GNU Radio: Multiple USRP configurations 配置多个USRP设备
Introduction 引言
Some USRP devices are capable of being grouped to form a single, virtual device. A single uhd::usrp::multi_usrp instantiation can control such a compound of devices.
一些 USRP 设备能够组合成 multi_usrp 设备。并通过一个 uhd::usrp::multi_usrp 实例对象来控制这个 multi_usrp 设备。
Currently, the following devices support this capability:
目前，支持这种组合能力的设备有：
- USRP2 / N2x0 Series
- X3x0 Series
Note that only USRPs of the same type can be combined.
注意: 只有同类型的 USRP 设备才可以组合使用。
Setting up devices 设置设备
A description of a multiple-USRP setup can be found on the respective device's manual pages.
在相关的设备手册页可以看到关于 multi_usrp 设置的描述。注：指的是下图这里:
Addressing of a compound of devices is done by listing multiple addresses, e.g.:
通过列出多个设备的地址，完成这种 multi_usrp 设备的地址配置：
addr0=192.168.10.2,addr1=192.168.20.2
Channel and Device Numbering 信道&设备编号
Assume we have combined 2 X310 USRPs into a single multi_usrp using the address string given above, maybe using the following command:
假设我们使用上面给出的地址字符串，将两个USRP X310设备组合，配置成一个 multi_usrp 设备，可能使用下面的命令：
uhd::device_addr_t args("addr0=192.168.10.2,addr1=192.168.20.2");
uhd::usrp::multi_usrp::sptr usrp = uhd::usrp::multi_usrp::make(args);
Some uhd::usrp::multi_usrp commands require passing a device index. This is simply the index in the address list, so say we want to check the master clock rate on both devices, this would be valid:
一些 uhd::usrp::multi_usrp 方法需要将设备索引(母板的编号)作为实参。索引就是地址列表对应的编号，即地址为addr0=192.168.10.2的母板对用的索引为0，addr1对用的索引为1，因此，如果我们想要查看2个母板的时钟率，需要使用下面的合法语句。
double mcr0 = usrp->get_master_clock_rate(0);
double mcr1 = usrp->get_master_clock_rate(1);
Some methods default to applying to all devices, so the following command would set the time on all devices to zero:
一些方法默认可以应用于所有的设备，因此下面的命令可以设置所有设备的时间为０。
usrp->set_time_next_pps(uhd::time_spec_t(0));
So, device indexes run from 0 to N-1 where N is the number of devices.
因此，设备索引取值范围为 0 - N-1，N为设备数量。
Channels are indexed in a similar way. Channel indexes run from 0 to M-1 where M is the total number of channels on all devices.
信道(天线数)索引与设备索引(母板数)类似，范围为 0 - M-1，Ｍ是所有设备总的信道数。
The number and order of channels per device depends on the subdev spec (see also Specifying the Subdevice ). In the current example, assume all the X310 USRPs are using their standard configuration, and all have two daughterboards inside.
每个设备的信道数量和编号依赖于子设备的规格(see also Specifying the Subdevice )。在当前例子中，假设所有的X310设备都使用标准配置，并且每个母板都有两个子板。
In this case channels 0 and 1 map to slot A and B of the first USRP, respectively. Channels 2 and 3 map to slots A and B of the second USRP, and so on.
在这种条件下，信道０，１分别的映射到第1个USRP的插槽A，B。信道２，３分别映射到第２个USRP的插槽A，B，以此类推。
However, by changing the subdev spec on individual devices, this can change. Say we have this unusual piece of code next:
但是，通过改变一个子设备规格。也就是说，我们有下面代码段，通常我们不这么做：
usrp->set_rx_subdev_spec("A:0 B:0", 0);
usrp->set_rx_subdev_spec("A:0", 1);
usrp->set_rx_subdev_spec("B:0 A:0", 2);
The first device uses the default configuration. The second device artificially disables slot B, giving this USRP a single channel only. The third device uses both devices, but flips their order.
第１个设备使用默认配置，第２设备人为的不用第２个插槽，只用这个USRP的一个信道。第３个设备使用两个信道，但是改变他们的顺序编号。
Now, there's a total of 5 channels, mapped as:
现在，我们的到5个信号，映射如下：
- Channel 0: Slot A of Device 0
- Channel 1: Slot B of Device 0
- Channel 2: Slot A of Device 1
- Channel 3: Slot A of Device 2
- Channel 4: Slot B of Device 2
While valid, this kind of configuration is not recommended unless heavily documented. It is usually simplest to call set_rx_subdev_spec() without a device index, which will set the same subdev spec on all devices. This assumes all devices have a similar daughterboard configuration
除非有正当的理由，否则不建议使用这样的配置。通常最简单的是调用不带设备索引的 set_rx_subdev_spec() (第二个参数有默认值)，这样就会设置所有设备相同的子设备。这是假设所有的设备都有相同的子板配置。
MIMO Operation MIMO 操作
When a multi-channel streamer is generated from a compound multi_usrp, and a streamer with multiple channels is generated, MIMO operations is automatically chosen. This means samples will be aligned between streams automatically.
当multi_usrp产生 multi-channel 流的时候，MIMO操作会自动的将各个流的样点对齐。
In order for this to work, all devices must use a common time and frequency reference. This can be achieved in different ways, e.g. by daisy-chaining devices (for a small number of X-Series devices), using the MIMO cable (when only 2 N2x0 devices are used), or using a clock distribution system, e.g. an OctoClock. See Device Synchronization and the individual device manuals on more details on how to do this.
为了对齐各个channel 流的样点，所有的设备必须有一个共同的时间和频率参考。这可以通过不同的方式实现。例如，1. 通过daisy-chaining devices (对于少数X-系列设备)；2. 使用MIMO Cable(两个N2x0设备)；或者使用时钟分布系统，详见 Device Synchronization。
