Microwave Engineering Europe - March 2009 - (Page 16) 16 FOCUS ON RADIO • A receiver chain capable of working in the downlink band • Switching of a downlink filter to the receiver input when in ‘Listen’ mode. Flexibility is an inherent requirement in the listen mode, covering all the bands. It is also prudent to use the existing antenna filter used for the transmitter when listening to the same band. Low power levels in femtocell make the addition of a low cost RF switch possible so that when required the Tx filter can be used as a receive path. If the system is limited to using a single receive path to implement then it is also necessary to also add a switch to the main receive path (figure 1). This can have a number of negative effects on receiver performance: • The switches insertion loss will reduce Rx sensitivity (≈0.5 dB) • Tx to Rx isolation of the diplexer requirement is >45 dB, hence switch isolation is dominates the Tx to Rx isolation (≈2 × 20 dB = 40 dB). Whilst these issues are insurmountable, any additional components required adds to the cost and complexity. By utilising an additional receiver input at the transceiver IC, the downlink listen path can be kept separate from the main receiver removing all the identified risks and reducing part count. Additional network listening modes As well as the requirement to listen to the downlink of a Macrocell at the same frequency and modulation type as itself, the femtocell may also be placed in a location where such option may not be available. Therefore, it is desirable to be able to obtain network information from other standards and bands. Hence, further requirements are placed on the listen mode path, primarily that it must be able to cope with different modulation standards (most commonly GSM) and frequencies that are possibly an octave or more separated from the primary transceiver band. Broadband operation Multi-band transceivers often use multiple LNAs each tuned to deal with different RF frequency bands, however in the femtocell market the frequency of the required listen mode needs to remain flexible to the geographical location of the deployment. A minimum set of receive frequencies to cover the popular European and U.S. bands would be: Microwave Engineering Europe ● March 2009 ● Figure 3: Minimum system implantation for targeted band and minimum BOM cost. Figure 4: Lime transceiver set up for multi-band, multi-standard operation covering WCDMA bands I and V with downlink listen mode and GSM900/1800 listen mode offering single BOM covering dominant European bands. • Main Receive o WCDMA (band I ,U.S. band class 6), Uplink – 1920 to 1980 MHz o WCDMA (band V, U.S. band class 0), Uplink – 824 to 849 MHz • Listen Modes o WCDMA (band I, U.S. band class 6), downlink - 2110 to 2170 MHZ o WCDMA/GSM850 (band V, U.S. band class 0), downlink – 869 to 894 MHz o GSM900 (band VII, U.S. band class 9), downlink – 925 to 960 MHz o GSM1800 (band III, U.S. band class 8), downlink – 1805 to 1880 MHz o GSM1900 (band II, U.S. band class 1), downlink – 1930 to 1990 MHz Clearly, to provide a fully flexible system it becomes unfeasible to keep adding receiver inputs for every band, since other bands will become available in the future. The cost due to additional silicon and pin count (hence package size) of the transceiver IC will start to dominate. www.mwee.com The improved performance obtained by tuning the LNA is desirable for the main receive path, however for the listen modes (which in general are listening to the broadcast channels of the local macro network) there are a number of factors which can allow a slightly relaxed noise figure: • Listen mode receivers are required to meet mobile sensitivity levels. • When in listen mode the transmitter is off, so there is no degradation due to transmit noise. • Additional filtering is not required in receive path to deal with transmitter signal rejection (hence lower front end loss). The optimum solution therefore is to offer high performance receiver inputs for the main receive paths which can be dedicated to this task and a wideband LNA input for listen mode which accepts all bands. Modulation, GSM receive The narrowband nature of GSM signals requiring low noise receivers and offering http://www.mwee.com
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