EMRFD Message Archive 10737

Message Date From Subject
10737 2015-01-19 11:31:23 Chris Howard w0ep QEX free for shipping
I was the recipient of a stack of QEX magazines recently,
probably half of which are duplicates of what I already
own. I haven't checked the exact overlap yet but
likely about 20 issues from 2004-ish to 2010-ish.
Not guaranteed to be a complete series.

If anyone is interested in paying shipping, I'll send
them to you in a USPS priority mail box (under-$20)

10738 2015-01-19 11:49:46 Chris Howard w0ep Re: QEX free for shipping

10756 2015-01-27 04:08:51 radioaustralia123 Spatial Multiplexing ....(SM or SMX)....question.
Hi all,

Spatial Multiplexing is a system that is used in G4 mobile systems that has two antennas. One with polarization at 45 deg and the other at 135 deg. My understanding is that two separate RF signals that have the same frequency are transmitted (or received), one in each antenna.  The fact that the antennas don’t have the same polarization means that they don’t interfere with each other. Well I guess not all the time for line of sight transmission paths. Because there are two data streams the data rate is doubled.

My question is:

But in a mobile phone that you have in your pocket, How do they make the antennas have the correct polarization, they are not fixed…??? OK at the Cell station as they are fixed.

Roderick Wall, vk3yc.

10767 2015-02-07 04:12:52 Jim Amos Re: Spatial Multiplexing ....(SM or SMX)....question.
HI Roderick,

The inherent multi-path in the channel will destroy the absolute polarization of the signals anyway, so will just moving the handset.  Most modern systems are based on some form of OFDM modulation and MIMO, this includes  Wi-Fi 802.11N, 802.11AC, and also Cellular LTE (4G).   As you point out, MIMO based system are used to send multiple signals on the same channel .  To do this MIMO systems require a certain amount of isolation between the signal paths for the signals to be receive on the different antennas.  Mathematically this is called decorrelation of the channel.    You can achieve this isolation by also just moving the antennas apart, so if they are adequately spaced the polarization does not matter, which is the configuration of the cell tower.  It does matter on the closer spacing of a handset where you can only move the antennas apart so far.   The other thing this does is to point the antenna radiation patterns away from each other on the hand set, also helping with the isolation or decorrelation. 

Jim, N8CAH

10768 2015-02-07 06:06:58 radioaustralia123 Re: [emrfd] Spatial Multiplexing ....(SM or SMX)....question.
Hi Jim,

Thanks for your reply. Very interesting with regard to moving the antennas apart where polarization does not matter. I suppose this would then be called Space Division Multiplexing. Where the same frequency is used in different space.

Would I be correct in saying that MIMO based systems rely on both Space Division Multiplexing and Spatial Multiplexing. It also looks to me that Spatial Multiplexing would only work when you are close to the Cell station. If there is a multi-path problem it then falls back to just using the channel with one data stream instead of using two data streams with Spatial Multiplexing?


Roderick, vk3yc.

10769 2015-02-08 04:17:40 Jim Amos Re: Spatial Multiplexing ....(SM or SMX)....question.
Hi Roderick,

There is no need for two terms, spatial multiplexing is "space" multiplexing.   Keep mind that different polarization is used where it helps, when the antennas can't be placed far enough apart to maintain the decorrelation of the multiple streams. 

With regard to the multipath problem, there are several techniques being deployed that mitigate this problem.   Unlike earlier modulation techniques, the OFDM signal is designed to tolerate multipath by having a intersymbol gap.  

MIMO systems also deploy a form of beam forming that is used to mitigate destructive interface when signals arrive out of phase at the receiver.   (I'll use an 802.11 example that can be purchased by anyone here)  You use a 4x4 802.11n or 802.11ac access point to beam form to a 1x1 client, or a 2x2 client.   In MIMO terminology 4x4 implies 4 transmit and 4 recieve antennas.   So, if you have your original 2 antenna design, we'll assume this is capable of 2x2 operation.  With a 4x4 access point (or cell tower) you can use 2 of the 4 antennas to beam form to each antenna, or all 4 antennas to beamform to one or the other of the antennas.    

And you are correct, their is a "fall back" position, created by a technique called rate shifting.   The radios are designed to operate at their best possible rate.  So, taking your 2 antenna example, with strong signals you can operate using two spatial streams (two tx antennas per one receive antenna), but under weak signal conditions this will drop down to a single spatial stream (4 tx antennas to 1 receive antenna).    This is done automatically by the radios, but if you pay attention to the data rates, you can see when you are using multiple spatial streams or not.

In the reverse direction with a 4x4 access point to a 2x2 client you will have 1 TX antenna to 2 RX antenbas, or 1 TX antenna to 4 RX antennas.  Typically down link rates or greater than uplink rates, and Access Point or Base station power levels are greater then then the radios in your handset.    This is OK, since most of the time you have more downstream packets anyway.

4x4 access points are common for 802.11, but for 4G base stations they can use even more antennas.