Coexistence solutions for smartphones
Working together on a shared antenna
Today’s smartphones must provide users with an increasing number of connectivity options. A broad range of advanced EPCOS coexistence solutions ensure the simultaneous operation of cellular services, GPS, and WLAN.
The coexistence of several communication services in compact mobile devices presents manufacturers of smartphones and RF components alike with a complex set of RF challenges. Today’s smartphones must typically support a broad array of cellular and wireless services over a very broad frequency range. In addition to cellular services such as GSM, UMTS, or LTE, for example, a smartphone must also offer GPS and Bluetooth/WLAN connectivity. In order to save space and improve performance, the standard architecture calls for the use of a primary cellular antenna for both Tx and Rx and a diversity antenna for Rx. As shown in Figure 1, a typical diversity antenna must support both GPS and WLAN along with cellular bands.
The RF challenges in smartphones are compounded by user expectations: Not only must the smartphone be slim and compact, but these multiband, multifunctional devices are expected to offer full functionality for all RF-based services simultaneously. In other words, users expect to be able to conduct a voice call via LTE and navigate in real-time with GPS, while a large file is being uploaded via WLAN – all at the same time and without any delays or comprises in performance. These expectations place very high demands on the RF filtering and processing in smartphones, especially on the filters and modules used to extract GPS and WLAN signals on the diversity antenna.
Based on the frequency bands needed to support cellular, GPS, and WLAN operations, RF designs must address three main coexistence challenges (Figure 2):
- Interference from extremely close adjacent communication bands (≤ 20 MHz)
- Second harmonic interference to GPS signals from lowband cellular Tx signals
- Intermodulation interference with GPS signals from WLAN Tx signals
|Figure 2: Coexistence challenges in smartphones|
Coexistence case 1: Extremely close adjacent bands
Spectrum is a limited resource and, with the continuously growing number of communication services and protocols, it is becoming ever more densely populated. In particular, the band used for WLAN and Bluetooth is separated by ≤ 20 MHz from the new bands 7, 40, and 41 that are used for LTE cellular service.
Under such circumstances, the demands placed on RF filters for the respective cellular and WLAN bands are very high. They must prevent the WLAN and highband cellular signals from interfering with each other. This calls for RF filters that combine very high selectivity and low insertion loss. Efficient RF filtering also has an impact on power consumption in smartphones. Minimized interference between adjacent bands means that transmit power can be reduced without affecting performance.
Coexistence case 2: Reliable filtering of second harmonics
Harmonic effects are a major challenge for the RF design of smartphones. Satellite navigation systems such as GPS, the Russian GLONASS, or Chinese BeiDou (also known as Compass) operate at frequencies between 1561 MHz and 1605 MHz. Moreover, the signal strength at the navigation receiver is very low. As a result, the navigation signals are susceptible to interference from the second harmonic of lowband cellular Tx signals, which are coupled from the main antenna to the diversity antenna. For this reason, additional filtering is required for the entire GPS receive path using high-linearity SAW filters in combination with a low noise amplifier (LNA) in order to prevent these lowband cellular signals from interfering with the GPS signal.
Coexistence case 3: Reliable filtering of intermodulation effects
Since the diversity antenna is also used to transmit WLAN signals, intermodulation can occur due to the frequency difference between WLAN signals and lowband cellular signals, and the resulting intermodulation products can interfere with the GPS Rx signals. Such nonlinearity effects can severely impair GPS performance, especially since WLAN Tx signals are typically > 150 dB stronger than the GPS signals. Effective filtering again requires highly linear filters and amplifiers.
How extractors enable joint use of antennas
The use of a single antenna to send and receive signals in multiple frequency bands requires the use of what are known as extractors. These frequency multiplexers separate the Rx signals (e.g. cellular, GPS) so that they arrive at their respective receivers. EPCOS already pioneered the development of GPS extractors years ago. A GPS extractor, for example, combines a GPS bandpass filter with a GPS notch filter. The bandpass filter permits only the GPS frequency bandwidth to pass through to the GPS receiver, while the notch filter allows the only the cellular signals to pass through to the cellular receiver. Figure 3 shows a typical GPS extractor with a bandpass filter for the 1575 MHz GPS Rx signal and a 1575 MHz notch filter to block the GPS signal at the cellular port.
|Figure 3: Basic circuit diagram of a GPS extractor|
A WLAN extractor can be realized in a similar fashion by employing a WLAN bandpass filter in place of the GPS bandpass filter. It is also possible to design a combined GPS+WLAN extractor into a single multiplexer, which simultaneously extracts both GPS and WLAN signals. The combined GPS+WLAN extractor shown in Figure 4 features two bandpass filters for the GPS and WLAN frequency bands at 1575 MHz and 2450 MHz, respectively, and a dual notch filter for both bands.
|Figure 4: Basic circuit diagram of a GPS+WLAN extractor|
High-performance coexistence solutions
TDK has applied these signal extraction concepts to create an innovative range of coexistence solutions designed especially for use in smartphones. Examples of key products are summarized in the Table.
Based on the success of the first EPCOS WLAN/Bluetooth BAW filter (B9604), which was launched two years ago and is one of the leading filter solutions for phones with LTE band 7, the filter performance has now been improved even further in the new EPCOS LS70 filter. This filter employs advanced second-generation BAW technology (BAW2), and thus achieves a much better Q factor and features a significantly lower insertion loss than the B9604 in a smaller package size. With a miniaturized footprint of just 1.1 mm × 0.9 mm, the 1109 package of the LS70 is also extremely thin with a height of just 0.4 mm.
The new EPCOS R159 GPS/GLONASS front-end module is a fully integrated front-end solution that enables the cellular receiver to share the cellular diversity antenna with the GPS receiver. The new module combines an LNA and a low-loss SAW GPS/GLONASS extractor for GPS and GLONASS (1575 MHz to 1605 MHz) in single 2.5 mm × 2.5 mm × 0.8 mm package. Thanks to the integrated matching and linearity enhancement circuits, no further external components are required.
The EPCOS R157 GPS/GLONASS/BeiDou front-end module, which supports full coexistence GPS, GLONASS, and BeiDou with cellular services and WLAN, represents today’s most advanced GPS front-end solution. The module combines a best-in-class SAW bandpass filter for GPS/GLONASS/BeiDou, matching and linearity enhancing circuits, and an LNA with a very low noise figure and supply current. This module measures in at a compact 2.5 mm × 2.5 mm × 0.8 mm. The module package of the R157 is covered with metal plating, which protects the GPS receive path from interference from spurious signals (received via air interface) into the GPS receiver chain. Thanks to its full shielding, the R157 is able to offer optimum sensitivity of the GPS receive path to signals that are at thermal noise level.
EPCOS coexistence products
Coexistence case 1 using dedicated WLAN antenna
EPCOS LS70 BT/WLAN BAW filter
Coexistence case 2 using shared GPS/diversity antenna
EPCOS R159 GPS-GLONASS frontend module
Coexistence cases 2 and 3 using dedicated GPS antenna
EPCOS R157 GPS-GLONASS-BeiDou frontend module