1/4/2017
Physi ca cal R es esour ce ce Bl oc ock - Ask the Exper t - Ask the Exper t!t! - LT E U ni ni ve ver si si ty ty
Ask the Expert! Members can sign in to post a question to our Subject Matter Experts.
ASK THE EXPERT
Phys Physical ical Res Resource Block Block This question is answered
Posted by PhdStudent on 10 Aug 2010 12:56 PM
Hi, I am beginner in LTE and woud like to know how many packets (voice,video or data) could be sent in physical resource block (PRB)? From reading through the literature, I came across a PRB consists of 12 subcarriers. Each subcarrier has 7 symbols per 0.5ms slot. And if I am considering 64 QAM, it is 6 bits per symbol. So a subcarrier will have 48 bits which gives a total of 576 bits per PRB. Assuming my above above understan understanding ding is correct correct and if my packet size is 424 bits, can I send the packet in that t hat PRB PR B and what happens to the rest of the bits in the PRB. Does it goes wasted? What if my packet size is bigger than 576 bits. Sorry if it it sounds silly but I need to understand for me to proceed further. Thanks hanks in Advance PhdStudent
Posted by Vishal on 10 Aug 2010 9:50 PM Verified by Vishal
Verified Answe Answer r
Hi PhdStudent, On the LTE air interface the unit of allocation is a PRB. Like you mention, a PRB is 12 subcarriers by 7 OFDM symbols = 84 modulation symbols. The instantaneous throughput achieved will depend on the modulation scheme used for transmission of the PRB (lets ignore MIMO). Now, given that the PRB is a Unit of allocation the Minimum allocation to a single UE during a subframe (1 ms) are 2 PRBs with one PRB in each slot of the subframe. Hence a UE will get a total of 2 PRBs/subframe = 84 x 2 = 168 modulation symbols/subframe. Now, not all the 168 modulation symbols can be used to transmit user information because one needs to take out the modulation symbols used for Reference Signals, PDCCH, PCFICH, PHICH and PSS, SSS, PBCH if applicable. The amount of overhead will depend on number of MIMO antennas, bandwidth .etc. Once can safely take approximately a 25% http://lteuniver si ty.com /ask_the_exper t/f/59/t/2652.aspx
1/3
1/4/2017
Physi cal R esour ce Bl ock - Ask the Exper t - Ask the Exper t! - LT E U ni ver si ty
overhead out so. About 144 usable modulation symbols per subframe is a reasonable approximation for a "back of the envelope calculation" if you consider a 2x2 MIMO 10 MHz deployment. Please note, I am approximating to motivate the associated concepts. Now one can calculate the raw throughput based on a chosen modulation scheme. Please remember the bits that you calculate includes the FEC Turbo/Convolutional coding overhead. Also on a subframe basis to a UE the MAC and RLC will coordinate to maximize the use of the resource allocation - though rate matching is always an option available while assembling the MAC PDU for transmission. Hope that helps. Vishal
Posted by Vishal on 10 Aug 2010 9:50 PM Verified by Vishal
Verified Answer
Hi PhdStudent, On the LTE air interface the unit of allocation is a PRB. Like you mention, a PRB is 12 subcarriers by 7 OFDM symbols = 84 modulation symbols. The instantaneous throughput achieved will depend on the modulation scheme used for transmission of the PRB (lets ignore MIMO). Now, given that the PRB is a Unit of allocation the Minimum allocation to a single UE during a subframe (1 ms) are 2 PRBs with one PRB in each slot of the subframe. Hence a UE will get a total of 2 PRBs/subframe = 84 x 2 = 168 modulation symbols/subframe. Now, not all the 168 modulation symbols can be used to transmit user information because one needs to take out the modulation symbols used for Reference Signals, PDCCH, PCFICH, PHICH and PSS, SSS, PBCH if applicable. The amount of overhead will depend on number of MIMO antennas, bandwidth .etc. Once can safely take approximately a 25% overhead out so. About 144 usable modulation symbols per subframe is a reasonable approximation for a "back of the envelope calculation" if you consider a 2x2 MIMO 10 MHz deployment. Please note, I am approximating to motivate the associated concepts. Now one can calculate the raw throughput based on a chosen modulation scheme. Please remember the bits that you calculate includes the FEC Turbo/Convolutional coding overhead. Also on a subframe basis to a UE the MAC and RLC will coordinate to maximize the use of the resource allocation - though rate matching is always an option available while assembling the MAC PDU for transmission. Hope that helps. Vishal
Posted by PhdStudent on 10 Aug 2010 9:57 PM
many thanks.it cleared my confusion.
http://lteuniversity.com/ask_the_expert/f/59/t/2652.aspx
2/3
1/4/2017
Physi cal R esour ce Bl ock - Ask the Exper t - Ask the Exper t! - LT E U ni ver si ty
Posted by manic on 2 Apr 2014 12:35 PM
Hi i also have a question, in 5MHz there are 25 PRB every TTI so the eNB can serve up to 25 UEs ?
http://lteuniversity.com/ask_the_expert/f/59/t/2652.aspx
3/3
