Sunday, November 4, 2012

Multiuser MIMO: A brief introduction


  • Simultaneous transmissions to multiple stations. Extends the idea of MIMO where multiple streams are sent to a single stations. Here the multiple streams are sent to different stations.
  • Other cases where the MU-MIMO model applies- "The downlink of a DSL system with crosstalk between the wires for each user is one scenario where the transmitter terminals can cooperate, but the far end of the MIMO channel cannot."
  • Capacity of MU-MIMO channel in accordance with the dirty paper approach: It has been shown that  in the case where the interference on the channel is known before hand, the achievable capacity is similar to that which can be achieved without interference.
  • Two approaches to MU-MIMO: (1) Signal processing  and (2) Dirty paper approach.
  • MIMO channels are represented by the standard equation: y = Hx + w, where H is the transfer function of the channel whose dimensions are determined by the receivers(nr) x transmitters(nt).
  • MU-MIMO is more suited to WLANs rather than cellular because the channel is rich with multipath and is quasi-static. Cellular is difficult because of cost constraints, mobility,and small cell size.
  • Most recent MU-MIMO works assumes that the channel state information (CSI) is available at the transmitter.
  • SU-MIMO benefit from CSI only when nt > nr or when operating with low SNRs.
  • MU-MIMO systems always benefit from CSI.
  • Obtaining CSI: For TDD systems, use of training or pilot data UL (because same channel is used - assuming reciprocity). For FDD systems, explicit feedback from the receiver is used based on the training data sent downlink.
  • Multiple access interference (MAI) is the interference caused to one user because of simultaneous transmission to other users. Techniques like multiuser detection (MUD) could be used to detect signals.
  • Ideally, using CSI, MAI should be mitigated at the transmitter. 
  • Capacity of MU-MIMO channel is based on the fraction of power allocated to each of the users of the system.
  • Channel inversion is a linear processing technique used at the transmitter to mitigate MAI. x = H†d = H* (HH*)–1d
  • Expected capacity improvement with MU-MIMO is min(nT,nR). However, if the channel matrix H is ill conditioned, then the gains with linear processing are not achieved.

Difference between WiMAX and LTE Physical and MAC layers

This post will cover the technological differences between these two technologies and contains a brief bullet point description as to why LTE took off and WiMAX did not:

  • Slot times: LTE uses much smaller slot times - 1ms as opposed to 4ms which gives much worse delay performance with multiple users and does not scale just as well.
  • Uplink modulation: LTE introduced SC-FDMA which dramatically improved uplink performance for cellular systems. This modulation technique combines the advantages of low peak to average ratio of traditional systems (such as GSM) and multipath resistance of newer modulation schemes (such as OFDM). SC-FDMA also provides savings for the mobile users (on their uplink).
  • Timing: WiMAX was the first to start off. Hence most of the experiments were performed on this and LTE could learn from their experience and MISTAKES. WiMAX was initially designed for fixed systems rather than mobile systems and they were not able to adapt well for usage with cellular providers.

Other non MAC/PHY differences:
  • WiMAX is based on IEEE standards (specifically, the 802.16 family), and then managed by the WiMAX Forum. LTE is defined by 3GPP.  
  • WiMAX was originally designed for fixed networks and has gradually evolved into a mobile network. But this has resulted in some changes not being  made correctly. LTE was designed as a mobile network from the first day. This particularly impacts the power at the receiver (handhelds). WiMAX handhelds are slated to consume more power as compared to LTE.

What is a metal spin or chip spin?

A silicon wafer is "photo-etched", where the masking needed to etch out the different layers to produce the desired circuitry pattern is placed upon to wafer by "projecting" the image on the wafer with light (like film photography), before being chemically processed to actually etch out the pattern. The light-sensitive chemical for the masking is placed on the wafer while it is spinning to produce a uniform coat.

Respins would be where this light sensitive material is reapplied to a previously processed wafer so a new masking can be applied and the wafer re-etched to correct bugs detected in the previous run.

That probably refers to "spin-coating", a method of adding layers to a wafer. In this, a wafer is fixed on a turntable, and the photochemically active lacquer is sprayed on it's surface. The wafer then is spinned at the necessary speed to evenly distribute the material to the desired thickness.
The layer is then exposed to light through a mask, and after that, the non-exposed parts of the layer are etched away. If a new layer has to be added, that is a respin.

A brief history of the Indian subcontinent BC

     3300 BC - 1700 BC      Indus Valley Civilization.
     1700 BC - 1300 BC      Late Harappan Culture.
     1500 BC - 500 BC       Vedic Civilization..
     1200 BC - 316 BC       Kuru dynasty.
               1000 BC      Aryans expand into the Ganga valley.
               900 BC       Mahabharata War.
               800 BC       Aryans expand into Bengal. Beginning of the Epic Age:
                            Mahabharata composed. First version of Ramayana.
      700 BC - 321 BC       Maha Janapadas
      684 BC - 321 BC       Madadha Empire
               550 BC       Composition of the Upanishads
               544 BC       Buddha's Nirvana
               327 BC       Alexander's Invasion
               325 BC       Alexander marches ahead till Multan
               324 BC       Chandragupta Maurya defeats Seleacus Nicator
               322 BC       Rise of the Mauryas: Chandragupta establishes first Indian
                            Empire
      321 BC - 180 BC       Mauryan Empire
               298 BC       Bindusara Coronated
               272 BC       Ashoka begins regin
               180 BC       Fall of the Mauryas. Rise of the Sungas by 
Pushyamatra Sunga