MIMO Transmission schemes for LTE and HSPA Networks, 3G

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3 PRINCIPLES3.1 MULTI‐ANTENNA‐TRANSMISSION BASICSRoughly speaking, multi-antenna transmission has two partly separate aims:• Improving SINR• Sharing SINRIn principle, the first approach (improving SINR) is mainly targeting low SINR scenarios while the secondapproach (sharing SINR) is mainly targeting high SINR scenarios.3.1.1 IMPROVING SINRThe classical technique of using an antenna array for transmitting energy in the direction of the intendedreceiver falls into the category of improving SINR. Such classical beamforming achieves increased SINR byphase adjustments of the signals transmitted on the different antennas with the aim of making the signalsadd-up constructively on the receive side. In a cellular network, this may also cause interference for otherusers since the transmission is confined to a rather narrow beam as opposed to being uniformly distributedover a whole sector.Transmit diversity using e.g. the Alamouti space-time block code or space-frequency block codes alsostrives for improving the SINR, albeit not in the same sense as beamforming. In contrast to beamforming,transmit diversity does not improve the average SINR but rather, due to diversity, reduces the variations inthe SINR experienced by the receiver.3.1.2 SHARING SINRCompared with improving SINR, the reasons for sharing SINR are a bit less obvious. An improved SINR canbe exploited by transmitting with a more aggressive modulation and coding rate, thus achieving a higher linkthroughput. However, when the SINR level becomes high enough the throughput versus SINR curvesaturates, giving diminishing returns for a further increase in the SINR, as illustrated in Figure 1.6
Figure 1. Throughput curve versus SINR saturates at high SINR values.However, by the use of multiple antennas at both sides (transmitted and receiver) of the radio link, multipleparallel channels can be created. These parallel channels share the overall SINR, thus avoiding the abovementioned throughput saturation. In the simplest of cases, such a multiple-input multiple-output (MIMO)antenna configuration uses two antennas at the transmitter and two antennas at the receiver resulting in a2x2 MIMO channel. Here, the notation NTxNR is taken to mean NT transmit antennas (channel inputs) andNR receive antennas (channel outputs). The SINR may be shared by simply encoding and modulating twoblocks of information bits separately, and transmitting the resulting two symbols streams from differentantennas. This is referred to as Spatial Multiplexing. In practice, the signal on each receive antenna will be asuperposition of the signals stemming from all transmit antennas. Neglecting the inter-stream interference, the2x2 MIMO link can be modeled as two parallel channels, each having some specific SINR. Keeping the totaltransmitted power the same (reducing each transmitted stream by 3 dB) and further assuming the twochannels have the same amount of noise, that SINR value will be half of the SINR value for a single streamtransmission using the same total power of only one antenna. On the other hand, there are now two datapipes to use, offsetting the reduction in the common SINR value. The overall gain depends on where on thethroughput curve the operating point is but for sufficiently high SINRs, the throughput can be doubled, just asusing twice as much bandwidth can double the throughput. If the two channels are not balanced i.e. theirSINR values are different, then a higher overall SINR level is needed to approach a factor of two gain inthroughput. In general, a NTxNR MIMO channel supports transmissions of at most rmax = min{NR,NT}simultaneous symbol streams for sufficiently high SINR values.However, in practice the inter-stream interference needs to be taken into account. The inter-streamorthogonality largely depends on the actual MIMO channel realization. The number of simultaneouslytransmitted streams the MIMO channel can support is commonly referred to as the channel rank and theactually transmitted number of streams is correspondingly termed transmission rank. The transmission ranktypically needs to be adapted to suit the current channel characteristics and hence avoid excessive interstreaminterference.In the most general context, transmission rank can be defined as the number of complex-valued independentmodulation symbols transmitted per time-frequency resource. Beamforming, transmit diversity based onAlamouti codes (as defined below) and even single-antenna transmissions are all considered single-rankschemes and naturally fit within the definition of transmission rank. Obviously, spatial multiplexing schemesprovide the possibility to use transmission ranks higher than one.7
Page 2 and 3: Table of Contents1 TERMINOLOGY ....
Page 4 and 5: 1 TERMINOLOGYIt is assumed that the
Page 8 and 9: 3.1.3 PEAK RATE OR COVERAGEIn a cel
Page 10 and 11: sufficiently accurate channel infor
Page 12 and 13: Figure 5. Examples of Power Delay P
Page 14 and 15: 3.2 BS ANTENNA CONFIGURATIONSFigure
Page 16 and 17: NOTE:Implementation complexity refe
Page 18 and 19: 3.3.4 ADVANCED‐ANTENNA CONCEPTS F
Page 20 and 21: and interference. By considering th
Page 22 and 23: ank over all the feasible 1 set of
Page 24 and 25: Typically a constant delay increase
Page 26 and 27: the columns of Λ U r rxrfor some o
Page 28 and 29: to blindly estimate this power rati
Page 30 and 31: Figure 16. MMSE Receiver for UL MU-
Page 32: transmitter has information about t
Page 35 and 36: Intra-site CoMPBS3BS2Inter-site CoM
Page 37 and 38: 5.5.4 UL COMPUplink coordinated mul
Page 39 and 40: 6.2 DL SYSTEM PERFORMANCE6.2.1 THE
Page 41 and 42: Compared to baseline, SE gains for
Page 43 and 44: 6.2.3. CL‐MIMO WITH DIV ANTENNA C
Page 45 and 46: 6.2.4. MIMO WITH ULA‐4VWith the U
Page 47 and 48: 0.035Baseline Uplink Cell Edge Sect
Page 49 and 50: 7 CONCLUDING REMARKSThis report pro

MIMO Transmission schemes for LTE and HSPA Networks, 3G

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