In this post we present 3GPP radio prototyping, using Nutaq’s Radio420X. The content of this blog is taken directly from our new whitepaper which you can download here.

 

Abstract

This application note addresses 3GPP radio design and prototyping when using the Nutaq Radio420X FPGA mezzanine card (FMC). It discusses the most critical radio requirements as they relate to 3GPP radio conformance testing, namely TS 51.021 (GSM) and TS 36.141 (LTE). Being similar to some extent to LTE, we invite the reader to apply the same methods and analysis to WCDMA. The main focus is on the GSM DCS1800 pico base station (BTS) and the LTE local area and home eNB. The scope is limited to FDD in UMTS bands 1, 2, 3 and 4, with a 5 MHz bandwidth for LTE enhanced node B (eNB).

Introduction

RF transmitters must be designed to generate a clean signal within the assigned spectrum while keeping unwanted spurious products within allowable levels. Likewise, receivers must reliably demodulate the wanted weak signal while also rejecting interference from neighbouring channels. Performance requirements for these RF characteristics aim to ensure that equipment authorized to operate on GSM or LTE carriers meets certain minimum standards

[1]-[2].

This document focuses on the frequency bands and arrangements for FDD as shown in Table 1-1. Other frequency bands and arrangements can be found in TS 36.141 [2].

GSM-LTE-Frequency-Bands-FDD

The design of the LTE physical layer (PHY) is heavily influenced by requirements for a high peak transmission rate (100 Mbps downlink/50 Mbps uplink), multiple channel bandwidths (1.25 to 20 MHz), and spectral efficiency. To fulfill these requirements, orthogonal frequency division multiplexing (OFDM) was selected as the basis for the physical (PHY) layer. The use of OFDM and multiple-input/multiple-output (MIMO), two key technologies, significantly differentiate LTE from other 3G systems such as WCDMA. LTE adopts different modes of operation (FDD/TDD) and different downlink and uplink access technologies (OFDMA, SC-FDMA). GSM, on the other hand, uses GMSK modulation in both uplink and downlink directions. Time division multiple access (TDMA) was adopted in GSM as a multiple access scheme wherein one time frame can support up to eight users.

Radio420X capabilities and RF performance

The Radio420X FPGA mezzanine card (FMC) is a powerful multi-mode software-defined radio (SDR) RF transceiver module. The Radio420X is designed around the state-of-the-art, multi-standard, multi-band Lime Microsystems LMS6002D RF transceiver IC, which supports broadband coverage, as well as TDD and FDD full duplex modes of operation [3].

The LMS6002D RF transceiver IC’s bandwidth (1.5–28 MHz) is selectable on-the-fly, suitable for a large number of narrowband and broadband applications, and offers excellent channel selectivity.

Figure 2-1 shows the Radio420X FMC with its shield removed.

Radio420X FMC

Figure 2-1

 Figure 2-2 shows the Radio420X’s functional block diagram.

Radio420X's functional block diagram Figure 2-2

Supporting multiple references and synchronization modes, the Radio420X is the right choice for applications like multi-mode software-defined radio (SDR), advanced telecommunication systems (MIMO systems, cognitive radios, WiMAX, white space, Wi-Fi, GSM, WCDMA), and signal intelligence (SIGINT). The Radio420X complies with VITA 57.1, a widely used standard in the digital signal processing industry, making it easier for developers to integrate FPGAs into embedded system designs.

The Radio420X is completely integrated with the Nutaq uTCA Perseus AMCs, but it can just as easily be used with other FMC carriers. It is compatible with both low pin-count (one RF transceiver) and high pin-count (two RF transceivers) FMC interfaces.

The Radio420X’s TX and RX analog paths are designed to offer the best versatility-to-performance ratio, addressing the high demands of multi-mode RF applications. At the transmitter end, a software-selectable RF switch enables the LMS6002D’s low-band TX1 output or the high-band TX2 output. This switch is followed by a 6-bit, 4 GHz broadband variable gain amplifier where the gain is adjustable from -13.5 to 18 dB (in addition to the LMS6002D’s TX VGAs), yielding a maximum output power of 20 dBm.

Figure 2-3 shows the TX RF performances in terms of output compression point, third-order intermodulation products, RF harmonics level, unwanted sideband rejection, and local oscillator leakage level. Using the low band TX path, one would expect +20 dBm OP1dB, better than 40 dB and 45 dBc for harmonic filtering and LO leakage, and unwanted sideband suppression (using auto-calibration routines) while keeping intermodulation products around -60 dBc.
a) low band from 300 MHz to 2000 MHz

a)

b) high band from 1500 MHz to 3800 MHz

b)

Figure 2-3 Radio420X transmitter performance:

a) low band from 300 MHz to 2000 MHz and b) high band from 1500 MHz to 3800 MHz

At the receiver end, a similar 6-bit, 4 GHz broadband variable gain amplifier is present on top of the integrated LMS6002D’s RX VGAs. The amplifier is followed by a software-selectable RF switch that enables the LMS6002D’s low-band RX1 path or the high-band RX2 path. Each RX path has eight software-selectable filter banks.

Figure 2-4 describes the filter banks on each path.

FMC Radio420X RX filter banks
Figure 2-4 FMC Radio420X RX filter banks

The receiver filter bank uses band-pass SAW filters that provide greater than 40 dB of out-of-band-blocker filtering and transmitter signal leakage rejection when operating with the right duplexing separation. The filter bank supports most relevant 3GPP and IEEE standard radios.

Figure 2-5 shows typical RX RF performance curves. These consist of the input compression point, intermodulation products, and sensitivity. For the UMTS bands of interests (see Table 1-1), using the same minimum gain settings, one would expect a typical noise figure, 3rd-order intermodulation products, and input compression point of 10 dB, -58 dBc and -28 dBm, respectively.

a) Low band from 300 MHz to 2000 MHz

a)

b) high band from 1500 MHz to 3800 MHz

b)

Figure 2-5 Radio420X receiver performance:

a) Low band from 300 MHz to 2000 MHz and b) high band from 1500 MHz to 3800 MHz

In the subsequent sections of this document, we will demonstrate the Radio420X’s suitability for 3GPP radio design and prototyping by linking its RF performance values to frequently used 3GPP metrics, including but not limited to EVM and ACPR for the transmitter, and sensitivity and intermodulation attenuation for the receiver.

The content of this blog is taken directly from our new whitepaper which you can download here.