An RF Front-end Module for LTE/EUTRAN

The third generation partnership project (3GPP) responsible for the universal mobile telecommunication systems (UMTS) 3G system has defined the high-level requirements for the next generation of cellular telecommunications services. These requirements include reduced cost per bit, increased service provisioning, flexible use of existing and new frequency bands, simplified architecture and open interfaces as well as reasonable terminal power consumption. Dubbed LTE for Long Term Evolution, this service will enable much higher speeds along with much lower packet latency, which is necessary for the growing use of services such as VoIP.

The first technical specifications for LTE radio access were approved in late 2007. Final approval of the remaining specifications is occurring now (early 2008) with the initial conformance test specifications scheduled for this September. Operators and equipment vendors have started to announce their timelines for LTE rollout and are planning the first equipment shipments for 2009 with the initial commercial deployments to begin in 2010. As system specifications for LTE and evolved UMTS terrestrial radio access network (EUTRAN) applications become available, RF integrated device manufacturers are busy developing power amplifiers (PA) and front-end modules (FEM) for user equipment (handsets). First to market is Skyworks Solutions’ SKY77445 Band VII (2.6 GHz) high integration FEM, first introduced at the Mobile World Congress in Barcelona this past February.

LTE Requirements

As a “3.9G” or 4G technology, LTE will try to gain market share in a field that includes: HSPA+ (an evolved version of 3GPP HSPA); 3GPP EDGE Evolution; 3GPP2 Ultra-Mobile Broadband (UMB) (an evolution of CDMA2000 and 1xEV-DO); and Mobile WiMAX. LTE will include support of at least 200 active users in every 5 MHz cell along with HSPA (High Speed Packet Access), a combination of HSDPA and HSUPA with download rates of 100 MPs and upload rates of 50 MPs for every 20 MHz of spectrum. LTE uses an OFDM (Orthogonal Frequency Division Multiplex) signal depicted in Figure 1, comprised of 2048 different sub-carriers with 15 kHz spacing. The modulation format used within the OFDM signal is based on QPSK, 16QAM or 64QAM depending upon the prevailing operating conditions. LTE allocates a sub-carrier(s) to the mobile device for its link to the base station. An OFDMA access scheme for both the uplink and downlink is used; however, the uplink uses an implementation of OFDMA called Single Carrier Frequency Division Multiple Access (SC-FDMA). This form of modulation overcomes the high peak-to-average power ratio (PAPR) associated with the 3G systems using CDMA. The high PAPR of CDMA systems led to a considerable reduction in the transmitter power amplifier efficiency, which in turn reduced the battery life. SC-FDMA overcomes this problem and results in greater PA efficiency and longer battery life.

LTE also utilizes Multiple Input Multiple Output (MIMO) for enhanced data throughput and spectral efficiency. MIMO employs multiple antennas on the receiver and transmitter, taking advantage of multi-path affects and resulting in more reliable signal quality and greater bandwidth for greater range and capacity. The variable channel bandwidths specified for LTE increases the system’s flexibility and capability, but also add to its complexity. The use of multiple antenna configurations and OFDMA (and SC-FDMA) adds further complication to the development of next generation devices and results in some challenges unique to LTE. With performance targets for LTE set exceptionally high, engineers have to make careful design trade-offs to cover each critical part of both transmit and receive chains.

Integrated Technologies

Addressing these challenges is the SKY77445, a highly integrated, fully matched, 16-pin surface-mount module. The LTE FEM integrates the power amplifier, inter-stage filter, input and output matching, power detection and duplexer in a single 4 x 7 x 1.1 mm package. The FEM provides excellent Tx attenuation in the Rx-band, and operates at a low voltage of 3.3 V with high linearity and efficiency. The SKY77445 meets the stringent spectral LTE/EUTRAN requirements up to 23 dBm output power at 3.3 V battery voltage and up to 25.5 dBm for WCDMA. The FEM incorporates Skyworks’ BAW Inter-stage Filter and Duplexer, InGaP BiFET PA, output power detector and MCM packaging. Control pads are available to enhance the FEM performance at different power levels, as shown in Figure 2. The FEM’s transmitter operates over the 2500 to 2570 MHz band while the receiver covers 2620 to 2690 MHz.

Integration of the RF front-end greatly simplifies the design of the 3.9G-compatible handset radio or data card as all critical matching between the inter-stage filter, PA, power detection and duplexer is optimized within the single module component. By optimizing the efficiency of the InGaP BiFET PA MMIC, reducing RF loss between the integrated components and within the duplexer itself, and improving the match between the PA and the duplexer, this FEM achieves low current at maximum output power that significantly reduces the power dissipated in the LTE-enabled handsets or data cards.

FEM Performance

One new challenge facing LTE user equipment (UE) will be the need to handle variable channel bandwidths. All previous 3GPP systems have had one channel bandwidth, but LTE is being defined with eight different channel bandwidths varying from 1.4 to 20 MHz. Such flexibility allows for a rich set of new possibilities in deployment. However, this flexibility also presents significant new challenges with regard to how in-channel and out-of-channel requirements are specified as well as the operational aspects related to radio resource management (cell selection/re-selection, handover, etc.). To address these specific LTE needs, the SKY77445 supports up to 100 resource blocks and 1.25, 2.5, 5, 10 and 20 MHz bandwidths. Figure 3 shows the FEM performance for the 16QAM-10 MHz LTE modulation at P_out = 23 dBm.

The SKY77445 addresses all LTE/EUTRAN transmitter requirements, including maximum output power (MOP) and maximum power reduction (MPR); frequency error; power control (minimum output power, transmit ON/OFF power, out-of-synchronization handling of output power); control and monitoring functions; occupied bandwidth; UE spectrum emissions mask and ACLR for LTE; spurious emission requirements for LTE; transmit inter-modulation; and transmit linearity requirements (EVM < 3%) (see figure 4).

Packaging

The device is packaged using Skyworks’ miniaturized, low cost, multi-laminate substrate technology and is approximately half the size of individually packaged component solutions, as shown in figure 5. The SKY77445 front-end module can save handset and data card designers significant board space and design-cycle time, and significantly simplify supply chain and sourcing of RF components.

LTE is a strong contender for defining the next generation of wireless services. Addressing the stringent performance specifications called for by LTE will be especially challenging for engineers developing the microwave devices inside user equipment. The first FEM addressing LTE specifications, including variable channel bandwidths, spectral efficiency (via OFDM and MIMO), linearity and low power consumption has been introduced into the market. The availability of this part represents a shift from developing specifications to hardware realization, a critical step in LTE deployment.