How SAW Filters are Used in a Typical LTE RF Front-end
3GPP, as the global regulatory body governing the global standards for cellular telecommunications network technologies, must consider and accommodate the wide range of frequency bands that have been licensed and allocated to network operators all over the world.
As a result, a device intended for worldwide operation must in theory support all of the allocated bands. The number of bands already allocated for LTE is large, and growing. To illustrate, the first 3GPP LTE standard release, release 8, includes 15 FDD bands and 7 TDD bands, covering a frequency range from 700 MHz to 2.6 GHz; the most recent release, release 14, includes approximately 30 FDD bands and 20 TDD bands, covering a frequency range from 450 MHz to 5.9 GHz.
To design a device capable of operating worldwide, it is necessary for the device to support most of these bands. In a typical design, the digital processing of the radio signal is more or less independent of the frequency, but the analog radio front end, i.e., the device technology that transforms the digital signal into analog radio waveform, must be designed for a specific frequency band. The main functions of the radio front end are:
- on the transmit path: to transmit the signal with good spectral shape, adequate EVM (error vector magnitude) and best possible efficiency, amplifying it at the right power level, and not polluting adjacent channels with unwanted transmission;
- on the receive path: to receive the analog waveform and filter out all unwanted noise.
To enable these functions, a typical LTE transceiver design architecture utilizes power amplifiers, switches, and sharp filters on the transmit and receive chains, that are specific to the band of operation. When changing bands, a new set of filters must be employed. Typically, these filters are implemented using surface acoustic wave (SAW) technology. See Figure 1.2
Figure 1: SAW Filter Technology
Surface acoustic wave filters are the most commonly used filters in cellular technology since they provide good rejection for relatively low insertion loss and they are quite well suited to the band and bandwidths used in cellular. They convert electrical signals into mechanical waves within piezoelectric material. Related technologies include BAW (bulk acoustic wave) and F-BAR (film bulk acoustic resonator) filters.
Front-end architecture design is also a function of the duplexing mode. In FDD, the device is transmitting and receiving at the same time on a frequency separated by what is called duplex spacing. Specific filters called duplexers are used to create significant attenuation that prevents the transmit signal from overloading the receive signal. To achieve this sharp and high attenuation, SAW filters are used in the duplexers, usually resulting in significant insertion loss.
In TDD mode, transmit and receive occur on the same frequency, but are separated in time. In this case, the front-end design does not use a duplexer, but rather a switch that requires a sharp filter to reject unwanted signals from adjacent bands.
To summarize, a worldwide LTE device requires multiple RF chains in the front-end, requiring a corresponding number of power amplifiers, filters, and switches. Such front-ends are quite complicated and costly in a complete modem solution. For example, a typical high-end smartphone must support 15 to 20 frequency bands, requiring approximately 10 power amplifiers, 10 to 15 duplexers and 15 to 20 SAW filters for a total overall cost of more than USD 5.00.
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