Praise for Fundamentals of WiMAX

8.9 Closed-Loop MIMO 2998.9.3 Codebook Based FeedbackCodebook based feedback allows the MS to explicitly identify a precoding matrix based on acodebook that should be used for DL transmissions. Separate codebooks are defined in the standardfor various combinations of number of streams N s and number of transmit antennas N t . Foreach combination of N s and N t , the standard defines two codebooks: the first with 8 entries andthe second with 64 entries. If it chooses a precoding matrix from the codebook with 8 entries,the MS can signal this to the BS by using a 3-bit feedback channel. On the other hand, if itchooses a precoding matrix from the codebook with 64 entries, the MS can indicate its choice tothe BS by using a 6-bit feedback channel. This choice of two codebooks allows the system toperform a controlled trade-off between performance and feedback efficiency. For band AMCoperation, the BS can instruct the MS to provide either a single precoder for all the bands of thepreferred subchannels or different precoders for the N best bands.The IEEE 802.16e-2005 standard does not specify what criteria the MS should use to calculatethe optimum precoding matrix. However, two of the more popular criteria are maximizationof sum capacity and minimization of mean square error (MSE). Link performances of the variouscodebook selection criteria and their comparison to more optimal closed-loop precoding techniques,such as based on singular-value decomposition precoding, are provided in Chapter 11.8.9.4 Quantized Channel FeedbackQuantized MIMO feedback allows the MS to explicitly inform the BS of its MIMO channel stateinformation. The MS quantizes the real and imaginary components of the N t × N r MIMO channelto a 6-bit binary number and then sends this information to the BS, using the fast-feedback channel.Clearly, the quantized channel feedback requires much more feedback bandwidth in the ULcompared to the codebook-based method. For example, in the case of a IEEE 802.16e-2005 systemwith four antennas at the transmitter and two antennas at the receiver, a quantized channelfeedback would require 16 × 6 bits to send the feedback as opposed to the codebook basedmethod, which would require only 6 bits. Owing to the high-bandwidth requirement of the quantizedchannel feedback mode, we envision this mode to be useful only in pedestrian and stationaryconditions. In such slow-varying channel conditions, the rate at which the MS needs toprovide this feedback is greatly reduced, thus still maintaining a reasonable bandwidth efficiency.Again, the IEEE 802.16e-2005 standard does not specify what criteria the BS needs to usein order to calculate an optimum precoder, but two of the most popular criteria are maximizationof sum capacity and minimization of MSE. Link performances of various optimization criteriaand their performance relative to other techniques are provided in Chapter 11.8.9.5 Channel SoundingAs defined in the standard for TDD operations, the channel-sounding mechanism involves theMS’s transmitting a deterministic signal that can be used by the BS to estimate the UL channelfrom the MS. If the UL and DL channels are properly calibrated, the BS can then use the ULchannel as an estimate of the DL channel, due to channel reciprocity.