Wireless Infrastructure Market Evolution Tightens Transmitter Requirements - Innovative Architectures Help

 

By Matthias Feulner, Texas Instruments

 

Drivers for future transmitter requirements

 

To start with, cost (or better cost efficiency) is the main driver for new wireless base stations in general and transmitter architectures in particular. But at the same time the desire to deploy flexible platforms that can be easily adapted to address a wide variety of air interfaces and frequency bands as well as the demand for higher capacity (larger signal bandwidth and carrier density) and enhanced performance (smart antenna systems) are strong influences.

Cost efficiency

 

Cost is the uppermost criterion for transmitter (Tx) implementations and the goal not only is to minimize pure component cost, but rather overall solution cost taking account of active signal chain components, passives, filters and particularly synthesizers for LO generation, board space and, ever more importantly, power consumption. Any meaningful attempt to reduce cost needs to take all of these aspects into account. One good example is the switchover from conservative, but well proven heterodyne Tx architectures with two intermediate frequency (IF) stages (see Figure 1) to direct up-conversion, also known as zero IF conversion, in which a quadrature modulator takes the in-phase and quadrature-phase signal components modulates them directly onto the RF carrier (see Figure 2).

 

Figure 1: Dual IF heterodyne transmitter block diagram

 

 

Figure 2: Direct up-conversion transmitter block diagram

 

Extended frequency coverage

 

With new air interface standards and new frequency bands being introduced in ever shorter cycles, and consequently the co-existence of  many parallel diverse wireless standards (see Figure 3), an essential requirement is the ability to support extensions beyond the traditional frequency bands both towards higher frequencies (UMTS Long Term Evolution in  2.62 – 2.69MHz band for BTS Tx and WiMAX in 2.3-2.7GHz and 3.3-3.8GHz bands) as well as towards lower frequencies (450MHz band, which are becoming more popular now both for GSM and UMTS).

 

 

Figure 3:  Evolution and co-existence of wireless standards

 

Flexible platforms and frequency agility

 

In today's competitive market place, the ability to respond quickly to market demand may be a major differentiator. System manufacturers are trying to respond by developing flexible platforms based on a common design that can be adapted easily to capture short-term opportunities without carrying significant inventory, thus optimizing lead times and inventory levels.

Demand for more bandwidth

 

Bandwidth demand is being driven primarily by  more users signing up for service,  WCDMA networks now moving from initial coverage installations to volume deployments, and the introduction of high-capacity data services like HSDPA and WiMAX. As a result, signal bandwidths of up to 20MHz and beyond are becoming more common. At the same time the desire to improve power amplifier and thereby overall system efficiency by means of advanced digital pre-distortion (DPD) expands the bandwidth requirements for wireless transmitters even further. The reason is that to implement an efficient DPD algorithm, the 3rd and 5th order harmonics being produced by the Power Amplifiers non-linear operation need to be detected (see Figure 4). Accordingly advanced transmitter architectures need to support so-called expansion bandwidths of up to 100MHz.

 

Figure 4: PA output spectrum with 3rd and 5th order harmonics

 

Channel density & smart antenna systems

 

In a standard configuration, a wireless base station typically supports three sectors with each sector being supported by a separate transceiver card. Both the advent of more compact mechanical form factors and the promise of enhanced system performance delivered by Multiple Input / Multiple Output (MIMO) antenna configurations (see Figure 5) lead to more transceivers being packed into the same physical space. Consequently both component level miniaturization (i.e. putting chips into smaller packages like QFN and BGA) as well as signal path integration (vertical, i.e. multi-channel components, and horizontal, i.e. several functions along one signal chain into a single component) and use of more compact architectures are being pursued.

 

 

Figure 5: Smart antenna transmitter architecture

 

An advanced architecture helps meet requirements

 

The answer to the aforementioned requirements is given below. It is a zero-IF direct up-conversion transmitter requiring only minimum component count, thus allowing for the most compact size, lowest overall system cost and dramatically reduced power consumption. It employs the following key components:

 

  • Dual interpolating with digital features (DAC5687)
  • Wideband I/Q quadrature modulator (TRF3703)
  • Integrated VCO/PLL RF synthesizer (TRF3761)
  • Sampling clock synthesizer (CDCM7005)

 

 

Figure 6: Implementation of a flexible direct up-conversion transmitter

 

The individual components have been designed to deliver an optimum overall solution and as a result, significant savings in space, power consumption and eventually cost can be achieved compared to a super heterodyne architecture. We will revisit the components, their key features and demonstrate overall solution performance in some key applications.

 

Direct Conversion Modulator

 

In a direct conversion architecture the output frequency range of the modulator directly determines the signal bands that can be supported. Hence a very wide RF frequency range is essential and the TRF3703 operates from 400MHz to 4GHz, covering basically all the important wireless frequency bands including GSM450, GSM900, DCS1800/PCS1900, UMTS bands I – VII (ranging from 869MHz to 2690MHz for BTS Tx) WiMAX bands from 2.3-2.7GHz and 3.3-3.8GHz.

 

Glue-less passive interface with DAC: To comply with the goal of minimum component count and prevent the use of buffer amplifiers potentially degrading performance, the I/Q modulator's common mode input voltage range needs to be matched to the common mode output range of the DAC. Since this has been accomplished at the interface between DAC5687 and TRF3703, a simple passive network is sufficient for coupling the two devices.

Wideband operation and linearity, carrier & side band suppression: Systems that operate both with wideband signals as well as across wide signal bands are asking for sustained performance of key performance parameters like carrier and side band suppression (both displayed in Figure 7 for the TRF3703) as well as linearity (referred to the 3rd order output intercept point OIP3) of the modulator across the entire operating frequency range.

 

 

Figure 7: TRF3703 I/Q modulator carrier & side band suppression and output power

 

Flexible interpolating DAC with integrated digital features

 

Figure 8: DAC5687 interpolating DAC functional block diagram

 

To expand further on the concept of flexibility, the digital-to-analog converter (DAC) used in this architecture is a dual channel device that readily integrates a whole array of digital features to simplify system design and allow a more compact implementation, including interpolation filters for up-sampling, a clock multiplier PLL to derive the output sampling clock from the input clock when using interpolation, a numerically controlled oscillator (NCO) and digital mixer to allow for creation of a digital IF frequency, a digital quadrature modulator to create a modulated output from the  I and Q input channels and necessary correction features for gain, phase and offset of the I and Q channel to correct for I/Q imbalance of the external analog quadrature modulator. The block diagram in Figure 8 describes the functional blocks and their arrangement. The major functional concepts are described in the following.

 

DAC-to-modulator IF concepts: Choosing the DAC output interface, there are three possible configurations:

 

  • Base band DAC output to external analog I/Q modulator
  • Real IF output, creating a digital IF with the integrated NCO and mixer
  • Complex IF output to external analog I/Q modulator

 

While each of the modes has its specific advantages (for a comparison see [4]), we will take a detailed look at the complex IF configuration, since it exploits the digital features of the DAC to their full extent. Operating the DAC in complex IF mode considerably improves LO suppression since the LO frequency of the modulator is moved away from the signal band. In addition, together with an analog I/Q modulator, a single side band up-conversion is performed, which enhances suppression of the undesired side band. By performing complex mixing on the I and Q input channels, two Hilbert-transform pairs are generated at the two DAC outputs and when these are fed to the modulator, one of the side bands is suppressed significantly (upper side-band up-conversion is shown in Figure 9), thus reducing requirements for filtering and calibration (particularly for the analog I/Q modulator) and resulting in a simplified  manufacturing process and associated cost savings.

 

 

 

Figure 9: Single side-band up-conversion mode of DAC + modulator

 

Additionally, a feature to synchronize multiple transmit DACs is integrated, which is considered to be particularly useful with emerging smart-antenna, or MIMO architectures that use multiple transmitters for beam-forming to enhance performance and to satisfy the need to maintain tightly the phase relationship between individual Tx chains.

 

Programmable Integrated LO Synthesizer

 

Another building bock that directly impacts frequency agility is the LO synthesizer. It has to support at least the full frequency range within one operating band; changing to other bands must be possible with the minimum of effort. The integrated VCO/PLL synthesizer approach featured here supports these requirements whilst significantly reducing system cost and board space, yet complies with stringent requirements on phase noise and spurious signals (details given in [2]).

 

Figure 10: TRF3761 integrated VCO/PLL block diagram

 

Tunability

 

For reducing synthesizer variants and simplifying logistics associated with stocking multiple frequency version of a transceiver board, the synthesizer must support at least the full width of its operating frequency band, for example UMTS band I from 2110 to 2170 MHz for BTS Tx.

 

 

Figure 11: Frequency ranges of individual TRF3761 synthesizer devices

 

In addition, changing to other frequency bands is easily possible by swapping another pin-compatible version of the device with a different frequency range. A total of 6 available synthesizers (see Figure 11) covers a frequency range from 1500 MHz to 2404 MHz, together with available /2 and /4 output divider modes this extends to 375 MHz to 2404 MHz.

 

Performance demonstrated

 

This innovative implementation has already been shown to save on system cost, space and power significantly, but now we still need to demonstrate that it delivers performance. Representative of the many possible applications, we have chosen a test case for a 3-carrier WCDMA signal, the DAC operating at complex IF output of 122.8 MHz to the modulator.

 

 

Figure 12: Tx output spectrum (after modulator) and ACPR for a WCDMA 3-carrier test case

 

We observe an ACPR (Adjacent Channel Power Ratio, the key measure for transmitter performance) of better than 66dBc at 5MHz offset and better then 68dBc at 10MHz offset respectively, and thus can conclude that it is possible to combine the benefits of this flexible highly integrated direct up-conversion transmitter architecture with the performance requested by base station infrastructure implementations.

 

References:

 

[1] 3GPP TS 25.101 version 7.3.0 Release 7

[2] New Breed of Low-Noise Integrated VCO/PLL RF-Synthesizers Suits Wireless Infrastructure, Matthias Feulner, Texas Instruments

[3] TRF3703 I/Q modulator data sheet http://focus.ti.com/docs/prod/folders/print/trf3703.html

[4] DAC5687 interpolating DAC data sheet http://focus.ti.com/docs/prod/folders/print/dac5687.html

[5] TRF3761 integrated VCO/PLL data sheet http://focus.ti.com/docs/prod/folders/print/trf3761.html

[6] CDCM7005 clock synthesizer data sheet http://focus.ti.com/docs/prod/folders/print/cdcm7005.html