System simulations of a 1.5 V SiGe 81–86 GHz E-band transmitter

Analog Integr Circ Sig Process DOI 10.1007/s10470-016-0902-2 System simulations of a 1.5 V SiGe 81–86 GHz E-band transmitter Tobias Tired1 • Per Sandrup2 • Anders Nejdel1 • Johan Wernehag1 • Henrik Sjöland1,3 Received: 21 September 2016 / Accepted: 9 December 2016 Ó The Author(s) 2016. This article is published with open access at Springerlink.com Abstract This paper presents simulation results for a sliding-IF SiGe E-band transmitter circuit for the 81–86 GHz E-band. The circuit was designed in a SiGe process with fT = 200 GHz and uses a supply of 1.5 V. The low supply voltage eliminates the need for a dedicated transmitter voltage regulator. The carrier generation is based on a 28 GHz quadrature voltage oscillator (QVCO). Upconversion to 84 GHz is performed by first mixing with the QVCO signals, converting the signal from baseband to 28 GHz, and then mixing it with the 56 GHz QVCO second harmonic, present at the emitter nodes of the QVCO core devices. The second mixer is connected to a threestage power amplifier utilizing capacitive cross-coupling to increase the gain, providing a saturated output power of ?14 dBm with a 1 dB output compression point of ?11 dBm. E-band radio links using higher order modulation, e.g. 64 QAM, are sensitive to I/Q phase errors. The presented design is based on a 28 GHz QVCO, the lower frequency reducing the phase error due to mismatch in active and passive devices. The I/Q mismatch can be further reduced by adjusting varactors connected to each QVCO output. The analog performance of the transmitter is based on ADS Momentum models of all inductors and transformers, and layout parasitic extracted views of the active parts. For the simulations with a 16 QAM modulated baseband input signal, however, the Momentum models were replaced with lumped equivalent models to ease & Tobias Tired 1 Lund University, Lund, Sweden 2 Ericsson AB Business Unit Radio, Lund, Sweden 3 Ericsson Research, Lund, Sweden simulator convergence. Constellation diagrams and error vector magnitude (EVM) were calculated in MATLAB using data from transient simulations. The EVM dependency on QVCO phase noise, I/Q imbalance and PA compression has been analyzed. For an average output power of 7.5 dBm, the design achieves 7.2% EVM for a 16 QAM signal with 1 GHz bandwidth. The current consumption of the transmitter, including the PA, equals 131 mA from a 1.5 V supply. Keywords E-band  MM-wave  EVM  Transmitter  Power amplifier  16 QAM  SiGe 1 Introduction High capacity Gb/s wireless point-to-point communication links can be implemented in the E-band at 71–76 and 81–86 GHz. Optical fiber has previously been preferred for the backhaul networks [1, 2]. However, it is not always possible to deploy an optical fiber due to regulations, installation time and cost [1, 2]. In the upcoming 5G heterogeneous networks the number of base stations will increase, making a wireless backhaul more favorable. In Europe, the 5 GHz spectrum of each sub-band is divided into 250 MHz channels [2, 3] which can be merged if higher data capacity is required. In the Unites States the bands are instead divided into 1.25 GHz channels [4]. A typical E-band transceiver product consists of several MMwave ASICs plus external power amplifiers (PAs). In [2, 5] a SiGe E-band transceiver product is presented, demonstrating a 3.18 Gbps radio link using 256-level quadrature amplitude modulation (QAM) [6] in a 500 MHz RF channel bandwidth, with 8 dBm output power at the antenna. The architecture consists of separate receiver and 123 Analog Integr Circ Sig Process transmitter ASICs, an external phase locked loop (PLL) together with an external power amplifier (PA) and low noise amplifier (LNA) in GaAs-technology. In industry, there has so far been less focus on integration level of E-band transceivers. Compared to chipsets for cellular communication, the integration level for E-band transceivers is therefore still low. As the volumes of wireless links will increase with the deployment of the upcoming 5G networks, integration level will be a key driver for product cost reduction. To address this, in this paper, a 1.5 V E-band transmitter for the 81–86 GHz E-band is presented. The transmitter is fully integrated, i.e. it consists of upconversion mixers together with an integrated PA that share a common supply. The upconversion is based on an on-chip 28 GHz QVCO [7–9], which creates four LO phases for an I/Q upconversion mixer for the baseband signal. In a second mixing stage, the 56 GHz second harmonic, present at the emitter nodes of the QVCO core devices, upconverts the 28 GHz signal to 84 GHz [7–10]. Using a single supply voltage of only 1.5 V for the entire transmitter eliminates the need of a dedicated voltage regulator, since, the supply can then be shared between the transmitter and the digital control circuits. The low supply three-stage PA uses capacitive cross-coupling [11–15] to increase the power gain and isolation of each stage. Early E-band transmitters used simple modulation schemes such as binary phase-shift keying (BPSK) or on–off keying (OOK) [1]. These modulation schemes do not require a high linearity transmitter but are on the other hand less spectral efficient [16]. In E-band systems of today, to support spectral efficient transmission with high data rates, M-ary QAM is used. For low bit-error (BER), data links using QAM modulation put more stringent requirements on transmitter nonidealities, resulting in tight error-vectormagnitude (EVM) specifications [17–22]. In this paper, the effects on simulated EVM, for a 2 GHz 16 QAM signal, of local oscillator (LO) phase noise, I/Q imbalance, and PA compression are therefore investigated. Transient simulations were performed using parasitic extracted views of the circuit parts and lumped model equivalents of the inductors and transformers. The EVM was calculated by importing the demodulated data into MATLAB. For each modulation scheme, there is a known relationship between EVM and bit-error-rate (BER) [22]. Using the EVM as a metric to evaluate the performance is advantageous, since more time consuming BER calculations can then be avoided at an early stage of the design phase [22]. The transmitter was designed in a 0.18 lm SiGe HBT process, with four Cu metal layers with a top layer thickness of 2.8 lm, and with an fT of 200 GHz. The process does not have any MOS devices. In this paper the presented transmitter and the EVM simulation setup are first briefly discussed. In Sect. 2, the transmitter architecture is described together with a 123 comparison to other transmitter topologies. The design of the different circuit parts is then discussed in Sect. 3. In Sect. 4, the design and layout of the inductors and transformers are presented, together with the layout of the complete transmitter, including the power amplifier. The simulation results for a non-modulated baseband signals are provided in Sect. 5. In Sect. 6, the EVM as a metr (...truncated)