With LTE and WiMAX becoming the latest ingredients in a wireless standards melting pot that already included W-CDMA and WiFi, the greatest challenge for any programmable CPE and basestation is to implement RF components that are sufficiently wideband to encompass all the different frequency bands.

In this article, requirements for key components within RF transceiver design are outlined that will dramatically reduce both component count and power consumption, and therefore costs and space, in small cell programmable basestations.

A programmable basestation is distinct from a software-defined one in that it switches the radio frequency band between a set of pre-defined standards and bandwidths using a bank of frequency synthesizers and filters, whereas a software-defined basestation would distinguish from the incoming signal which standard it was operating to and demodulate it digitally.

Normally, the switching process substitutes different transceivers, front end modules (FEM) and basebands for each standard, but the drive to reduce costs and bill of materials (BoM) is increasing the pressure for these three elements to also be capable of multi-standard, and thus wideband, performance.

This would remove the need for a complex power management strategy to turn off the unused channels, as well as saving on board space and bill of materials. This is especially important in high volume small cell basestations – femtocells and picocells.
The requirement of multi-standard femtocells is only just emerging, but it is expected that these would have to be programmable to cope with coexistence of 3G, 2.5GHz Mobile WiMAX and 3.5GHz Mobile WiMAX. 

An additional factor that is driving the requirement for wider system bandwidth is the increasing pressure to optimize the power amplifier efficiency – and hence the overall system efficiency – in order to reduce operating costs and to address environmental concerns over carbon footprint. Increasingly, this is being achieved by means of digital pre-distortion (DPD) technology. For DPD to work efficiently, both third and fifth order harmonics of the amplifier output need to be taken into consideration.

Among commercial broadband transceivers already on the market, the typical operating bandwidth is up to 25%, e.g. 2.3 – 2.7GHz or 3.3 – 3.9GHz, which is insufficient to cover all the bands shown in Figure 1 with a single device – for this, a multi-octave transceiver is required. In many cases it is the characteristics of the synthesizer used to create the local oscillator (LO) signal that is the limiting factor to the bandwidth.

A single-chip transceiver has been developed that operates over the full frequency range of 345MHz to 4GHz. The chip combines LNA, PA driver, Rx/Tx mixer, Rx/Tx filters, synthesisers, Rx gain control, and Tx power control with a minimum requirement for external components. The key challenges are achieving the required wide frequency range while maintaining the phase noise requirements of the synthesiser, multiple bandwidth filtering with sharp roll-off at the baseband, along with linearity and noise figure budget for the transceiver chain. The baseband filter used within the design provides a very sharp roll-off for multiple bandwidths covering both WiMAX and LTE bands, the design of which is based on a modified transconductance amplifier-capacitor (gm-C) cells.

The wideband characteristics of the transceiver have been obtained by using a broadband synthesizer. The synthesiser is based on a fractional-N sigma-delta based architecture covering a continuous range between 345MHz - 4GHz. The sigma delta modulator is used with a very low oversampling ratio simplifying the design and relaxing the specification of the analogue components within the synthesizer.  The phase noise performance of the implemented synthesiser measured at the output nodes meet all requirements of the key broadband wireless standards including WIMAX and LTE standards. The frequency synthesiser employs a digitally controlled 40MHz reference crystal oscillator.

The zero-IF transceiver chip can handle OFDM modulation up to 64QAM, and supports both FDD and TDD full duplex, with a sensitivity of -70dBm at 7MHz bandwidth under 64QAM. Operating current is typically 300mA under FDD operation at 1.8 V and 3.3V, with a standby current of less than 1mA with power-down modes being software-selectable. Modulated Tx RF output is -10dBm.

Based on this chip, a unique reference design (Figures 2 and 3) has been developed for a reconfigurable multi-standard MicroTCA wireless transceiver system, which has 6 user-selectable channel bandwidths from 1.5MHz to 28MHz and can be digitally configured to operate in bands from 345MHz to 4GHz. The re-configurable design supports a variety of network configurations – WCDMA/HSPA, WiMAX and LTE – as well as different bandwidths.

The multi-standard design allows the component count to be reduced from 4 ICs and 184 passives for a typical existing design to a single transceiver IC with 120 passives, leading to a 60 per cent reduction in BoM cost.

Until now, most transceivers available on the market have only covered a fraction of the frequency range required for a multi-standard small basestation covering all the current and planned band allocations for 3G, WiMAX and 3GPP-LTE. In general the bandwidth is limited both by the synthesizer architecture and the filter network. 

In contrast, the transceiver architecture developed by Lime Microsystems and described above is capable of transmitting and receiving data across all WiMAX bands, as well as those used for W-CDMA and HSPA, and those that are planned for LTE.

This removes the need for individual transceiver chips for each of the different bands, and allows a small basestation to be reconfigured rapidly and simply. Because this chip design replaces multiple transceivers in a programmable basestation, it enables a considerable reduction in power consumption as well as in bill of materials.

Assaad Borjak, Danny Webster, Srdjan Milenkovic are from Lime Microsystems