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1. 제 목 : WiBro 물리 계층과 성능개선을 위한 다중안테나 기술
2. 발 표 자 : 성 상 훈 박사 (삼성전자)
3. 일 시 : 2005년 5월 6일(금) 10:30 ~ 11:30
4. 장 소 : 공대5호관 104호
5. 초청교수 : 김 남 철 교수
6. 강사약력 :
   2002 ~ 현재,   삼성전자 (책임연구원)
   2002 AWARE, MA (Sr. Engineer)
   2001 Northeastern University, MA (공학박사)
   1995 University of Pittsburgh, PA (공학석사)
   1992 경북대학교 전자공학 (공학사)
7. 내용요약 :
   4G is a vast subject, encompassing a variety of new services, applications, and air interfaces. To address in depth the hottest technical issues worldwide nowadays, I shall limit this talk on WiBro (IEEE802.16e), one of the evolving 4G candidates.  WiBro operates on TDD (Time Division Duplexing) OFDMA (Orthogonal Frequency Division Multiplexing Access) at a center carrier of 2.3GHz. WiBro enhances the mobile spectral efficiency by functioning multiple PHY features such as AMC, Hybrid ARQ, SA/MIMO, OFDM.
   WiBro AMC: The principle of AMC is to change the modulation and coding format in accordance with instantaneous variations in the channel conditions, subject to system restrictions. AMC extends the systems ability to adapt to good channel conditions. Since the channel conditions vary over time, the receiver collects a set of channel statistics which are used both by the transmitter and receiver to optimize system parameters such as modulation and coding, signal bandwidth, signal power, channel estimation filters, automatic gain control, etc
   WiBro Hybrid ARQ:A successful broadband wireless system must have an efficient co-designed medium access control (MAC) layer for reliable link performance over the lossy wireless channel. The corresponding MAC is designed so that the TCP/IP layers sees a high quality link that it expects. This is achieved by an automatic retransmission and fragmentation mechanism (ARQ), wherein the transmitter breaks up packets received from higher layers into smaller sub-packets, which are transmitted sequentially.
   WiBro MIMO & Smart antenna: Increasing demand for high performance 4G broadband wireless mobile calls for use of multiple antennas at both base station and subscriber ends. Multiple antenna technologies enable high capacities suited for Internet and multimedia services and also dramatically increase range and reliability. The broadband channel is typically non-LOS channel and includes impairments such as time-selective fading and frequency-selective fading. Multiple antennas at the transmitter and receiver provide diversity in a fading environment. By employing MIMO, multiple spatial channels are created and it is unlikely all the channels will fade simultaneously. Meanwhile, smart antenna exploits the directivity of user and cannot only support spatial multiplexing but also actively suppress intercell interference.
   WiBro OFDM(A): OFDM(A)is chosen over a single carrier solution due to lower complexity of equalizers for high delay spread channels or high data rates. A broadband signal is broken down into multiple narrowband carriers (tones), where each carrier is more robust to multipath.  In order to maintain orthogonality amongst tones, a cyclic prefix is added which has length greater than the expected delay spread. With proper coding and interleaving across frequencies, multipath turns into an OFDM system advantage by yielding frequency diversity. OFDM can be implemented efficiently by using FFT''''s at the transmitter and receiver. At the receiver, FFT reduces the channel response into a multiplicative constant on a tone-by-tone basis.  With MIMO, the channel response becomes a matrix. Since each tone can be equalized independently, the complexity of space-time equalizers is avoided. Multipath remains an advantage for a MIMO-OFDM system since frequency selectivity caused by multipath improves the rank distribution of the channel matrices across frequency tones, thereby increasing capacity.
   As an engineer who has been working on WiBro solutions for past a couple of years, I shall talk how the described PHY features are implemented in WiBro.