By the end of 2012, 150 LTE networks will be on air in 64 countries worldwide. Compared to previous technology roll-outs (2G, 3G, 3.5G), this is by far the wireless industry’s fastest. Despite marketing claims by network operators advertising LTE deployments as 4G, from a strictly technical perspective, they are not. LTE as defined by 3GPP Release 8 does not meet all IMT-Advanced requirements set by ITU for a true 4G technology1(ITU-R M.2134). 3GPP (3^rdGeneration Partnership Project), the standardization body behind LTE, is addressing and exceeding these requirements, while standardizing LTE-Advanced as part of its Release 10. This article describes one of the most requested features of LTE-Advanced in greater detail: carrier aggregation.

Carrier Aggregation and LTE-Advanced

While the industry still faces challenges with LTE (that is providing circuit-switched services, such as SMS and voice via the “All-IP”-based network architecture) standardization is enhancing LTE to meet the IMT-Advanced requirements outlined in Table 1. Two steps are required to achieve the spectral efficiency and requested peak data rates for downlink and uplink. First, enhancing the multi-antenna capabilities in downlink (up to 8×8 Single-User MIMO²) and allowing multi-antenna support in the uplink (up to 4×4 Single-User MIMO); second, applying carrier aggregation. LTE-Advanced as specified by 3GPP Release 10 (Rel-10), 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 backward compatibility, each carrier is configured to be 3GPP Release 8 (Rel-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, 3, 5, 10, 15 or 20 MHz. This is dependent on each network operator’s spectrum availability. Currently RAN4 discusses constellations with 5, 10, 15 and 20 MHz channel bandwidth. Since no service provider owns continuous spectrum of 100 MHz, three carrier aggregation modes are possible within LTE-Advanced: intra-band contiguous and non-contiguous as well as inter-band carrier aggregation (CA). Rel-10 already comprises intra-band contiguous and inter-band CA but intra-band non-contiguous CA must wait for Rel-11. Intra-band describes the aggregation of component carriers within the same frequency band in a contiguous or non-contiguous way. For inter-band carrier aggregation, the two component carriers reside in different frequency bands. Figure 1 shows the different modes of carrier aggregation.

Inter-band carrier aggregation has resulted in many band combinations requests by network operators worldwide. Especially in the U.S., there is a competitive situation among carriers over how much spectrum is available to each of these service providers: contiguous, non-contiguous as well as in different frequency bands. Carrier aggregation is clearly considered as the best possible way to combine frequency allocations and therefore is often referred to as spectrum aggregation. Table 2 shows the band combination that RAN4 is currently considering. As mentioned above, U.S. operators have submitted the majority of band combinations. Most combinations call for aggregation of currently deployed LTE networks at 700 MHz or, in general, lower frequencies with frequency blocks around 2 GHz, mostly in the so-called Advanced Wireless Services (AWS) spectrum. In 3GPP terminology, AWS corresponds to frequency band 4. The Federal Communication Commission (FCC) auctioned AWS frequencies in 2006, whereas 700 MHz frequency band licensing occurred in February 2008.

Are All Component Carriers Equal?

The single most important question is: How does the network activate carrier aggregation? The answer is simple: only in connected mode. Before this can actually happen, a Rel-10 supporting mobile device must execute the generic access procedures defined for LTE as of Rel-8: cell search and selection, system information acquisition and initial random access. All these procedures are executed on the so-called primary component carrier (PCC) for downlink and uplink. Secondary Component Carrier (SCC) – in total up to four, initially two – are considered as additional transmission resources. The basic linkage between the PCC in downlink and uplink is signaled within system information block type 2 (SIB Type 2). The PCC is device-specific, not cell-specific. That means, for instance, two terminals of the same network operator could have their PCC on different frequency bands. If one sticks with the U.S.-based example, terminal #1 could have its PCC in the AWS spectrum, whereas terminal #2’s PCC could be at 700 MHz. However, the most likely initial deployment scenario is that the PCC is configured for 700 MHz because the carriers’ initial LTE deployments are in the lower frequency bands. Nevertheless, the network can change the PCC for a terminal while executing the handover procedure. Besides being used for initial access, only the PCC in the uplink can carry the Physical Uplink Control Channel (PUCCH), for uplink control information transmission. Any additional SCC in the uplink provides only the Physical Uplink Shared Channel (PUSCH).

However, even if carrier aggregation in the uplink is defined with Rel-10, it is most likely that initial LTE-Advanced deployments for FDD make use of carrier aggregation only in the downlink. That means there is an asymmetric aggregation of component carriers: two in the downlink and only one in the uplink, for example. For TD-LTE, carrier aggregation typically results in a symmetric aggregation. But first systems may feature two DL and one UL CA for TDD as well. In FDD systems, which are the majority of current LTE deployments, symmetric carrier aggregation with up to five CCs, as shown in Figure 2, as well as intra-band non-contiguous CA, is supposed to be rolled out in a second step. Reasons are quite simple. Uplink carrier aggregation, for inter-band, requires a second transmit chain,¹which leads to a more complex device design and higher power consumption. In addition, things like power control per (uplink) component carrier, related power head room reporting, buffer status reports as well as timing advance are more challenging to implement and must be thoroughly tested and verified, which results in longer time to market. This would not be in line with the aggressive roadmaps of some network operators for carrier aggregation. Therefore, this two-step deployment approach seems plausible and is backed up with the submitted work items (WI) to RAN4, where only one network operator requested inter-band carrier aggregation for FDD including aggregation of two uplink component carrier³[RP-120364, Rapporteur: SK Telecom (South Korea)].

What Type(s) of Carrier Aggregation Does a Device Support?

It is important to note that there are certain limitations to which frequency band combination a Rel-10-capable terminal can support. A device enabled for global roaming and with multi-technology support must at least support four GSM frequency bands, five 3G/WCDMA frequency bands and three LTE bands, if just support of technologies defined by 3GPP is considered. In addition, support of GPS, FM, Bluetooth, WiFi and, eventually, Near Field Communications (NFC)⁴are important. Each technology requires its own transmit-receive (TRX) chain and space is limited due to the size of today’s smartphones. The more TRX elements, the higher the power consumption. Because of these limitations, a device that supports LTE-Advanced will submit additional information to the network during the UE capability procedure. The UE capability transfer is part of the Default EPS bearer establishment – shown in Figure 3 – that takes place after contention resolution of the initial random access procedure. The submitted UE capabilities, already enhanced with Rel-9, have been extended by Rel-10.

With regards to the supported band combinations, the RF-Parameters-v1020 information element provides this important detail to the network. Capabilities are signaled per frequency band, separately for downlink and uplink. Furthermore, so-called bandwidth classes are indicated on a per band basis, including the support of either intra-band (contiguous or non-contiguous) and inter-band carrier aggregation. Table 3 shows supported bandwidth classes for LTE-Advanced as defined by the actual version of the related 3GPP specification. Figure 4 provides an example of which terminology the device uses to indicate carrier aggregation support for a particular frequency band or frequency band combination (R4-122764, CA configuration acronyms for non-contiguous intra-band CA, Nokia Corp.).

Let’s take the intra-band non-contiguous case with CA_25A_25A as an example. It tells the network that this device can receive (or transmit) two separate carriers in frequency band 25, each with a maximum bandwidth of 100 RB, or in other words 20 MHz. If this device could aggregate two carriers in that frequency band, but continuously, the acronym would change to CA_25C. Bandwidth class C defines an aggregated transmission bandwidth between 100 and 200 RB, allocated to two component carriers. Obviously, 3GPP has work to do for bandwidth classes D, E and F, which are marked FFS – For Further Studies. It is fair to say that, initially, an aggregated bandwidth of 200 RB maximum, equal to 40 MHz as required by IMT-Advanced using two separate component carriers, will be used. Once the network is aware of the carrier aggregation capabilities of the device, it can add, modify or release SCC by means of the RRC-Connection Reconfiguration message that has been enhanced with Rel-10. But, let us take a step back.

Impact of Carrier Aggregation on LTE Signaling Procedures

Generally speaking, carrier aggregation signaling 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, are considered additional transmission resources. For the Packet Data Convergence Protocol (PDCP) and Radio Link Control (RLC) layer, carrier aggregation signaling is transparent. The latter, compared to Rel-8, needs only to support higher data rates with a larger buffer size. The buffer size is defined by the UE category the device belongs to. With Rel-10, three new categories have been added (see Table 4).

Carrier aggregation is not limited, however, to these new categories. Rel-8 device categories 2 to 5 can be also capable of carrier aggregation. A terminal is configured on the Radio Resource Control (RRC) layer to handle secondary component carriers provided by secondary cells. Moreover, on RRC the parameters of the SCell(s) are set, that is 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. In case of activation in subframe n, then 8 subframes (8 ms) later, the resources are available to the device and it can check for scheduling assignments. At this moment, a newly introduced timer (SCellDeactivationTimer r-10) will be started. Assuming an SCC has been configured for a device using RRC signaling and has been activated via MAC, but no scheduling information in a certain period via the PDCCH is received, this SCell will be deactivated on MAC after the timer expires. The timer can be set to infinity, however, to override deactivation. When the MAC acts as multiplexer, each component carrier has its own Physical Layer (PHY) entity, providing channel coding, HARQ, data modulation and resource mapping. On the primary and each secondary component carrier, both types of synchronization signals are transmitted to allow the device detection and synchronization. Figure 5 shows the control plane signaling, highlighting the layers involved in activating carrier aggregation for a particular handset. Returning to the extension of the RRCConnectionReconfiguration message at RRC layer, a maximum of four secondary cells can be activated. For each cell, its physical cell identity is sent, the explicit downlink carrier frequency as an Absolute Radio Frequency Channel Number (ARFCN) as well as common and dedicated information. For the two latter ones, the transferred information is separated for downlink and uplink. Common information (That is information applicable to all devices to which this carrier will be added) includes its bandwidth, PHICH and PDSCH configuration and, in case of TD-LTE, the UL-DL configuration and special subframe configuration. Further, the MBSFN subframe configuration is part of the downlink information. With Rel-9, broadcast/multicast capabilities have been fully defined for LTE and summarized as enhanced Multimedia Broadcast Multicast Services (eMBMS). With this feature, a mixed mode is possible, where certain subframes of an LTE radio frame are used for broadcast purposes. In terms of carrier aggregation, this is important information to the device, because it need not check subframes, which are assigned to MBSFN. Similarly, for the uplink, carrier frequency and bandwidth information are signaled, as well as power control-related information and uplink channel configuration (PRACH, PUSCH).

Dedicated information (That is information applicable to a particular terminal) includes the activation and use of so-called cross-carrier scheduling, which is an optional device feature. Its support is also indicated to the network during the UE capability transfer procedure. The use of cross-carrier scheduling is linked to Heterogeneous Network (HetNet) deployment scenarios with carrier aggregation, where it is used to measure interference reduction. In brief, HetNet’s aims to improve spectral efficiency per unit area, using a mixture of macro-, pico-, femto-cell base stations and relays. In these deployment scenarios, interference control and management is introduced. The question remains of how to schedule resources when carrier aggregation is activated for a device. The answer is in the definition of cross-carrier scheduling. Instead of decoding 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 SCC. This is implemented by extending the Downlink Control Information (DCI) formats (which carry scheduling assignments) with a so-called Carrier Indicator Field (CIF). This new 3-bit field enables the terminal to clearly identify the component carrier intended by the decoded scheduling decision. Figure 6 illustrates this principle. As previously noted, cross-carrier scheduling is enabled by RRC signaling. 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 subframe are for control data. Thus this information, referred to as PDSCH-Start, must be signaled to the device during activation of cross-carrier scheduling and is therefore part of the related information element. Dependent on the bandwidth of the component carrier, this could be 1 to 4 OFDM symbols. It is also important to point out that for cross-carrier scheduling, when resources on a component carrier are scheduled via another carrier (Such as the PCC), no resources on that SCC for that terminal can be scheduled by any other component carrier.

However, initial deployments with carrier aggregation will utilize resource allocation according to Rel-8. This means that the terminal will check on the PCC as well as on all activated SCC’s for the PDDCH to decode the associated DCI format and demodulate the assigned PDSCH resources. HetNets with cross-carrier scheduling will be deployed in a second phase.

Testing Requirements for Carrier Aggregation and LTE-Advanced

LTE-Advanced is a complex and powerful technology enhancement. The variances permitted in carrier aggregation increase mobile device complexity. The major design challenge is at the transceiver front end, which must support multiple band combinations. This requires the use of highly flexible switches, wideband power amplifiers and tunable antenna elements. That places exceptional performance requirements on test and measurement equipment. Without adequate planning during the selection process, test equipment can prove inadequate or quickly become obsolete. The introduction of intra-band (contiguous, non-contiguous) and inter-band aggregation with two component carriers, for instance, calls for a single instrument that supports all 3GPP frequency bands that LTE can utilize. The tester should also be capable of handling all combinations of inter-band carrier aggregation, including 2×2 MIMO and support of different bandwidths per component carrier (up to 20 MHz each). Other test design challenges are at the PHY layer, signaling and mobility. Testing these functions requires a comprehensive set of test scenarios best provided by a T&M company with broad experience in physical-layer testing. Complexity increases additionally, when talking about mobility testing for carrier aggregation including multi-antenna technology, known as MIMO that is 2×2. In today’s test labs, often multi-box setups are used to simulate multiple cells of different technologies (such as LTE, 3G/WCDMA or 2G/GSM) for various types of mobility testing, PLMN and cell selection scenarios or neighbor cell measurements. Such setups will pay off another time, while being used testing mobility for carrier aggregation. Such a multi-box setup using the R&SCMW500 Wideband Radio Communication Tester is illustrated in Figure 7.

Vector signal generators also play an initial, critical role in testing carrier aggregation functionality at the PHY layer. 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. Configuring cross-carrier scheduling and the PDSCH, start offset of the secondary component carriers is also supported in addition to the generation of AWGN, fading and MIMO support.

Time alignment error (TAE) measurement presents additional test challenges. Frames of LTE signals at a base station antenna port are not perfectly aligned, but must fulfill certain timing requirements. The test setup in Figure 8 shows how this can be accomplished. A high-end signal and spectrum analyzer acts as master and is controlled by a software application inside the instrument. The program synchronizes the capture of IQ data from master and slave (a midrange spectrum analyzer).

Conclusion

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 PHY/MAC layer and the adaptations to the RRC layer must be thoroughly tested.

References

1. 3GPP TR 36.815 Further Advancements for E-UTRA; LTE-Advanced Feasibility Studies in RAN WG4, V9.1.0, see section 5.3.
2. 3GPP TS 36.141 E-UTRA Base Station (BS) Conformance Testing, V11.1.0.
3. 3GPP TS 36.321 Medium Access Control (MAC) Protocol Specification, V10.5.0.
4. 3GPP TS 36.331 Radio Resource Control (RRC) Pro