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LTE (Diversity - 2) - 6b

Diversity - 2 [Under LTE]

Spatial/Space diversity

What if the receiver has multiple antennas, would receiver be able to catch better or more signal ?

Layman's answer would be "YES" as two antennas mean double signal reception. But the idea is not receiving the signal twice; because that way signal-to-noise ratio would still be same. The idea is rather one antenna catching the signal which other antenna is not able to catch due to fading (or undesirable radio conditions) and vice versa !! So that when signals received by antennas are "combined", we will have much better signal-to-noise ratio.

To achieve this, it is correct to say that two antennas should not be at same place; that way both antennas are likely to experience same radio conditions. So two antennas are to be "appropriately" put (far apart); that way overall best signal-to-noise ratio can be received. This is called Spatial/Space diversity.

Spatial diversity can better be used at base station site rather than at UE (where space is limited) i.e. in UL direction.

Above method can also be called (antenna) Receive diversity.

Similar is the idea when multiple antennas are put at transmitter. There is an interesting point to be noted here: transmission from multiple antennas would work almost like multiple paths at receiver. Rake receiver (used for CDMA solutions) already has capability to catch these multiple paths and amplify them. So in UMTS, if we use multiple antennas at transmitter, no major modification required in reception at UE !!

This method of spatial diversity is called Transmit (antenna) diversity.

In transmit antenna diversity, antennas are located a certain distance apart; but that leaves out whether the same signals are to be transmitted at the same time. When the signals transmitted with certain delay between multiple antenna transmissions, it is called Delay diversity. When instead of linear delays, cyclic shifts are used between multiple antenna transmissions, it is called Cyclic Delay diversity (CDD).

Delay diversity is used in UMTS (WCDMA) and CDD is used in LTE (OFDM) for DL transmission.

Spatial diversity idea can be extended to put multiple antennas in different directions; it is then called as Polarisation diversity. This can be used in both transmit and receive antenna diversity configurations.

Study of above diversity techniques form basis for practical applications like MIMO (Multiple Input and Multiple Output), Beam forming.

UMTS has improved HSPA to HSPA+ with addition of MIMO in Rel 7 (we will look at it more detail in later articles on HSPA+).

References: UMTS by Sanchez and Thioune, WCDMA for UMTS by Holma and Toskala, 3G Evolution: HSPA and LTE by Dahlman, Parkvall, Sköld, and Beming, and WiMAX handbook by Ahson and Ilyas.

Copyright © Samir Amberkar 2010-11§

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