13 May 2013

Carrier Aggregation – How to Test the Key Enabler for LTE Advanced

Meik Kottkamp, Technology Manager at Rohde & Schwarz describes the key issues behind LTE Carrier Aggregation and shows how they can be tested effectively

By March 2013 a total of 156 LTE networks in 67 countries were commercially operated. LTE has become a global phenomenon, but with only three years of real-life deployment the technology is still in its infancy compared with 2G/3G technologies.

However, things are changing quickly in mobile communications, and thus it is not surprising that the first LTE improvements have already been discussed and agreed on in 3GPP standardization. Moreover, the technology components specified in 3GPP Release 10 – also known as LTE-Advanced (LTE-A) – are gaining industry wide traction.

The feature that has top priority in LTE-A is carrier aggregation (CA). The focus on CA, specifically supporting two downlink (DL) carrier frequencies and one uplink (UL) carrier frequency, was also obvious at this year’s Mobile World Congress in Barcelona. This article gives a brief introduction to carrier aggregation and then describes the impact on testing both base station and end user devices.



Carrier aggregation and LTE-Advanced

LTE-Advanced was specified in 3GPP Release 10 in order to satisfy IMT-Advanced requirements set by ITU. LTE-A comprises essentially four features:

  • Enhanced MIMO schemes allowing operation of up to 8x8 MIMO in downlink and 4x4 MIMO in uplink .

  • Enhanced intercell interference coordination (eICIC) introducing the capability to escape interference in the time domain, which is particularly important in heterogeneous network deployments.

  • Enhanced SC-FDMA, an improvement in the uplink transmission scheme to increase uplink capacity at the burden of increased linearity requirements for the end user device transceiver.

  • Carrier aggregation, which is explained in more detail below LTE-A allows the aggregation of up to five component carriers with up to 20 MHz of bandwidth to attain a total transmission bandwidth of up to 100 MHz. However, 3GPP’s RAN Working Group 4 (RAN4) presently limits aggregation to two component carriers for a maximum aggregated bandwidth of 40 MHz – still in line with IMT-Advanced requirements.

To assure downward compatibility, each carrier is configured to be 3GPP Release 8 compliant. Each of the aggregated component carriers can use a different bandwidth. In fact, one of the six supported bandwidths within LTE: 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz or 20 MHz. This is dependent on each network operator’s spectrum availability. 3GPP RAN4 specified constellations with 5 MHz, 10 MHz, 15 MHz and 20 MHz channel bandwidth. Three carrier aggregation modes are possible within LTE-Advanced: intraband contiguous and non-contiguous as well as inter-band carrier aggregation (CA). Fig. 1 shows the different modes of carrier aggregation.

Modes of carrier aggregation

Fig. 1: Modes of carrier aggregation.

One reason for introducing CA was to achieve the 1 Gbps data rate requirement set by IMT-Advanced. However, what has been driving the implementation of CA is the feature’s capability to pick up and combine spectrum portions in the generally fragmented frequency space an operator owns. Inter-band carrier aggregation has therefore resulted in many requests for band combinations by network operators worldwide. CA is clearly considered as the best possible way to combine frequency allocations and therefore is often referred to as spectrum aggregation. Nevertheless, limitations in terms of the number of carriers and simultaneously received frequency bands as well as total bandwidth have to be considered.

The end user device will need to run two receiving chains, which has impact on complexity and power consumption. For this reason, initial implementations will be limited to two DL carriers comprising in total 20 MHz bandwidth (e.g. 10 MHz + 10 MHz or 5 MHz + 15 MHz) and only one UL carrier. In a second step, the total bandwidth is increased to 40 MHz.

What is important to know about carrier aggregation? A Release10 supporting mobile device executes the generic access procedures defined for LTE as of Release 8: cell search and selection, system information acquisition and initial random access. All these procedures are executed on the primary component carrier (PCC) for downlink and uplink.

Additional carriers – specifically, secondary component carriers (SCC) – are considered additional transmission resources. The basic linkage between the PCC in downlink and uplink is signalled within system information block type 2 (SIB type 2).

LTE-A CA SMU200A GUI

Fig. 2 R&S SMU200A vector signal generator GUI for configuring an LTE-A CA signal including cross-carrier scheduling

An LTE-A user device will submit additional information to the network during the user equipment (UE) capability procedure. With regard to the supported band combinations, the RF-Parameters-v1020 information element provides this important detail to the network. Capabilities are signalled per frequency band, separately for downlink and uplink.

Furthermore, bandwidth classes are indicated on a per band basis, including the support of intra-band (contiguous or non-contiguous) and/or inter-band carrier aggregation. Table 1 shows supported bandwidth classes as defined by the current version of the related 3GPP specification. Once the network is aware of the carrier aggregation capabilities of the device, it can add, modify or release SCCs by means of the RRCConnectionReconfiguration message that has been enhanced with Release 10.

Dedicated information (i.e. information applicable to a particular terminal) includes the activation and use of cross-carrier scheduling, which is an optional device feature. The use of cross-carrier scheduling is linked to heterogeneous network (HetNet) deployment scenarios with carrier aggregation, where it is used to facilitate interference reduction. Instead of decoding the physical downlink control channel (PDCCH) on each associated component carrier, the device just decodes the PDCCH on one carrier, presumably the PCC, to identify allocated resources on associated SCCs.

This is implemented by extending the downlink control information (DCI) formats (which carry scheduling assignments) with a carrier indicator field (CIF). Cross-carrier scheduling is enabled by radio resource control (RRC) signalling. Since the terminal no longer decodes the PCFICH on the associated (secondary) component carrier, it does not know how many OFDM symbols at the beginning of each sub-frame are for control data.

This information, referred to as PDSCH-Start, must be signalled to the device during activation of cross-carrier scheduling and is therefore part of the related information element. However, initial deployments with carrier aggregation will utilize resource allocation in line with Release 8. This means the terminal will check on the PCC as well as on all activated SCCs for the PDCCH to decode the associated DCI format and demodulate the assigned PDSCH resources.

From a base station perspective, the introduction of CA has limited impact. LTE base stations as deployed today already transmit multiple carriers depending on the spectrum available to a specific operator. 2x2 MIMO operation in downlink has been a requirement right from the start, and operation of two TX antennas is therefore a standard implementation as well.

Carrier aggregation bandwidth classes

 Table 1: Carrier aggregation (CA) bandwidth classes.

Testing carrier aggregation

LTE-Advanced carrier aggregation is a complex and powerful technology enhancement. The variances permitted in carrier aggregation increase mobile device complexity. Receiving multiple frequencies with an overall increased bandwidth requires significant redesign in the receiver chain.

Primarily the increased data rate capabilities need to be tested on all layers (physical layer, protocol stack and E2E). It also requires verifying the correct end user behaviour in terms of correctly responding to RRC messages. At the base station, the major design challenge is at the transceiver frontend, which must support multiple band combinations. This requires the use of highly flexible switches, wideband power amplifiers and tuneable antenna elements.

Base station testing

In order to verify functionality of components as well as for single-ended base station receiver testing, signal generators are generally used. Ideally, the instrument combines two complete signal generators – each with baseband section and RF up-conversion. As carrier aggregation signals can be exceptionally complex, an intuitive configuration is essential. The R&S SMU200A vector signal generator offers a user interface that allows the configuration of up to five component carriers with variable bandwidths of up to 20 MHz (Fig. 2). Configuring cross-carrier scheduling and the PDSCH start offset of the secondary component carriers is also supported, in addition to the generation of AWGN and in-build fading support.

From an RF perspective each single component carrier is identical to an LTE Release 8 carrier. Consequently, measurements such as ACLR, spurious emission and modulation accuracy need to be carried out. Multiple carrier frequency and also multiple standard radio analysis (MSRA) as offered by the R&S FSW are widely used. However, measuring the time alignment error (TAE) presents additional test challenges.

Frames of LTE signals at a base station antenna port are not perfectly aligned but must fulfil certain timing requirements. The test setup in Fig. 3 shows how this can be accomplished even in complex scenarios, when four TX antennas per component carrier (CC) are used. In the example, the R&S RTO oscilloscope is used to capture the I/Q data from the eight transmit antennas. The I/Q data is then analysed by the R&S FS-K10xPC LTE analysis software, providing the TAE in relation to CC1 on TX1.

Measuring TAE for carrier aggregation

Fig. 3: Set-up measuring TAE for carrier aggregation with the R&S RTO oscilloscope and R&S FS-K102PC LTE analysis software.

End user device testing

Testing the end user device that supports carrier aggregation focuses on the capability to cope with the eventually increased amount of data now received in parallel through two receiving chains. Testing has to be conducted on all relevant layers.

On the physical layer, the hybrid automatic repeat request (HARQ) procedure is to be verified by counting ACK/NACKs from the device under test. It is essential to allow easy configuration of different band combinations, to apply different modulation and coding schemes and/or to vary absolute power levels.

Carrier aggregation signalling affects only certain layers of the protocol stack. For instance, the device is permanently connected via its PCC to the serving primary cell (PCell). Non-access stratum (NAS) functionality such as security key exchange and mobility information are provided by the PCell. All secondary component carriers, or secondary cells (SCell), are considered additional transmission resources. For the packet data convergence protocol (PDCP) and radio link control (RLC) layer, carrier aggregation signalling is transparent.

The main testing impact resulting from carrier aggregation is related to the RRC layer. A terminal is configured on the RRC layer to handle secondary component carriers provided by secondary cells. Moreover, on RRC the parameters of the SCell(s) are set, i.e. configured. The medium access control (MAC) layer is the multiplexing entity for the aggregated component carriers as they are activated or deactivated by MAC control elements.

If activation is in sub-frame n, then eight sub-frames (8 ms) later the resources are available to the device and it can check for scheduling assignments. Fig. 4 shows the control plane signalling, highlighting the layers involved in activating carrier aggregation for a particular handset. Returning to the extension of the RRCConnection Reconfiguration message at the RRC layer, a maximum of four secondary cells can be activated.

Carrier aggregation signalling

Fig. 4: Carrier aggregation signalling, involved protocol layers (control plane).

For each cell, its physical cell identity, the explicit downlink carrier frequency as an absolute radio frequency channel number (ARFCN) as well as common and dedicated information are sent. For the common and dedicated information, the transferred information is separated for downlink and uplink. Common information (i.e. information applicable to all devices to which this carrier will be added) includes its bandwidth, PHICH and PDSCH configuration and, in the case of TD-LTE, the UL-DL configuration and special sub-frame configuration. Similarly, for the uplink carrier, frequency and bandwidth information, power control-related information and the uplink channel configuration (PRACH, PUSCH) are signalled.

Last but not least, the data rate performance also needs to be verified on the application layer when an E2E service is using the underlying LTE-A CA functionality.

Configuring an LTE CA signal


Fig. 5a and 5b: Configuring an LTE CA signal on the R&S CMW500 and performing physical layer throughput measurements.

Testing these functions requires a comprehensive set of test scenarios best provided by a T&M company with broad experience. The R&S CMW500 wideband radio communication tester provides all functionalities as described above. Using the test device as an RF call box tester allows physical layer throughput testing (Fig. 5). A number of examples of carrier aggregation LLAPI / MLAPI scenarios are available to verify the complete protocol stack implementation of an end user device (Fig. 6). E2E application layer testing is available using the data application unit (DAU) integrated in the R&S CMW500.

Carrier aggregation signalling

Fig. 6: RRCConnectionReconfiguration message adding an SSC for protocol stack testing executed on the R&S CMW500

In March 2013 the Global Certification Forum (GCF) started working on the relevant test cases for certification of LTE-A CA-capable devices. Upon finalization the work will include RF, RRM as well as protocol test cases. Although details remain to be specified, Rohde & Schwarz has already implemented first test cases on its R&S TS8980 test system based on 3GPP RAN5 specifications. Due to the increasing complexity of technology, leading operators have specified their own simulator-based interoperability testing (IOT) requirements for mobile handset testing in the lab. The focus of lab-based IOT device testing has shifted from pure conformance to defined minimum performance. This allows flexible and fast delivery of test solutions suitable for introducing new technologies such as carrier aggregation; however, individual investments and resources are required. Conformance (e.g. GCF), operator IOT and field tests complement each other. They need to be balanced depending on operators’ requirements and their specific business models. Thanks to its experience in the industry, Rohde & Schwarz can help operators to create their own IOT testing scheme and to successfully put it into operation.

Summary

Carrier aggregation is a key enabler for LTE-Advanced to achieve the peak data rates of the IMT Advanced requirements. It is highly desired by network operators because it enables the aggregation of spectrum fragments and offers a way out of the spectrum crunch.

The major design challenge is on the terminal side. Support of higher bandwidths and aggregating carriers in different frequency bands tremendously increases transceiver circuit complexity, including the design of components such as wideband power amplifiers, highly efficient switches and tunable antenna elements.

The additional functionality provided to the PHY/MAC layer and the adaptations to the RRC layer must be thoroughly tested. As a premium supplier of test and measurement solutions to the wireless industry, Rohde & Schwarz already offers today a comprehensive test portfolio to guide the design engineer through these challenges.

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About the author

Meik Kottkamp is Technology Manager within the Test & Measurement Division of Rohde & Schwarz in Munich. He is responsible for strategic marketing and product portfolio development covering the enhancements in existing and new 3GPP technologies. He started out with GSM/EDGE Evolution and HSPA+, followed by LTE. Now his focus is on LTE-Advanced and the emerging 5G research projects. Meik joined Rohde & Schwarz in August 2007 after working 11 years for Siemens/NSN. He holds a Dipl.-Ing. “Elektrotechnik, Hochfrequenztechnik”, i.e. degree of Electrical Engineering from the University of Hanover, Germany.

Rohde & Schwarz is an independent group of companies specializing in electronics. It is a leading supplier of solutions in the fields of test and measurement, broadcasting, radio monitoring and radio-location, as well as secure communications. Established almost 80 years ago, Rohde & Schwarz has a global presence and a dedicated service network in over 70 countries. It has approx. 8700 employees and achieved a net revenue of € 1.8 billion (US$ 2.3 billion) in fiscal year 2011/2012 (July 2011 to June 2012). Company headquarters are in Munich, Germany.

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