GNU Radio: Synchronization and MIMO Capability with USRP Devices

Application Note

Synchronization and MIMO Capability with USRP Devices

Ettus Research

Introduction

　　Some applications require synchronization across multiple USRP (Universal Software Radio Peripheral) devices. Ettus Research provides several convenient solutions for synchronization. For example, two USRP N210s can be synchronized using a MIMO cable. It is also possible to synchronize more than two units using the Ettus Research OctoClock. Optional GPS-disciplined oscillators provide the capability to synchronize devices to the GPS standard over a large geographic area. This document will introduce and explain the synchronization features of the USRP product family and how to meet the requirements of multi-channel applications.

　　一些應用研究(如MIMO)須要多個USRP設備同步, Ettus公司提供了多種同步方案: 如兩個USRP N210可使用MIMO Cable, 多於兩個設備可使用Ettus公司的OctoClock, 一個大的地理區域內的多個設備可使用GPS-disciplined oscillators (GPSDO), 本文介紹USRP產品家族的同步特性以及如何知足多通道(多天線)同步需求. 

MIMO System Requirements – Time and Frequency Synchronization

　　For a transceiver to be considered MIMO-capable, each channel in the system must meet two basic requirements:

• • The sample clocks must be synchronized and aligned.
  • DSP operations must be performed on samples aligned in time – from the same sample clock edge.

　　The Ettus Research USRP N200/N210 is recommended for MIMO applications. Two USRP N200/N210s can be synchronized with the USRP MIMO cable. It is also possible to build larger MIMO systems (up to 16x16) by distributing an external reference. 

　　Ettus建議使用USRP N200/N210研究MIMO系統. 兩個USRP N200/N210能夠經過MIMO Cable同步, 也能夠經過分佈式外部時鐘參考實現多達16×16的大型MIMO系統. 

Beamforming and Direction Finding Requirements

　　Some applications, such as beamforming and direction finding, place additional requirements on the system. In addition to sample time and sample clock alignment, the system must maintain a known phase relationship between each RF input or output. Due to phase ambiguities caused by phased-locked loops which are used for up and down-conversion, some calibration may be required to determine this phase relationship.

　　例如波束成型和定向系統, 還須要添加額外的需求. 除了抽樣時間和抽樣時鐘的同步之外, 系統還必須在每一個射頻輸入輸出的之間維持一個已知的相位關係. 在上/下變頻時, 鎖相環會產生的相位模糊, 所以須要一些校準來肯定相位關係. 

　　It is possible to calibrate a multi-channel system by producing a tone that is distributed to the inputs of each USRP device with match-length RF cables. An illustration of a system that uses this methodology is shown in Figure 1. User-developed software running on the host PC is used to measure the phase and amplitude differences of each channel and apply a correction.

　　能夠經過提供一個基準來校訂多天線系統(校準多天線系統的相位偏移), 這個基準信號經過等長的RF Cable輸入到每一個USRP設備, 如圖1所示, 用戶程序運行在主機上, 該程序用來測量每一個通道的相位差和幅值差而且進行相位校準. 

Figure 1 - Multi-Channel USRP-Based System for Direction Finding (note the calibration signal)

Other Variables That Effect Phase Alignment

　　As mentioned, other components aside from local oscillators contribute to phase error. Filters, mixers, amplifiers and other components produce phase offsets that vary with time, temperature, mechanical conditions, etc. These types of errors can generally be calibrated out with intermittent, low-rate routines that detect the channel-to-channel phase with a calibration tone. These errors will not change with each PLL retune but may change with time and temperature variation. Thus, applications that require RF phase alignment may require periodic calibration. 

　　如前文所述, 除了本振之外的其餘組件也會致使相位錯誤: 濾波器, 混頻器, 放大器等其餘組件也會產生相位偏移, 而且相位偏移隨時間, 溫度, 自身機械特性等條件變化. 這種類型的相位錯誤能夠經過間歇性, 規律性的校準信號來校準相位偏移. 而且這種錯誤不會隨着每一個PLL的從新調諧而變化, 可是會隨着時間和溫度而變化. 所以須要RF相位同步的應用須要按期校準. 

USRP B100 and E100 – Not Recommended for MIMO

　　Like the USRP N200/N210, the USRP B100 and E100/110 can be synchronized with an external reference and PPS source. However, this does not imply the USRP B100 and E100/E110 are MIMO capable. The flexible frequency clocking architecture used in these devices produces phase ambiguity in ADC/DAC sample clocks. Therefore, sample edges will not be aligned. Despite the sample edge misalignment, it is still possible to produce samples with relatively accurate time stamps. This is useful for time-difference-of-arrival or similar algorithms.

　　USRP B100 和 E100/110 雖然能夠像N200/N210同樣使用外部參考信號和PPS源來實現同步, 可是這並不意味着 B100和 E100/110具備MIMO能力. 在這些設備中使用靈活的時鐘頻率架構致使ADC/DAC採樣時鐘的相位模糊. 所以採樣邊沿並無對齊. 儘管如此, 這些設備依然可能提供相對正確的採樣時間戳. 這是因爲採用時間差定位法或者相似的算法. 

　　One exception to these statements is when you make use of a daughterboard containing more than one channel in a single slot. For example, LFRX/TX, BasicRX/TX, and TVRX2 can all be used to achieve MIMO capability with a USRP B100 or E100/110.

　　有一個例外是, 當母板一個插槽上的子板有多餘一個通道時, 例如子板LFRX/TX, BasicRX/TX, and TVRX2均可以使用B100 or E100/110實現MIMO. 

Synchronization with GPS Disciplined Oscillator 

　　It is possible to provide time-synchronization over a wider geographic area with a GPS disciplined oscillator (GPSDO). A GPSDO derives 10 MHz/PPS signals from the GPS system. The GPSDO is accurate to approximately +/-50 ns across the globe. Ettus Research provides an optional GPSDO module with the USRP N200/N210. There is also an upgraded version of the OctoClock, which includes an internal GPSDO. 

　　可使用GPSDO在一個很大的地理區域內提供時間同步, 一個GPSDO生成的10MHz/PPS信號源於GPS系統, 精度大約+/- 50 ns. Ettus爲USRP N200/N210提供可選的GPSDO模塊, 同時也提供GPSDO的升級版OctoClock, 它包含一個內部的GPSDO. 

MIMO Capability of Ettus Research USRP Devices

　　Table 1 provides an overview of the synchronization and MIMO capabilities of the USRP product line.

　　The RF daughterboard selection also impacts the synchronization experience. Most daughterboards use a fractional-N synthesizer to generate the local-oscillator signals. Generally, these fractional-N synthesizers introduce a random phase offset after each retune. If phase alignment between all RF channels is required, this random phase offset will need to be measured and compensated for in software. The SBX PLL includes a resync feature that resets to a fixed phase after each retune.

　　子板的選擇一樣會影響同步性能. 大多數子板採用N分頻的頻率合成器生成本振信號. 一般這些N分頻頻率合成器每一次從新調諧都會產生一個隨機的相位偏移. 若是必須對齊全部通道的相位, 這個在軟件中測量這個隨機相位偏移而且對它進行補償. SBX子板的PLL擁有一個再同步特性, 能夠將每次從新調諧後的相位重置爲一個固定的相位. 

　　Thus, it may not need to be calibrated after each retune, but may require periodic calibration. This makes the SBX daughterboard the most ideal option for phased-array applications that fall within its frequency range. Also, the BasicRX/TX and LFRX/TX boards do not contain local oscillators that introduce phase errors.

　　所以可能沒必要每次從新調諧都校準, 可是須要按期校準. 這使得SBX子板在其頻率範圍內成爲相控陣最理想的選擇. BasicRX/TX 和 LFRX/TX子板也不含有引發相位錯誤的本振. 

USRP N200/N210 – Plug and Play 2x2 MIMO System

　　The easiest method to implement a high-performance 2x2 MIMO system is to utilize two N200/N210s synchronized with an Ettus Research MIMO cable. In this configuration, a single Gigabit Ethernet(GigE) interface can be used to communicate with both USRP devices. The USRP connected to the GigE acts as a switch and routes data to/from both USRP devices. It will also handle time synchronization of the data so the sample alignment process is transparent to the user. The total sample rate for USRP devices connected to a single GigE port on the host computer cannot exceed 25 MS/s in 16-bit mode, or 50 MS/s in 8-bit mode.

　　實現一個2×2高性能MIMO系統, 最簡單的方法是使用MIMO Cable鏈接兩個N200/N210. 在這一配置下, 一個吉比特以太網接口能夠用來與兩臺USRP設備通訊. 鏈接網口的那臺USRP設備充當兩臺設備的數據網關和路由. 而且會處理數據的時間同步, 所以對於用戶來講, 樣點對齊是顯然的. 單網口鏈接模式(共享網絡模式)下, 與主機通訊的總速率在16-bit模式不能超過 25 MS/s, 或者8-bit模式不能超過50 MS/s. 

Application Example – Visual Phase Alignment with N210 2X2 MIMO System

　　The UHD API allows you to select synchronization settings for each USRP device. These settings are also exposed through GNU Radio blocks. This allows for a quick, illustrative example of how to use the N200/N210 to build a MIMO system. GNU Radio Compnion (GRC) is used for this basic illustration.

　　UHD的API容許用戶爲每一個設備選擇同步設置. 也能夠經過GNU Radio 的block進行設置.  

　　Figure 3 shows a flowgraph that receives two streams from two unsynchronized USRP devices. A 400.01 MHz tone is injected into both USRP devices, which are tuned to 400.00 MHz. The flow graph separates the real component of each complex baseband signal and plots them together on a WX GUI Scope. When the two USRP devices are synchronized, the scope sill show two, 10 kHz tones with constant relative phase. In this test case, the USRP devices are unsynchronized. Notice in Figure 4 that there are obvious phase and frequency differences between the two signals. This is a result of variations in the two unsynchronized reference clocks. Heat is applied to one of the USRP device’s internal reference crystals to amplify the frequency variation between the two units.

　　圖3是一個從2路不一樣步的USRP設備接收2路 stream 的流圖(flowgraph). 一個400.01MHz的單一信號輸入到2箇中心頻率爲400MHz 的 USRP設備中, 流圖分離出每路復基帶信號的實部而且畫在同一個示波器中. 當2個USRP設備同步以後, 示波器仍然顯示兩個10KHz的單一信號, 而且相位差保持不變. 在這一測試條件下, USRP設備是不一樣步的. 在圖4 中能夠看到兩個信號之間有明顯的相位和頻率誤差. 這是因爲2 個不一樣步的參考時鐘致使這一變化的結果. 其中一個USRP設備因爲熱量致使其的內部參考晶振放大了兩個設備之間頻率的變化. 

　　Next, plug-and-play 2×2 system is illustrated in Figure 5 . Notice the relative simplicity of the system. A MIMO system is created by connecting two N200/N210s with a MIMO cable, which shares Ethernet connectivity and common 10 MHz/PPS signals. Like the unsynchronized setup that produce the scope view in Figure 4 , a signal generator drives the receiver inputs of both inputs. A block diagram of the synchronized system is shown in Figure 5 . The flowgraph is shown in Figure 6.

　　圖5所示一個即插即用的相對簡易2×2的MIMO系統. 兩個 N200/N210經過 MIMO Cable鏈接, 他們共享網絡鏈接和10MHz/PSS, 流圖如圖 6 所示.

　　A single UHD block is used in the GRC flowgraph. The block parameters are configured to set up a 2x2 MIMO system. The settings of interest are:

• • Device addr: addr0=192.168.10.2,addr1=192.168.10.3
  • Sync = don't sync
  • Num Mboards = 2
  • Mb0 Clk Src = Default
  • Mb0 Time Src = Default
  • ...
  • Mb1 Clk Src = MIMO Cable
  • Mb1 Time Src = MIMO Cable

　　These settings configure the first USRP device, Mb0, which corresponds to the first entry in the address string, to use its default reference for clocking and timing. The second USRP(Mb1) is configured to accept its frequency and timing reference from the MIMO cable. The signals are provided by Mb0. All other standard settings such as center frequency and gain assignments apply as well.

　　IP地址爲192.168.10.2的設備爲Mb0, 另外一個爲Mb1. 按照上面的方法設置USRP Source相應的參數. Mb0使用默認時鐘參考和時間參考, Mb1經過MIMO Cable 從Mb0獲取頻率和時間參考. 信號是由Mb0提供. 全部其餘標準設置(例如中心頻率和增益)都保持同樣便可. 

　　Like the first flowgraph shown, the real part of each USRP stream is displayed on a scope. A phase correction block is included to compensate for the random, but constant, phase offset discussed earlier in this paper. Figure 7 includes a screenshot of the resultant display both before phase adjustment. Note the phase is constant and the tones are the same frequency.

　　正如第一個流圖所示, 每一個USRP 數據流的實部都顯示在示波器中. 一個相位校訂模塊(點擊這裏查看如何編寫相位偏移模塊)用於補償這個隨機可是保持恆定的相位偏移. 圖7 所示相位調整以前的波形. 注意相位差保持恆定, 而且頻率相同. 

　　Figure 8 shows the signals with phase correction applied. In this plot, it is clear the MIMO connection has enabled the frequency and time references to be synchronized. The random phase offset is corrected with a complex phase shift in the flowgraph. In real applications, this phase correction would be implicitly generated with algorithms such as maximum-ratio combining (MRC), or periodic calibration. 

　　圖8 所示使用相位校訂模塊調節相位以後的波形, 顯然, MIMO 鏈接是的頻率和時間參考獲得同步. 隨機相偏獲得糾正. 在實際應用中, 相偏校訂是經過最大比合並或者按期校準等算法隱式完成的. 

　　This is a simple illustration of the MIMO capability provided by the USRP N200/N210. In most applications the phase compensation is implemented with some automated process. However, calibrating the devices with this type of manual correction is certainly an option. 

　　以上是USRP N200/N210的MIMO能力的簡單說明. 一般, 相位補償是一個自動實現的. 可是, 也能夠手動校訂設備(即不經過GRC, 直接調用UHD 的API).

　　All principles illustrated in this example are applicable to operations in the transmit direction.

 

Synchronization with 10 MHz and 1 PPS Signals

　　While the USRP N200 and N210 provide plug-and-play MIMO capability with the Ettus Research MIMO cable, it is also possible to synchronize multiple devices using external 10 MHz and 1 PPS distribution. This is useful if the developer would like to use a high-accuracy external reference such as a Rubidium source. It is also helpful if the developer must build a MIMO system with more than two channels.

　　使用MIMO Cable可使N200/N210支持2天線MIMO. 使用外部10MHz和 1 PPS支持多於2天線的MIMO同步. 

　　If common PPS and 10 MHz signals are distributed to USRP N-Series devices, it is theoretically possible to build arbitrarily large MIMO systems. In practice, developers have built systems with up to 16 synchronized USRP devices.

　　若是公共 PPS和 10 MHz信號用於N-系列設備, 理論上支持大型MIMO系統. 事實上, 開發者能夠同步多達16臺設備的同步系統. 

　　The Ettus Research OctoClock and OctoClock-G make it easy to distribute 10 MHz and 1 PPS signals for multi-channel operation. The OctoClock serves as an 8-way PPS and 10 MHz reference splitter. The user must provide a single 10 MHz and 1 PPS signal. The upgraded OctoClock-G includes a high-accuracy, internal GPS-disciplined oscillator and does not require external signals to be supplied. Figure 9 illustrates the use of an OctoClock-G to create an 8x8 MIMO system.

　　OctoClock 和 OctoClock-G 在多天線系統中很容易產生10 MHz 和 1 PPS信號. OctoClock 有 8 路 PPS和 10MHz參考信號分離器, 用戶必須提供一個單一的10 MHz和 1 PSS信號, 升級版的OctoClock-G 包含一個高精度的GPSDO而且不須要外部信號. 圖 9 顯示經過OctoClock-G建立一個8×8的MIMO系統. 

　　All timing signals from the OctoClock should be connected to the USRP devices with matched length cable of the same type and connectorization. This ensures that there is low skew between all of the channels.

 

Synchronization Signals – Input Levels

　　To achieve optimum performance and prevent damage to the USRP devices, the designer must assure that the input levels fall with specified limits. Guidance for input voltage levels and power levels for the 1 PPS and 10 MHz inputs are shown in Table 3 .

Additional Resources

UHD API For Synchronization http://files.ettus.com/uhd_docs/manual/html/sync.html
OctoClock-G Product Page https://www.ettus.com/product/details/OctoClock-G
GPSDO Module for USRP N200/N210 https://www.ettus.com/product/details/GPSDO-KIT

原文下載地址: https://pan.baidu.com/s/1o894eRC