Công nghệ CDMA cho hệ thống điện thoại tổ ong

Chapter 7. CDMA Technologies for Cellular Phone System Công nghệ CDMA cho hệ thống điện thoại tổ ong. Sources:1) July 7th, 2004 - Takashi INOUE –KDDI R&D Laboratories Inc. 2) IS-95 - ECE 371VV - Erik Stauffer Yu Li Yan Sun Contents Introduction (giới thiệu) Spread Spectrum Technology (công nghệ trải phổ) DS-CDMA Spreading Codes (mã trải phổ) Features of CDMA Đặc tính của cdma RAKE Receiver (bộ thu rake) Power Control (điều khiển công suất) Frequency Allocation (cấp phép tần số) Soft Handoff (chuyển giao mềm) Contents CDMA Links Forward Link (cdma đường xuống) Reverse Link (cdma đường lên) Special Features of IS-95 CDMA (đặc tính đặc biệt của cdma is-95) Brief Comparison to other second-generation standards Possible Improvements Conclusion 7.1. Introduction (giới thiệu) Modern Wireless Systems(hệ thống ko dây hiện đại) Outdoor voice/data communication (thoại ngoài trời, giao tiếp dữ liệu) Cellular phone, PCS (điện thoại tổ ong, PCS) Paging, Satellite (tìm gọi, vệ tinh) Indoor data communication (giao tiếp dữ liệu trong nhà) Cordless phone WLAN Low cost low power: Bluetooth, Zigbee, UWB Outdoor data broadcast (quảng bá dữ liệu ngoài trời) Wide area wireless data service (dịch vụ dữ liệu ko day băng rộng) Broadband wireless access: WLL, LMPD (truy nhập không dây băng hẹp) 7.1. IntroductionEvolution of Cellular Systems (phát triển của hệ thống điện thoại tổ ong) IMT-2000 CDMA2000 W-CDMA Cellular System Two-way voice/data communication with large coverage (hai đường thoại/dữ liệu với vùng phủ sóng rộng) Spectrum efficiency: frequency reuse (since power falls off with distance) (hiệu quả phổ, tần số tái sử dụng) (công suất tỉ lệ nghịch với khoảng cách) Macrocells and microcells: trade-off among capacity, interference, cost (tính thương mại giữa dung lượng, nhiễu, giá cả) System is interference limited (hệ thống giới hạn nhiễu) advaced techniques for interference reduction can enhance performance: cell sectoring, directional/smart antennas, multi-user detection, dynamic resource allocation (công nghệ tiên tiến giảm nhiễu có thể thực hiện bằng cách sector hóa, hướng/anten thông minh, hướng đa thuê bao, cấp phép tài nguyên động) 1G Cellular network (mạng tổ ong thế hệ thứ nhất) AMPS: FDMA/FDD 2G Cellular network (mạng tổ ong thế hệ thứ 2) TDMA/FDD, or CDMA/FDD GSM, 8 users/200KHz channel, TDMA IS-136, 3 users/30kHz channel, TDMA IS-95 (CDMA One): 64 users/1.25MHz channel, CDMA 2.5G Cellular Network (thế hệ 2,5G) General objective: support packet data service with higher data rate (tính năng chung: hỗ trỡ dịch vụ dữ liệu gói với tốc độ cao hơn) GPRS: 170 kbps (max), TDMA (enhanced GSM by channel aggregating) EDGE: >384 kbps, TDMA (enhanced GSM by high-level modulation)(nâng cấp GSM bởi điều chế mức cao hơn) IS-95B: 115 kbps, CDMA (enhance IS-95 by aggregating spreading codes) (nâng cấp IS-95 bởi mã trải phổ) 3G Cellular Network (thế hệ thứ 3) Objective: support higher data rate: from 384kbps (pedestrian) or 144 kbps (vehicular) to 2 Mbps (indoor)( hỗ trợ tốc độ dữ liệu cao) General: work in 2GHz band, ~2Mbps, FDD/TDD WCDMA (UTRA), evolved from GSM (phát triển từ GSM) Always on (packet radio), 2Mbps, ~8Mbps future, 5MHz bandwidth(băng thông 5M) CDMA 2000, evolved from IS-95 CDMA Use the same 1.25MHz bandwidth as IS-95(sử dụng cùng băng thông 1.25 như IS-95) Multi-carrier mode 3xTT (đa sóng mang) 5 MHz bandwidth (băng thông 5M) 2 Mbps and higher (tốc độ 2M và cao hơn) 3G incompatible with 2G, need much investment for deployment, such as the cost for buying spectrum( thế hệ thứ 3 tương thích với thế hệ thứ 2 phải đầu tư phát triển giống như chi phí mua phổ tần số) 3G first deployed in Japan (thế hệ 3G đầu tiên được triển khai ở Nhật) Introduction Spread Spectrum Communications(giao tiếp trải phổ) Multiple Access Technique(công nghệ đa truy nhập) Direct Sequence DS-CDMA (trực tiếp tuần tự) CDMA standards(chẩn cdma) CDMAOne (2G) – IS 95A, (2.5G) IS-95B CDMA 2000 and WCDMA (3G) – IMT2000 4G in the future (tương lai thế hệ thứ 4 Conclusions (kết luận) Requirements for 3G mobile systems High Capacity (dung lượng cao) Tolerance for interference (loại bỏ nhiễu) Privacy( bảo mật) Tolerance for fading(giảm pha đing) Ability to various data rate transmission(có khả năng truyền với tốc độ dữ liệu thay đổi) Flexible QoS (đảm bảo hệ số dịch vụ) IMT-2000 systems approved by ITU-R Popular name Access method Body of Technical Spec production IMT-DS IMT-MC IMT-TC IMT-SC IMT-FT (Direct Sequence) (Multi Carrier) (Time Code) (Single Carrier) (Frequency Time) W-CDMA CDMA2000 UTRA-TDD UWC-136 DECT CDMA-FDD CDMA-TDD CDMA-FDD TDMA-TDD TDMA-FDD 3GPP(FDD） 3GPP2 3GPP(TDD) CWTS IS-136 DECT ESTI ESTI TIA TTA T1 CWTS ARIB/TTC ARIB/TTC CWTS TTA ESTI TTA T1 CWTS TIA Approved in 2000 as ITU-R M.1457 TD-CDMA TD-SCDMA Organization Partners What is CDMA ? Summary of Multiple Access 7.2. Features of CDMA Systems đặc tính của hệ thống cdma IS-95 CDMA - Radio Aspects Spread spectrum techniques adapted from military (used since 1950) —Narrowband signal is multiplied by very large bandwidth signal (spreading signal)(tín hiệu băng hẹp được ghép bởi tín hiệu băng rộng) —All users, each with own pseudorandom codeword approximately orthogonal to all other codewords, can transmit simultaneously with same carrier frequency(tất cả thuê bao, mỗi người có một mã giả ngẫu nhiên và trực giao với nhau có thể truyền đồng thời trên cùng một tần số sóng mang) —Receiver performs a time correlation operation to detect only desired codeword (bộ nhận thực hiện một lần tương quan để chỉ xác định mã mong muốn) —All other codewords appear as noise due to decorrelation(tất cả các mã khác xuất hện như là nhiễu do bộ giải mã tương quan) —Receiver needs to know only codeword used by transmitter (bộ nhận chỉ cần biết mã hóa sử dụng ở bộ phát) —In other words, users are separated by their codes rather than frequency and time slot (Nói một cách khác, mỗi người dùng được phân biệt bởi mã hơn là tần só và thời gian) IS-95 CDMA Interesting Features IS-95 CDMA Interesting Features IS-95 CDMA Interesting Features Drawbacks of CDMA Drawbacks of CDMA So far, CDMA looks like a step backwards: 􀂾 Tight synchronization is required to use orthogonal codes, which then break in a multipath channel anyway 􀂾 Quasi-orthogonal codes cause self-interference, which dominates the performance in most CDMA systems 􀂾 Near-far problem is a serious hindrance, requiring fast and accurate power control (that uses up bits we could otherwise send information with) 􀂾 And for all this, the required bandwidth is now J times larger than it was before, so there doesn’t appear to be a capacity gain PHYSICAL PROPERTIES OF WIRELESS CHANNELS thuộc tính vật lý của kênh vô tuyến COS598u: Pervasive Information Systems > data rate of message. All users use the same carrier frequency and may transmit simultaneously. The k-th transmitted signal is given by: CDMA Receiver (.)dt   Principles of operation-receiver At the receiver, the received signal is correlated with the appropriate signature sequence to produce desired variable. Message Signal m(t) is a time sequence of non-overlapping pulses of duration T, each of which has an amplitude (+/-) 1. The PN waveform consists of N pulses or chips for message symbol period T. NTC = T where TC is the chip period. Example:  Assume N=4 PN Wave for N =4 1 -1 -1 1 Correlator output for first user The multiplied signal will be p2(t) = 1 for the correct signal and will yield the dispersed signal and can be demodulated to yield the message signal mi(t). Probability of bit error xác suất của bít lỗi Probability of bit error Pe = Q {1/ [(K –1)/3N + (N0/2Eb)]1/2} K = Number of users số người sử dụng N = Number of chips/ symbol số chip/ký tự When Eb/No   Pe = Q{[3N/(K-1)]1/2 } Concept of CDMA Systems Diversity IS-95 Air Interface Standards Space diversity Frequency diversity Path diversity Time diversity Macro-diversity Near-Far Problem CODE B CDMA Transmitter DATA B CODE A CDMA Receiver CODE A CDMA Transmitter DATA A P Desired Signal Power = P/Lp-a Interfered Signal Power = P/Lp-b/(processing gain ) Demodulated DATA P Lp-a Lp-b When user B is close to the receiver and user A is far from the receiver, Lp-a could be much bigger than Lp-b. In this case, desired signal power is smaller than the interfered power. The Near-Far Problem Users may be received with very different powers: - Users near the base station are received with high power - Users far from the base station are received with low power - For a path loss exponent of 4 and a cell size of 1 km, example: Nearby users will completely swamp far away users Solution: Power Control Power Control Power Control … Power control adjust power level while conveying data to the base station in that way that all received signals at the base station have the same signal strength (not trivial) This process needs time to adjust in case of changing in the cell in terms of power/interference Power Control … In case of misadjusted power control entities some WTS might have better/worse quality conditions The highest cell capacity can be achieved all WTs have exactly the same power level For support of heterogeneous QoS support the power control is a viable option All signals (even the undesired ones) from the base station will receive the WT with the same strength (some path) Power Control … IS-95 Air Interface Standards Power Control... Time Detected Power from A from B When all mobile stations transmit the signals at the same power (MS), the received levels at the base station are different from each other, which depend on the distances between BS and MSs. Moreover, the received level fluctuates quickly due to fading. In order to maintain the received level at BS, power control technique must be employed in CDMA systems. Power Control (continued) ② ① Open Loop Power Control Closed Loop Power Control transmit transmit about 1000 times per second ① ② Effect of Power Control Ａ Ｂ Time Detected Power from MS B from MS A closed loop power control for MS B. for MS A. Effect of Power Control Power control is capable of compensating the fading fluctuation. Received power from all MS are controlled to be equal. ... Near-Far problem is mitigated by the power control. Frequency Allocation (1/2) In FDMA or TDMA, radio resource is allocated not to interfere among neighbor cells. Neighbor cells cannot use the same (identical) frequency band (or time slot). The left figure shows the simple cell allocation with seven bands of frequency. In actual situation, because of complicated radio propagation and irregular cell allocation, it is not easy to allocate frequency (or time slot) appropriately. Frequency Allocation (2/2) In CDMA, identical radio resource can be used among all cells, because CDMA channels use same frequency simultaneously. Frequency allocation in CDMA is not necessary. In this sense, CDMA cellular system is easy to be designed. Handoff in CDMA Two types of handoffs —hard handoff —Soft handoff Hard handoff is needed when the call is moved from one frequency to another and when the mobile moves the coverage area of another MSC Soft handoff Two base stations receive signals from the mobile. The signals are sent to the MSC that decides which one has lowest bit error rate. Vocoder in CDMA is in the switch. Mobile receives signals from two base stations and combine them before decoding. Uses rake receiver. Each tunes to one base station. This requires synchronization of the base stations. All base stations are equipped with GPS and receive information from it, including lat.., long and time. It also requires that the mobile dedicates one correlator for searching other pilot channels. Soft Handoff (1/2) Handoff : Cellular system tracks mobile stations in order to maintain their communication links. When mobile station goes to neighbor cell, communication link switches from current cell to the neighbor cell. Hard Handoff : In FDMA or TDMA cellular system, new communication establishes after breaking current communication at the moment doing handoff. Communication between MS and BS breaks at the moment switching frequency or time slot. Soft Handoff (2/2) Σ Cell B Cell A Soft handoff : break (old cell A) after connect (new cell B) transmitting same signal from both BS A and BS B simultaneously to the MS Soft Handoff : In CDMA cellular system, communication does not break even at the moment doing handoff, because switching frequency or time slot is not required. Soft Handoff CDMA and Soft Handover A unique advantage of CDMA is soft handover  more reliable All cells use same carrier Mobile contact with 6~7 nearby cells simultaneously No need to stop contact with cell 1 before switching to cell 2 Soft-Handover IS-95 Air Interface Standards Mobility Management in CDMA Five type of registration —Periodic —Power up —Power down —Zone change —Distance. When the distance between the current base station and the previously registered base station exceeds a certain limit. Capacity of CDMA Systems Voice Activity • In TDMA and FDMA systems: - If a user doesn’t have anything to send, the time/frequency slot allocated to them is wasted - It is typically very difficult to dynamically allocate time and frequency slots In CDMA systems: - If a user doesn’t have anything to send, it causes less interference to other users of the system - Typically, each user needs to transmit less than half the time - Since interference-limited, this doubles the capacity Sectorized Antennas Cells can use directional antennas to “sectorize” the cell At right, 120 degree antennas create 3-sector cells – very common For CDMA, this reduces the interference by a factor of three - Capacity is increased by a factor of three! FDMA/TDMA also use sectored antennas, but just to decrease reuse distance Uplink Single-cell System Model ... . . . ... . . . User 1 User 2 User k User Ku User n Assumptions Total active users Ku The intra-cell MAI can be modeled as AWGN Perfect power control is assumed Random sequences Coarse estimate of the reverse link (uplink) capacity Assumptions: Single Cell. The interference caused by other users in the cell can be modeled as AWGN. Perfect power control is used, i.e. the received power of each user at the base station is the same. If the received power of each user is Ps watts, and the background noise can be ignored (ex: microcells), then the total interference power (MAI) at the output of the desired user’s detector is where Ku is the total number of equal energy users in the cell. Suppose each user can operate against Gaussian noise at a bit-energy-to-noise density level of Eb/Io. Let W be the entire spread bandwidth, then the interference spectral density can be expressed as: Interference limited Also, the bit energy Eb is Thus, Now, if we consider the factors of voice activity (Gv), sectorized antenna gain (GA), and other-cell interface factor (f), where Gv  1/v = 2.67 GA (three sectors)  2.4 f = (Interference form other cells)/(Interference from given cell)  0.6 In this case, Ku can be approximated by Ex: If Gv  2.67, GA  2.4, f  0.6 If (Eb/Io) required is 6 dB (i.e. Eb/Io = 4) which will be larger than the TDMA or FDMA systems in the cellular environment. Capacity Comparison Comparing the capacity of TDMA/FDMA/CDMA is very controversial In 1991, a famous (notorious?) Qualcomm paper claimed that due to voice activity, frequency reuse, and sectorization, CDMA increased capacity by: - Factor of 18 relative to AMPS - Factor of 6 relative to US TDMA (and similar for GSM) This turned out to be optimistic, about 1/3 of this gain actually happened (still depends who you ask) Still, twice as many users is nothing to snear at! All 3G systems use CDMA for multiple access 7.3. Direct Spread Spectrum Technology Spread Spectrum Technique Low power spectral density Rejection to jamming signal and interference Pseudorandom sequence Randomness and noise properties Walsh, M-sequence, Gold, Kasami, Z4 Provide signal privacy Direct Sequence Spread Spectrum Spreading Source signal is multiplied by a PN signal Processing Gain: Despreading Spread signal is multiplied by the spreading code Spreading & Despreading Polar {±1} signal representation System Block Diagram How to spread spectrum... Demodulating DS Signals (1/2) Demodulating DS Signals (2/2) Feature of SS Feature of SS Direct Sequence Spread Spectrum:Transmission Technique Cross-Correlation Cross-Correlation between Code A and Code B = 6/16 Self-Correlation for each code is 1. Preferable Codes In order to minimize mutual interference in DS-CDMA , the spreading codes with less cross-correlation should be chosen. Synchronous DS-CDMA : Orthogonal Codes are appropriate. (Walsh code etc.) Asynchronous DS-CDMA : Pseudo-random Noise (PN) codes / Maximum sequence Gold codes Multiplexing using Walsh Code Synchronous DS-CDMA Synchronous CDMA Systems realized in Point to Multi-point System. e.g., Forward Link (Base Station to Mobile Station) in Mobile Phone. Asynchronous DS-CDMA In asynchronous CDMA system, orthogonal codes have bad cross-correlation. CDMA Code Division Multiple Access Start with data signal rate D (Called bit data rate) Break each bit into k chips by multiplying by a k bit user code (known as a Walsh code) Channel has chip data rate kD chips per second User code (Walsh code) is orthogonal to all other possible user codes User code 1 * User code 2 = 0 User code 1 * User code 1 = signal for user 1 Signals for several users can be added and sent as a single signal within the same band (multiplexed) CDMA user code and data Stallings 2003: Figure 9.10 CDMA Explanation Consider a user communicating with a base station Base station knows user A’s code Assume communication already synchronized Base station receives a message from A and wants to decode it. To extract the signal from A the basestation multiplies the signal by A’s code Decoder ignores other sources by using A’s code to decode For all other stations code station I * code station A = 0 so only the signal for station A remains CDMA for DSSS When the basestation sends messages to n users each message multiplied by a different orthogonal Walsh code sequence, those signals are added before transmission. At each receiving station, the signal for that station is extracted by multiplying by that stations Walsh code. CDMA: two-senders, eight bit Walsh codes Walsh Code 2 Walsh Code 1 Data Station 1 Data Station 2 Data multiplied by Walsh Code Data multiplied by Walsh Code (Sum of all stations) Transmitted data CDMA: eight bit Walsh codes Walsh Code 1 Received Data multiplied by Walsh Code Decoded Received Data Station 1 Receive Data (Sum) Summary of Channel Partitioning CDMA (Code Division Multiple Access) Used mostly in wireless broadcast channels such as cellular phones All users share same frequency band. Information from each user is spread throughout that frequency band Each user has their own orthogonal Walsh code ‘chipping’ sequence to encode data. encoded signal = (original data) X (Walsh code) Encoded signals from each channel are added, the summed signal is transmitted The orthogonal property of Walsh codes guarantees that (ignoring transmission errors) multiplying the received signal by a Walsh code will extract the data for the channel encoded using that Walsh code from the received (summed) signal. Decoded signal = (received summed signal X Walsh code) CDMA in a DSSS Environment Stallings 2003: Figure 9.11 7.4. DS-CDMA Systems DS-CDMA System Overview (Forward link) DS-CDMA System Overview (Reverse Link) 7.5. IS-95 CDMA SYSTEMS IS-95 CDMA SYSTEMS  Interim Standard 95 – CDMA  Viterbi, Qualcomm Outline  Forward Link  Reverse Link  Special Features of IS-95 CDMA  Brief Comparison to other second-generation standards · Possible Improvements 2G: IS-95A (1995) Known as CDMAOne Chip rate at 1.25Mbps Convolutional codes, Viterbi Decoding Downlink (Base station to mobile): Walsh code 64