10 Tm5117aen03gla1 Mimo for Lte Ppt

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    LTE Physical Layer services assume multiple port antenna systems are used. Multiple port antenna

    systems are implemented for the following reasons:

    • Improved transmission reliability 

    • Greater coverage or range 

    • Reduced UE power consumption 

    • Increased transmission throughput 

    Multiple port antenna systems include the following:

    • Single Input Multiple Output (SIMO) 

    • Multiple Input Single Output (MISO) 

    • Multiple Input Multiple Output (MIMO) 

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    In a SIMO configuration the transmitter (usually the UE) has one transmitter and the receiver (the

    eNodeB) has two physically separated antenna ports. The receiver picks up multiple versions of the

    same signal but separated spatially. SIMO receivers use the following techniques to compute the best

    received signal.

    Switched Diversity

    In Switched Diversity, the input with the best signal is chosen as the best source. The “best” signal 

    may be based on Signal-to-Noise Ratio (SNR) or Bit Error Rate (BER). Switched diversity is the most

    simple and inexpensive SIMO technique.

    Equal Gain Combining

    Equal Gain Combining is a summation of all available received signals.

    Maximum Ratio Combining

    In Maximum Ratio Combining (MRC), each received signal has compensation applied to it before being

    combined to produce a composite single signal. This technique is particularly effective where the signal

    undergoes deep fading. Because fading probably occurs at different frequencies on

    each antenna port,

    the reliability of the radio link is increased.

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     A MISO (eNodeB) transmitter has two or more physically separated antenna ports, while the MISO (UE)

    receiver has one antenna. Each Tx port transmits the same information bits. In addition to data signals,

    reference signals are also transmitted via both antenna ports. The normal reference signal pattern is

    sent via the first antenna port and the diversity reference signal pattern via the second antenna port.

    In Space-Time Transmit Diversity (STTD) the same data is transmitted simultaneously over both Tx

    ports. On each port, the channel-coded data is processed in blocks of four bits, then the bits are time

    reversed and complex conjugated. The physical separation of the antenna ports provides the space

    diversity, and the time difference derived from the bit-reversing process provides the time diversity. These

    features together make the decoding process in the receiver more reliable.

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    MIMO systems contain multiple antenna ports at both the transmitter and receiver. The MIMO transmitter

    transmits signals using time, frequency, and space diversity. The MIMO receiver recovers the data across

    multiple receiving antenna ports.

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    Space-Time Coding (STC) provides diversity gain to combat the effects of unwanted multipath

    propagation. Similar to STTD, time delayed and coded versions of the same signal are sent from the

    same transmitter antenna. The codes that are used are mainly: trellis and block (less complex) codes.

    This improves the SNR for cell edge performance.

    Spatial Multiplexing (SM)

    With Spatial Multiplexing, unique (different) data streams are transmitted over different antenna ports.

    Spatial multiplexing can double (2x2 MIMO) or quadruple (4x4 MIMO) capacity and throughput. This

    technique gives higher capacity when RF conditions are favorable and users are closer to the eNodeB.

    The graphic shows spatial multiplexing with a 2x2 MIMO configuration. The receiver can identify the

    transmitting antenna port for each received signal

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    MIMO supports single user MIMO and multi-user MIMO. Single User MIMO improves the performance

    for a UE (via space time coding), or increases the throughput for a UE (using spatial multiplexing).

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    In multi-user MIMO, the data for different users is multiplexed onto a single time- frequency resource, so

    the capacity of the cell can increase in terms of users without increasing the system bandwidth.

    Switching between SU-MIMO and MU-MIMO is supported on a per UE basis. The

    use of codes and reference signals not only allows the receiver to differentiate

    between antenna streams and users, but also allows accurate channel estimation

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    MIMO supports both open loop and closed loop control. Open loop MIMO transceivers adjust their

    transmission based on received (reference signal) measurements. This assumes no rapid feedback

    technique is available from the UE receiver back to the eNodeB transmitter. Unfortunately, in open loop

    operation, the transmitter receives no feedback regarding antenna port operation or signal strength in

    the forward direction.

    Closed loop MIMO supports a feedback loop describing eNodeB transmitter operation and UE

    recommendations. Both the eNodeB and UE contain a codebook which describes possible RF

    parameters, for example, the phase shift between antenna ports. In closed loop MIMO, the UE

    describes eNodeB transmitter operation by returning an index into the shared codebook.

    Closed loop operation uses the following steps.

    1. The eNodeB transmits a DL pilot channel as a reference signal on all antenna ports.

    2. he UE evaluates various codebook options that specify the RF parameters. 3. The UE transmits its recommendations in the form of a codebook index to the eNodeB.

    4. The eNodeB adjusts its DL transmission to the UE based on the recommended parameters.

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    In case of open loop spatial multiplex two cases have to be distinguished. If the transmitted rank indication (TRI) = 1 the transmission mode corresponds to transmit

    diversity.

    If TRI >1 large delay CDD is used. The number of layers is 2, 3 or 4.

    In case of closed loop spatial multiplexing feedback from the UE it is used.

    The UE feedbacks values of the RI = Rank Indicator and PMI = Precoding Matrix Indicator.

    In case of 2 antenna ports the codebook consists of 2 matrices, in case of 4 antenna ports there are 16 entries. A restriction may be signaled so that only a subset thereof can be used.

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