01-20-2016 12:31 AM
I want to design relaying system that use two USRP of NI USRP 2922 with the sample rates same as in LTE standard. But, there are limitation in LAN bandwidth utilisation of 1Gbps.
2) I also already try to change the sample width at the USRP property but it is not work well for Rx. What should I need to consider if I want to use 16 bits per sample of sample width. How can I reduce the LAN bandwidth utilisation that use for transmission from host to USRP?
3) Is it possible to make the data received by the USRP stored and processed in USRP instead of in PC?
4) Can Anyone give suggestions what type of device that I can use which have 4 antenna port and the processing can be done inside the device only without transferring to the PC
I am really appreciate for any suggestions and advices if you can help me with this.
Thank you.
Solved! Go to Solution.
01-25-2016 11:13 AM
I don't believe you would achieve full rate on a USRP without a dedicated link to the PC. The protocol used is UDP between the USPR and PC which can be disruped by network latency and cause packets to arrive out of order or be lost. Also you should assume that due to filter edge effects only 80% of the specified bandwidth is usable. (so 20 MHz of bandwdith requires an IQ rate of 25 MS/s) It takes a fast PC with a very high quality (we use intel) network card to achieve this. Additionaly you will need to use the NIC card manufactuer's driver and maximize RX and TX buffers. So far I've only acheived it on desktop PCs and not laptops. I would aslo assume 20% packet overhead, so on a 1 Gb/s link I would not expect to exceed 800 Mb/s.
25MS/s x (16bit I + 16 bit q) = 800 Mb/s
You can also, in the driver, switch to 8bit mode which will halve your dynamic range but also the data rate.
For a system like yours, you may consider placing a PC next to each usrp and installing 2 networks cards in each PC. You will add latency, but the extra computing will give you buffering against network issues and congestion. Also, relaying is not a 'constant' streaming activity... only the RX is constant. For TX you might use the bursty transmission approach.
Erik
01-26-2016 03:55 AM
Hi Erik,
Thanks for replying. Your explaination is easy to understand. Maybe I will try to use two network cards and use 8bit mode. By doing that, I think maybe I can achieved a LTE bandwidth up to 15MHz which using 23.04MS/s.
Thank you.
01-26-2016 06:39 AM
01-26-2016 08:48 PM
That's true. I can't achieve 23.04MS/s for IQ rate because it will coerce to 25MS/s. I see the bypassed stage is fix. So, you said I only can modified N from 1 to 128 at /N stage. How to do that, is it in the USRP properties?
01-26-2016 08:59 PM - edited 01-26-2016 09:01 PM
its calculated in the driver to give you the optimum setting based on requested IQ rate. I don't believe that you can choose them individually as half-band and CIC filter parameters are also factored into the calculations.
The USRP RIO devices (2942) have a larger fpga and a more flexible N/M aribtrary resampler so you have many more frequency possiblities. It also utlizes PCIe as a connectivity option so that you can get reliable, higher bandwidth, lower latency communicaiton from the radio to the PC.
Erik
01-26-2016 09:26 PM
Okay, from my understanding, the explaination about the coerced IQ rate is just for the information of how the IQ rate is coerce to nearest supported rate. So, there is nothing I can do with coerced IQ rate to get an accurate IQ rate same as desired IQ rate. Maybe I will consider to use USRP RIO next time but for now I only allowed to use NI USRP 2922 only to create relaying system.
Thank you Erik. This really help me understand better.