3GPP Release 10 to 12:  LTE features and its impact on telecom services

Avinash Hachappa of Keysight Technologies, says LTE-Advanced is an evolutionary step in the continuing development of LTE with one of the main targets being to ensure LTE would be fully compliant with the IMT-Advanced 4G requirements.

These include higher bandwidth, at least 40 MHz, higher bitrates with higher spectral efficiency, ability to handle a greater number of simultaneously active subscribers, and at the same time improved performance at cell edges.

The present technical note contains a high-level description of the 3GPP Release 10, 11 and 12 features with focus on LTE-Advanced air interface and Release 12 introduction of a new low complexity User Equipment (UE) category “Cat-0”.

LTE-Advanced is the evolved version of the Long-Term Evolution (LTE) standard developed by 3GPP to meet or exceed the requirements of the International Telecommunication Union (ITU) for a true fourth generation (4G) radio communication standard known as IMT-Advanced. LTE-Advanced is defined in 3GPP Release 10 and in subsequent 3GPP releases.

With the considerable level of development being undertaken into the Internet of Things, also referred to as cellular-IoT , there has been a growing need to develop an LTE category focused on these applications. Here, much lower data rates are needed, often only in short bursts with a requirement for the remote device or machine to be able to draw only low levels of current.

To enable the requirements of these devices to be met, new LTE Cat 0 was introduced in Release 12 of the 3GPP standards, and it is being advanced in further releases. This technical note gives an overview of the following topics:

# Release 10 and LTE-Advanced

# Release 11 LTE-Advanced enhancements

# Release 12 radio evolutions

# Cat 0 UE features with reference to 3GPP releases 12

Release 10 and LTE-Advanced

In the feasibility study for LTE-Advanced, 3GPP determined the existing Release 8 LTE could meet most of the IMT-Advanced requirements apart from minimum bandwidth of 40 MHz, uplink spectral efficiency and the peak data rates. These requirements were addressed with the addition of the following LTE-Advanced features introduced in Release 10:

Wider bandwidths, enabled by carrier aggregation

Higher efficiency, enabled by enhanced uplink multiple access and enhanced multiple antenna transmission (advanced MIMO techniques).

Carrier Aggregation

The most straightforward way to increase capacity is to add more bandwidth. LTE-Advanced extended the transmission bandwidth by means of so-called carrier aggregation (CA).  Each aggregated carrier is referred to as a component carrier (CC) and multiple CCs are aggregated and jointly used for transmission to/from a single UE. The CC can have a bandwidth of 1.4, 3, 5, 10, 15 or 20 MHz and a maximum of five CCs can be aggregated resulting in maximum bandwidth of 100 MHz.  However, in 3GPP Release 13, there is a goal to expand CA up to 32 CCs. The number of aggregated carriers can be different in DL (downlink) and UL (uplink); however the number of UL CCs is never larger than the number of DL CCs.  The individual CCs can also be of different bandwidths. LTE-Advanced is provided through aggregation of R8/R9 carriers, therefore, backwards compatibility can be maintained with R8/R9 UE. Carrier aggregation can be used for both FDD and TDD.

In R10 there are two CCs in the DL and only one in the UL (hence no carrier aggregation in the UL), in R11 there are two CCs in DL and one or two CCs in the UL when CA is used, R12 will include CA of FDD and TDD frequency bands, as well as support of aggregation of two CCs in uplink and three in downlink as shown in figure 1.

In order to enable exploitation of fragmented spectrum, CA combinations don’t have to be contiguous in frequency.  That is why there are intra-band (contiguous and non-contiguous) and inter-band (non-contiguous) CAs as depicted in Figure 2.

The easiest way to arrange aggregation is to use contiguous CCs within the same operating frequency band, called intra-band contiguous. For non-contiguous allocation it could either be intra-band, i.e. the CCs belong to the same operating frequency band, but are separated by a frequency gap, or it could be inter-band, in which case the CCs belong to different operating frequency bands.

Multiple Input Multiple Output (MIMO) or spatial multiplexing

MIMO  is used to increase the overall bit rate through transmission of two (or more) different data streams on two (or more) different antennas – using the same resources in both frequency and time, separated only through use of different reference signals – to be received by two or more antennas. A major change in release 10 is the introduction of 8×8 MIMO in the downlink and 4×4 in the uplink.

As shown in figure 3, in the DL there are nine different transmission modes, where TM1-7 were introduced in R8, TM8 was introduced in R9 and TM9 was introduced in R10. In the UL there are TM1 and TM2, where TM1, the default, was introduced in R8 and TM2 was introduced in R10.

The different transmission modes differ in a) number of layers/ streams, b) antenna ports used c) type of reference signal, Cell-specific Reference Signal (CRS) or Demodulation Reference Signal (DM-RS) and d) Precoding type.

In multi-antenna techniques, pre-coding is used to map the modulation symbols onto the different antennas. The type of pre-coding depends on the multi-antenna technique used as well as on the number of layers and the number of antenna ports. The aim with pre-coding is to achieve the best possible data reception at the receiver.

Additional functionalities introduced in R10 include relay nodes, enhanced inter-cell interference coordination (eICIC) and heterogeneous deployments.

Relay nodes are low power base stations wirelessly connected to the remaining part of the network and provide enhanced coverage and capacity at cell edges, hot-spot areas, and it can also be used to connect to remote areas without fiber connection. From a UE point of view, the relay node appears as an ordinary base station.

Enhanced inter-cell interference coordination (eICIC) is basic support for inter-cell interference coordination started in R8 and is enhanced in Release 10 with the eICIC work item.

Heterogeneous Networks (HetNets), are a combination of large macro cells with small cells resulting in heterogeneous networks. A variety of new base station (BS) types have been introduced including the local area BS (picocell), home BS (femtocell), and relay node. Even though HetNets were part of Release 8, Release 10 added new techniques to handle inter-layer interference that could occur between small cell and the overlaid macro cell.

Self-organizing networks (SON) improvements were also part of Release 10 including self-healing procedures.

Release 11 LTE-Advanced enhancements

Release 11 further extended the work of Release 10 with enhancements to LTE-Advanced as well as support for additional frequency bands.  New key features introduced in Release 11 with respect to physical layer aspects include:

Carrier aggregation enhancements

R11 introduced the capability for intra-band non-contiguous operation in FDD and TDD, and added additional numbers of CA bands as shown in Table 1. In addition, RF requirements were extended to cover the new CA bands in Release 11 as well as new RF tests such as “cumulative ACLR” and “cumulative Spectrum Emission Mask (SEM)” for the intra-band non-contiguous case to take into account additional emissions within the sub-block gap.

Table 1

Coordinated Multi Point operation (CoMP)

The main reason to introduce CoMP is to improve network performance at cell edges. In CoMP, a number of transmit points provide coordinated transmission in the DL, and a number of receive points provide coordinated reception in the UL. A TX/RX-point is made up of a set of co-located TX/RX antennas providing coverage in the same sector. The set of TX/RX-points used in CoMP can either be at different locations or co-sited, but providing coverage in different sectors, they can also belong to the same or different eNBs. CoMP can be done in a number of ways, and the coordination can be done for both homogenous networks as well as heterogeneous networks.

Figure 4 portraits two simplified examples for DL CoMP. In both these cases, DL data is available for transmission from two TX-points. When two or more TX-points transmit on the same frequency in the same subframe, it is called Joint Transmission.  When data is available for transmission at two or more TX-points but only scheduled from one TX-point in each subframe, it is called Dynamic Point Selection. For UL CoMP there is Joint Reception, a number of RX-points receive the UL data from one UE and the received data is combined to improve the quality. When the TX/RX-points are controlled by different eNBs, extra delay may be added as the eNBs must communicate in order to make scheduling decisions, for example. When CoMP is used, additional radio resources for signaling is required to provide UE scheduling information for the different DL/UL resources.

Enhanced downlink control channels (E-PDCCH)

The continued introduction of features such as carrier aggregation, CoMP and enhanced downlink MIMO has resulted in the need to enhance the capabilities of the physical downlink control channel (PDCCH). As defined in the Release 11 core specification, the enhanced PDCCH, provides more signaling capacity, supports frequency domain ICIC, improves the spatial reuse of the control channel, supports beamforming and diversity schemes, and operates in MBSFN subframes. Frequency-selective scheduling for the EPDCCH is also desirable as is mitigation of inter-cell interference.

Further enhanced inter-cell interference coordination (FeICIC)

Interference cancellation technique for UE (e.g. CRS canceller from Macro-cell) –  Some of the work on eICIC was not completed in Release 10; therefore, the FeICIC work item was created for Release 11. This includes specification of system performance requirements for scenarios involving a dominant downlink interferer.

Improved minimum performance requirements for E-UTRA: interference rejection

Rel. 11 LTE has introduced Minimum Mean Square Error Interference Rejection Combining (MMSE-IRC) receivers as a UE interference rejection and suppression technology to mitigate the effects of these interference signals, and increase user throughput even in areas that are recently experiencing high interference.

As depicted in Figure 5 receivers are able to use the multiple receiver antennas to create points, in the arrival direction of the interference signal, where the antenna gain drops (“nulls”), and use them to suppress the interference signal. The terminal orients a null toward the main interference signal, which is the signal that particularly affects the degradation of throughput, thereby improving the Signal-to-Interference-plus-Noise Ratio (SINR) and improving throughput performance.

Release 12 radio evolutions

R12 added further frequency bands and band combinations to the existing carrier aggregation modes. It introduced two new modes, enabling three downlink/one uplink carriers as well as two non-contiguous uplink carriers. In addition, for operators with both FDD and TDD spectrum, R12 started a framework for aggregation between FDD and TDD carriers.  Increasing the number of component carriers and the total bandwidth supported in both the downlink and uplink. Figure 6 shows the enhancements being made to LTE-A in R10 and above.

Further enhancement to LTE-Advanced features

Small cell enhancements

Small cells have been supported since the beginning with features like ICIC and eICIC in release 10. Release 12 introduces optimization and enhancements for small cells including support for 256 QAM modulation for low mobility sparse indoor scenarios, dense small cell deployments using multiple small cell clusters, small cell deployment without concurrent coverage of macrocells, higher frequency band support such as 3.5 GHz, reduced transition time for small cell on/off and dual connectivity i.e. inter-site carrier aggregation between macro and small cells.

Machine Type Communication (MTC)

Rel-12 enhanced LTE-Advanced ability to support MTC/M2M applications. Focused on low cost and extended coverage, on low cost enhancements, a new UE category with reduced data rate, half duplex support and single receive antenna was introduced. Huge growth is expected in machine type communication in the coming years, which can result in tremendous network signaling and capacity issues. To cope with this, new UE cat 0 is defined for optimized MTC operations.

WiFi integration with LTE

With integration between LTE and WiFi, operators will have more control on managing WiFi sessions. The intent is to specify mechanisms for steering traffic and network selection between LTE and WiFi.

Device-to-Device Proximity Services

D2D will facilitate the interoperability between critical public safety networks and ubiquitous commercial networks based on LTE. D2D fundamentally alters the cellular architecture, reducing the primacy of eNBs and enabling UE devices to transmit directly to nearby UE devices.

Network-Assisted Interference Cancellation and Suppression (NAICS)

NAICS is a new category of UE receivers with advanced interference suppression enhancement and interference cancellation- subtraction of interference replica from received signal – including cell edges. COMP and NAICS are two techniques being designed to improve the performance of LTE at the cell edge where inter-cell interference is at its worst.  Although they are targeting the same issue, they take different approaches.

Category 0 UE features with reference to 3GPP releases 12

The LTE UE categories define the performance specifications of the UE to enable base stations to be able to communicate effectively with them knowing their performance levels.

One major advantage of LTE Cat 0 is that the modem complexity is considerably reduced when compared to other LTE Categories. It is expected the modem complexity for a Cat 0 modem will be around 50% that of a Category 1 modem, further improvement is done on Release 13, comparison in Figure 8. This new category has a reduced performance requirement that meets the needs of many machines while significantly reducing complexity and current consumption. While Cat 0 offered a reduced specification, it still complied with the LTE system requirements.

Release 12 introduces important improvements for M2M with the below features:

# Low cost device

# Long battery life

# Enhanced coverage

# High capacity

# Support for narrow bandwidth

Figure 8

Further device complexity reduction is achieved (in R13) by reducing UE receive bandwidth to 1.4 MHz. The UE will still be able to operate in all existing LTE system bandwidths up to 20 MHz.

By Avinash Hachappa of Keysight Technologies
editor@telecomlead.com