional frequencies. The other main bands include 700 MHz (USA), 800 MHz (Europe), 1,700 MHz (USA) and 2,600 MHz (Europe, Asia, Middle East, Africa, Latin America). In order for an LTE device to be used worldwide, it must in future transmit for LTE in 700, 800, 1,800 and 2,600 MHz, for UMTS in 850, 900, 1,900 and 2,100 MHz and for GSM in 850, 900, 1,800 and 1,900 MHz can.
Which frequency bands an LTE device can handle depends on the equipment and certainly also on the price. Current mid-range devices should be able to handle around 10 frequency bands. There are also devices in the high-end area that can handle 22 or 23 frequency bands.

However, this would not mean that all obstacles would be overcome. The worldwide use of the various frequency bands still differs in the duplex method. This means how the frequency is used for the send and receive directions. The duplex method used decides whether the send and receive directions are assigned in the same or in different frequency blocks. Most LTE frequency bands are specified for the FDD method, in which the sending and receiving directions have their own frequency ranges. The other frequency bands are intended for TDD operation, in which the frequency block applies to both directions.
The use of the duplex method and the corresponding frequency band has consequences in practice. This makes it difficult to change network operators and restricts roaming. Under certain circumstances, the maximum achievable data rate cannot be used in all networks due to the duplex method.

CA - Carrier Aggregation (frequency carrier bundling)

Every new mobile radio technology must be able to interconnect as many carriers (frequency ranges) as possible. The frequency bands are typically 800, 2,000 and 2,600 MHz. With a carrier of 10 MHz in the 800 MHz band and a 20 MHz carrier in the 2,600 MHz band, a frequency range of 30 MHz would be possible. The data streams can be flexibly distributed to the individual frequency bands.

In order to keep the energy consumption and the complexity of the devices low, a primary frequency carrier is defined that every terminal device uses. If the need for higher transmission rates increases during a connection, the secondary frequency carriers are switched on within a few milliseconds. Such a secondary frequency carrier can also be located in a license-free frequency range (e.g. 5 GHz).

So that the frequency carrier bundling in Release 10 is backwards compatible with Release 8 and 9, the maximum bandwidth of a single frequency carrier is limited to a maximum of 20 MHz. However, up to 5 of these 20 MHz channels can be bundled and thus a frequency range of up to 100 MHz can be achieved. However, only in the downlink, not in the uplink.

In practice it is the case that not only the network operator has to interconnect the frequency ranges, but also the end devices. For LTE-A, devices from category 6 are required. It is also assumed that the selected tariff enables LTE-A, which you should see on the status bar on the display of a smartphone. But just because a device basically supports the frequency ranges does not mean that this device can interconnect these frequency bands via carrier aggregation.

MIMO - Multiple Input Multiple Output (Mehrantennentechnik)

Another measure is the MIMO multi-antenna technology. This is a spatial multiple access in which several signals are spatially separated and sent at the same time. The receiver receives various signals, from which it gets the most out of it through downstream signal processing. This increases the likelihood that a usable signal will arrive at the receiver even under unfavorable reception conditions. This increases the transmission rate significantly. LTE Advanced also uses MIMO in the uplink. So from the participant to the base station.

Whether the multi-antenna technology is built into a smartphone depends on the miniaturization and improvement of the battery technology. In stationary devices such as routers, a 4x4 MIMO transmitter and receiver unit should be easier to implement and install than in a smartphone. In addition to the increased space requirement, the additional electronics lead to a higher power consumption, so that only stationary end devices with their own power supply or end devices with a high battery capacity come into question.

MIMO technology usually only brings something to the base station. Only there is enough space and the necessary energy supply available. Smartphones usually only have space for 2x2 sender / receiver units with antennas. More is not possible, if only because the battery operation no longer allows. Only stationary cellular routers have enough power and space for more antennas and their electronics.

LTE transmission technology

LTE works with scalable and individual channels so that several mobile devices can transmit data at the same time. Specifically, this means that the frequency spectrum is divided and assigned to individual devices for a certain period of time.
OFDMA is used for the downlink. OFDMA divides the available frequency band into many narrow bands (channels). This means that LTE can manage with frequency bands of different sizes. The bandwidth is used flexibly in order to get the maximum transmission power out of the frequencies.

The frequency band (10, 15, 20 MHz) is divided into subcarriers of 15 kHz each. 12 subcarriers are combined to form a resource block (RB), which is the smallest unit that can be assigned to an LTE device. A device can occupy one or more resource blocks in each direction. The number depends on the capacity utilization of the cell and the signal quality. The upper limit results from the width of the frequency block that the base station uses. With a 10 MHz frequency block this is 50 resource blocks. At 20 MHz it is 100.

The transmission of a block is timed to 10 ms (frame). That's 10 blocks per second. Each frame in turn consists of 10 subframes. One transport block can be transmitted per subframe. This varies in size depending on the signal quality. The size of the transport block essentially depends on the signal quality. The signal quality determines which modulation is used, what the relationship between user data and error correction (code rate) is and how many resource blocks are used. These three parameters are directly related to one another.

Special algorithms select the suitable channels and take into account the influences from the environment. Only those carriers are used for transmission that are cheapest for the user.
SC-FDMA (Single Carrier Frequency Division Multiple Access) is used for the uplink. This is very similar to a single carrier access method and OFDMA. SC-FDMA has less power fluctuations and makes power amplifiers possible more easily. Above all, this saves the battery of mobile devices.

LTE also works with spatially separated data streams. The LTE specification provides for 4 antennas in the base station and 2 antennas in the end devices. The transmission signal is forwarded to several transmission antennas for transmission. The received signals are received by two antennas (MIMO). A better signal is then calculated from both signals. This achieves a better data throughput because both transmission and reception paths are not subject to the same disturbances (losses and interference). This procedure is also specified in a modified form in WLANs according to IEEE 802.11n. In addition, LTE, like HSPA, uses the same shared channel principle, as well as HARQ and AMC.

Calculate transmission speed

Assuming 50 resource blocks for a 10 MHz frequency block, the maximum transport block size is 36,696 bits. To calculate the data rate per second, multiply this by 1,000 subframes per second (10 ms x 10 x 10 subframes = 1,000 subframes per second).

  • 36,696 bits x 1,000 subframes / s = approx. 37 Mbit / s

In addition, different signals are sent on the same frequency using multiple antenna technology (MIMO). With two antennas per base station and mobile device, this doubles.

  • 36,696 bits x 1,000 subframes / s x 2 MIMO = 73.392 Mbit / s

The maximum data rate of an LTE cell at 10 MHz per second results in 73.392 Mbit / s (often rounded as 75 Mbit / s). However, so that the user data can be transmitted successfully, an additional error correction must be carried out, which depends on the signal quality. The worse the signal quality, the higher the error correction code component in the transmission. Without error correction, the code rate would be 1. But in practice it doesn't work without error correction. The lowest code rate is 0.93. The worse the signal quality, the more the code rate also drops.
In addition, with poorer signal quality, a switch is made to more robust modulation methods that include fewer bits per transmitted symbol. The further away a subscriber is from the base station, the poorer the signal quality and the more likely slower modulation methods are used and the lower the code rate.
In addition, the network needs some of the capacity for signaling and protocol headers. Assuming a very good connection in direct proximity to the base station, a data rate of a maximum of 50 Mbit / s per cell can be achieved on a 10 MHz channel. All participants in the cell have to share this data rate.

Overview: LTE

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