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Section 3 6 SDMA v1
Outline Introduction Precoding Scheduling (user selection) Chapter 3: Physical-layer transmission techniques Section 3.6: Space Division Multiple Access (SDMA) Instructor: Nguyen Le Hung Email: nlhung@dut.udn.vn; nnguyenlehung@yahoo.com Department of Electronics & Telecommunications Engineering Danang University of Technology, University of Danang Mobile Communications - Chapter 3: Physical-layer transmissions Section 3.6: Space Division Multiple Access (SDMA) 1 Outline Introduction Precoding Scheduling (user selection) 1 Introduction SDMA and OFDM Multiuser transmission 2 Precoding Precoding classification An example of linear precoding Power allocation in ZF precoding Possible research problems 3 Scheduling (user selection) Exhaustive selection Greedy selection Mobile Communications - Chapter 3: Physical-layer transmissions Section 3.6: Space Division Multiple Access (SDMA) 2 Outline Introduction Precoding Scheduling (user selection) SDMA and OFDM Multiuser transmission SDMA with OFDM The integration of multi-antenna and OFDM techniques has provided remarkable diversity and capacity gains in broadband wireless communications. In multiuser (MU) transmissions, the use of multiantenna array at the base station (BS) enables simultaneous transmission of multiple data streams to multiple users by exploiting spatial separations among users. ABS/eNB AMS/UE (a) IFFT SU-MIMO precoder ABS/eNB AMS/UE1 (b) IFFT MU-MIMO precoder AMS/UE2 Mobile Communications - Chapter 3: Physical-layer transmissions Section 3.6: Space Division Multiple Access (SDMA) 3 Outline Introduction Precoding Scheduling (user selection) SDMA and OFDM Multiuser transmission A simple example of multiuser (MU) transmission 1,1 h 2,1 h M h ,1 Base Station 1 s Modulation Coded bits of user 1 2 s Modulation Coded bits of user 2 1,2 h 2,2 h M h ,2 Antenna 1 Antenna M De-mod Channel estimator User 2 De-mod Channel estimator User 1 1 y 2 y 𝑦 1 = 𝑠 1 𝑀 𝑚=1 ℎ 1,𝑚 +𝑠 2 𝑀 𝑚=1 ℎ 1,𝑚 +𝑧 1 , and 𝑦 2 = 𝑠 2 𝑀 𝑚=1 ℎ 2,𝑚 +𝑠 1 𝑀 𝑚=1 ℎ 2,𝑚 +𝑧 2 Mobile Communications - Chapter 3: Physical-layer transmissions Section 3.6: Space Division Multiple Access (SDMA) 4 Outline Introduction Precoding Scheduling (user selection) Precoding classification An example of linear precoding Power allocation in ZF precoding Possible research problems Precoding classification In the so-called space division multiple access (SDMA), multiuser diversity is the primary factor that increases significantly the system sum-rate (throughput). As a result, an appropriate multiuser encoding technique (at the BS) is indispensable to attain the considerable sum-rate gain in SDMA. It is well-known that dirty paper coding (DPC) is an optimal multiuser encoding strategy that achieves the capacity limit of MU broadcast (BC) channels but at the cost of extremely high computation burden as the number of users is large. Recent studies have introduced several suboptimal multiuser encoding techniques with lower complexity (relative to DPC) that can be categorized into: nonlinear precoding such as: vector perturbation, Tomlinson Harashima techniques linear precoding such as: minimum mean squared error (MMSE), zero-forcing. Mobile Communications - Chapter 3: Physical-layer transmissions Section 3.6: Space Division Multiple Access (SDMA) 5 Outline Introduction Precoding Scheduling (user selection) Precoding classification An example of linear precoding Power allocation in ZF precoding Possible research problems Multiuser transmission techniques Broadband communications LTE (4G) system Broadband communications (high data rate and reliability) Diversity Time Freq. Signal Space Multi- user Space Multipath channel Modeling CSI feedback Analog Digital Vector quantization g Quasi-static Time-variant BEMs AR LBG Grassmannian Random Scheduling Precoding Exhaustive search Greed or iterative search Linear methods Non-linear methods Codebook- based ones MMSE BD DPC THP PU 2 RC Random user selection VP Mobile Communications - Chapter 3: Physical-layer transmissions Section 3.6: Space Division Multiple Access (SDMA) 6 Outline Introduction Precoding Scheduling (user selection) Precoding classification An example of linear precoding Power allocation in ZF precoding Possible research problems An example of linear precoding 1,1 h 2,1 h M h ,1 Base Station Feedback link of channel state information (CSI) 1 s X X X 1,1 w Modulation Coded bits of user 1 2,1 w M w ,1 2 s X X X 1,2 w Modulation Coded bits of user 2 2,2 w M w ,2 1,2 h 2,2 h M h ,2 Antenna 1 Antenna M De-mod Channel estimator User 2 De-mod Channel estimator User 1 1 y 2 y 𝑦 1 = 𝑠 1 𝑀 𝑚=1 𝑤 1,𝑚 ℎ 1,𝑚 +𝑠 2 𝑀 𝑚=1 𝑤 2,𝑚 ℎ 1,𝑚 +𝑧 1 , and 𝑦 2 = 𝑠 2 𝑀 𝑚=1 𝑤 2,𝑚 ℎ 2,𝑚 +𝑠 1 𝑀 𝑚=1 𝑤 1,𝑚 ℎ 2,𝑚 +𝑧 2 Mobile Communications - Chapter 3: Physical-layer transmissions Section 3.6: Space Division Multiple Access (SDMA) 7 Outline Introduction Precoding Scheduling (user selection) Precoding classification An example of linear precoding Power allocation in ZF precoding Possible research problems Inter-user interference The received signals at user-𝑢 can be determined by 𝑦 𝑢 = 𝑠 𝑢 𝑀 𝑚=1 𝑤 𝑢,𝑚 ℎ 𝑢,𝑚 + 𝑠 𝑢 ′ 𝑀 𝑚=1 𝑤 𝑢 ′ ,𝑚 ℎ 𝑢,𝑚 + 𝑧 𝑢 , 𝑢, 𝑢 ′ ∈ {1, 2}, (1) where 𝑠 𝑢 ′ 𝑀 𝑚=1 𝑤 𝑢 ′ ,𝑚 ℎ 𝑢,𝑚 is called as inter-user interference that would significantly degrade the performance of the system. Precoding design is to find the weighting coefficients {𝑤 𝑢,𝑚 } 2 𝑢=1 that satisfy the following condition 𝑀 𝑚=1 𝑤 𝑢 ′ ,𝑚 ℎ 𝑢,𝑚 = 0 with 𝑢, 𝑢 ′ ∈ {1, 2} (2) to eliminate the inter-user interference 𝑠 ′ 𝑢 𝑀 𝑚=1 𝑤 𝑢 ′ ,𝑚 ℎ 𝑢,𝑚 . The above technique is called as zero-forcing (ZF) precoding. The problem of finding the weighting coefficients {𝑤 𝑢,𝑚 } 2 𝑢=1 can be easily solved by expressing received signals in a vector form. Mobile Communications - Chapter 3: Physical-layer transmissions Section 3.6: Space Division Multiple Access (SDMA) 8 Outline Introduction Precoding Scheduling (user selection) Precoding classification An example of linear precoding Power allocation in ZF precoding Possible research problems Zero forcing (ZF) precoding formulation In the presence of two users, the previous equations become 𝑦 1 𝑦 2 = ℎ 1,1 . . . ℎ 1,𝑀 ℎ 2,1 . . . ℎ 2,𝑀 𝑤 1,1 𝑤 2,1 . . . . . . 𝑤 1,𝑀 𝑤 2,𝑀 𝑠 1 𝑠 2 + 𝑧 1 𝑧 2 . In the presence of 𝑈 users, the received signal can be expressed by: y = HWs + z, (3) where y = ⎡ ⎢ ⎣ 𝑦 1 . . . 𝑦 𝑈 ⎤ ⎥ ⎦ , H = ⎡ ⎢ ⎣ ℎ 1,1 . . . ℎ 1,𝑀 . . . . . . . . . ℎ 𝑈,1 . . . ℎ 𝑈,𝑀 ⎤ ⎥ ⎦ , s = ⎡ ⎢ ⎣ 𝑠 1 . . . 𝑠 𝑈 ⎤ ⎥ ⎦ W = ⎡ ⎢ ⎣ 𝑤 1,1 . . . 𝑤 𝑈,1 . . . . . . . . . 𝑤 1,𝑀 . . . 𝑤 𝑈,𝑀 ⎤ ⎥ ⎦ = [w 1 , . . . , w 𝑈 ] with w 𝑢 = [𝑤 𝑢,1 , . . . , 𝑤 𝑢,𝑀 ] 𝑇 , and z = [𝑧 1 , . . . , 𝑧 𝑈 ] 𝑇 . Mobile Communications - Chapter 3: Physical-layer transmissions Section 3.6: Space Division Multiple Access (SDMA) 9 Outline Introduction Precoding Scheduling (user selection) Precoding classification An example of linear precoding Power allocation in ZF precoding Possible research problems Zero-forcing precoding formulation (cont.) To eliminate inter-user interference, precoding matrix W can be determined by W = H 𝐻 HH 𝐻 −1 ≜ H † (4) so that y = HWs + z = s + z. (5) With precoding, the received signal can be written by y = Hx + z, (6) where x = [𝑥 1 , . . . , 𝑥 𝑀 ] 𝑇 = Ws are the transmitted signals in a vector form at 𝑀 antennas in the base station. Under the power constraint of 𝑃 max at the BS, one has 𝔼 𝑀 𝑚=1 ∣𝑥 𝑚 ∣ 2 = 𝔼 ∥ x ∥ 2 ≤ 𝑃 max , (7) Mobile Communications - Chapter 3: Physical-layer transmissions Section 3.6: Space Division Multiple Access (SDMA) 10 . Chapter 3: Physical-layer transmissions Section 3. 6: Space Division Multiple Access (SDMA) 3 Outline Introduction Precoding Scheduling (user selection) SDMA. VP Mobile Communications - Chapter 3: Physical-layer transmissions Section 3. 6: Space Division Multiple Access (SDMA) 6 Outline Introduction Precoding