A MULTICARRIER CDMA ARCHITECTURE BASED ON ORTHOGONAL COMPLEMENTARY CODES FOR NEW GENERATION OF WIDEBAND WIRELESS COMMUNICATIONS BY: COLLINS ACHEAMPONG GRADUATE STUDENT TO: Dr. Lijun Quin DEPT OF ELECTRICAL ENGINEERING PRAIRIE VIEW A&M UNIVERSITY PRAIRIE VIEW, TEXAS A RESEARH PROJECT WIRELESS NETWORKS ELEG:6203 FALL 2003 SEMESTER
OVERVIEW ABSTRACT INTRODUCTION PROBLEM STATEMENT METHOD OF SOLVING PROBLEM BASIC CODE DIVISION MULTIPLE ACCESS (CDMA) SYSTEM SPREAD SPECTRUM FUNDAMENTALS COMPLETE COMPLEMENTARY CODES MULTIPATH SELF-INTERFERENCE ANALYSIS OF MODEL PARAMETERS AND RESULTS LIMITATIONS, ADVANTAGES, AND REMARKS CONCLUSION
ABSTRACT The proposed Multicarrier Code Division multiple access (MC-CDMA) has a great potential for applications in future wideband mobile communications beyond Third Generation (3G), which is expected to offer a very high data rate in hostile environment. The paper is a continuation of on going research effort to construct a Multicarrier CDMA architecture based on orthogonal complementary codes, characterized by its unique spreading modulation scheme, uplink and downlink signature design, and receiver implementation for multipath signal detection. The ongoing research was first conducted by: Hsiao-Hwa, National Sun Yat-Sun University Jun-Feng, National Chung Hsing University Naoki Suehiro, Yeh, National University of Tsukuba
INTRODUCTION Code Digital Multiple Access (CDMA) is a predominant multiple access technique proposed for the 3G wireless communication systems worldwide. The maturing of 3G mobile communication technologies from concepts to commercially deliverable systems motivates us to think about the possible architectures for future generations of mobile wireless. The question is how to guarantee such high data rate in highly unpredictable and hostile channels? What types of air link architecture are qualified to deliver such high-data-rate services? The new MC-CDMA architecture that has a great potential for future mobile communications tackles this issues comprehensively. The new CDMA architecture ought to be technically feasible with current available digital technology. The proposed CDMA system should preferably have an inherent ability to mitigate path problems in mobile channels.
It can achieve spreading efficiency (SE) very close to one It offers Multiple Access Interference (MAI) free operation in both up and down link transmission in a MAI-AWGN (Additive White Gaussian Noise). Able to offer high bandwidth efficiency due to its use of unique spreading modulation scheme. It is particularly suited to multi-rate signal transmission due to the use of an offset stacked modulation scheme.
Mothod of Solving Problem Use of direct sequence orthogonal complete complementary codes. The bit error rate (BER) of the proposed CDMA system under MAI and Additive White Gaussian Noise (MAI-AWGN) is evaluated using computer simulations. The obtained BER performance of the new CDMA system will be compared to the conventional CDMA system using Gold codes and m-sequences under identical operation environments
BASIC CODE DIVISION MULTIPIPLE ACCESS (CDMA) SYSTEM CDMA IS A SPREAD SPECTRUM COMMUNICATIONS TECHNIQUE THAT SUPPORTS SIMULTANEOUS DIGITAL TRANSMISSION CDMA IS SIMILAR TO THAT OF FDMA (FREQUENCY DIV. MULTIPLE ACCESS) AND TDMA (TIME DIV. MULTIPLE ACCESS). CDMA HAS THE UNIQUE PROPERTY OF SUPPORTING A MULTIPLICITY OF USERS IN THE SAME RADIO CHANNEL CDMA HAS GRACEFUL DEGRADATION IN PERFORMANCE DUE TO MULTI-USER INTERFACE FREQUENCY RE-USE FACTOR IN CDMA CELLULAR ENVIRONMENT CAN BE AS HIGH AS UNITY. CDMA BEING A WIDEBAND SYSTEM CAN CO-EXIST WITH OTHER NARROWBAND MICROWAVE SYSTEMS THE MOST ADVANTAGE OF CDMA IS ITS ABILITY TO COMBAT FROM MULIPATH FADING.
CDMA system allows unique pseudo-noise codes to individual users Any user is allowed to access the air at ant time Spreading efficiency (SE) for all conventional CDMA equals 1/N which is less than one. Low rate data symbols are multiplexed by CDMA code resulting in a wideband waveform a process known as spreading Multi-code methods assigns one or more codes with fixed spreading factor to any one user. This results in a waveform with high peak power. A users requiring high throughput is allocated a CDMA code with a low spreading factor CDMA codes may be time slotted to regulate access.
Figure 1. CDMA in the code and time domain CDMA Code space User 0 S0 S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11S12 S13 S14 S15 S16 User 1 User 2 User 3 Time Figure 1: CDMA in the code and time domain
Spread Spectrum Fundamentals Originally develop for military applications to provide anti-jam. Qualcom Inc., developed it in the late 1980 s and early 1990 s which led to IS-95 cellular standard. Spread spectrum characteristic: Bandwidth used is much greater than the message bandwidth. Bandwidth spreading is achieve by using a spreading code or (pseudonoise) p-n sequence, while the spreading code is independent to the message signal. Two types of spread spectrum are: Direct sequence (DS) and Frequency hopped (FH) spread spectrum. DS uses p-n sequence to introduce rapid phase transition into the carrier containing the data. FH uses p-n sequence to pseudo randomly hop the carrier frequency throughput a large band.
The original information signal which occupies a bandwidth of B Hz is transmitted after spectral spreading to the bandwidth N times higher. N is known as the processing gain typically in the range of 10-30 db. Power of transmitted spread spectrum is spread over N times the original bandwidth while its density is correspondingly reduced by the same amount. Hence, the processing gain is given by N = Bs/B, where Bs is the bandwidth of spectral signal while B is the original information signal. The unique technique of spreading is the key to improving its detection in a mobile radio environment. This allows narrowband signals exhibiting higher spectral density to share the same frequency band. Hence, the spectral efficiency is defined as: η = Rd Brf bits / s / Hs where Rd is the transmitted data rate, and Brf the system bandwidth.
COMPLETE COMPLEMENTARY CODES Use of direct sequence orthogonal complete complementary codes The orthogonal complete complementary code is based on a flock of element codes jointly instead of a single code as in traditional CDMA. Each user in the MC-CDMA system will be assigned a flock of element codes as its signature code. Each bit is spread by one single continuous code comprising of N contiguous chips to attain certain processing gain or spreading factor. User that require higher throughput is allocated more codes with lower spreading factor Codes ought to be transmitted possibly via different frequency channels, and arrive at a receiver at the same time. The information bits correlator are spreading modulated by element codes that are offset stacked, each shifted by one chip relative to one another. Every signature code is spread into several segments (or element code)
New bits will start right after one chip is delayed relative to the previous bits. Bit is spread by element code length L. The bit error rate (BER) under the MAI and AWGN is evaluated using computer simulations
COMPLETE COMPLEMENTARY CODES Element code length L, = 4 Element code length L = 16 Processing gain are equal to 4x2 = 8 and 16x4 = 64 respectively Flock 1 A0: + + + - Flock 1 A0: + + + + + - + - + + - - + - - + A1: + - + - + + + + + - - + + + - - A2: + + - - + - - + + + + + + - + - A3: + - - + + + - - + - + - + + + + A1: + - + + Flock 2 B0: + + + + - + - + + + - - - + + - B1: + - + - - - - - + - - + - - + + B2: + -+ - - - + + - + + + + - + - + B3: + - - + - - + + + - + - - - - Flock 2 B0: + + - + Flock 3 C0: + + + + + - + - - - + + - + + - C1: + - + - + + + + - + + - - - + + C2: + + - - + - - + - - - - - + - + C3: + - - + + + - - - + - + - - - - B1: + - - - Flock 4 D0: + + + + - + - - + + + - - + D1: + - + - - - - - - + + - + + - - D2: + + - - - + + - - - - - + - + - D3: + - - + - - + + - + - + + + + Table 1. Two examples of complete complementary codes with element code lengths L = 4 and L = 16
Performance Analysis Under Multiple Access Interference Compared with traditional spreading modulation used in conventional CDMA systems, the new system has the following salient features: The new system is no longer aligned in time one bit after another. Instead a new bit will start right after one chip delay relative to the previous element L. The unique offset stacked spreading method can easily slow down data transmission by simply shifting more than one chip (at most L chips) between two neighboring offset stacked bits. Thus, the MC-CDMA architecture is capable of delivering much higher bandwidth efficiency than a conventional CDMA architecture under the same processing gain. The inherent ability of the MC-CDMA system to facilitate multirate transmissions is based on its innovative offset stacked spreading technique, which cannot be applied to traditional spreading codes. The MAI-independent property is significant in terms of its potential to enhance its system capacity in a multipath channel in MC-CDMA.
Multipath Self-Interference CDMA rake receiver achieves perfect diversity gain from multipath reception if it can perfectly decouple the different paths. In practice however, each path acts as interference for other paths. Interference becomes especially significant if the CDMA signal is transmitted over many code channels or the spreading factor is small. Conventional CDMA receiver usually uses a Rake receiver to collect dispersed energy among different reflection paths to achieve multipath diversity at the receiver. Therefore, the Rake receiver is a must for all conventional CDMA systems, including current operational 2G and 3G systems. In MC-CDMA architecture, the Rake receiver becomes inappropriate due to the nature of the unique spreading modulation technique employed in the system.
Power Control CDMA technology requires the use of power control for subscriber terminals. The effectiveness of power control determines the capacity of the network. In CDMA if one terminal transmit excessive power, it increases the interference on all other remaining terminals and reduces their link quality. In ODFM power control is not as critical as CDMA. There is no fundamental requirement for the use of power control since the user terminals in the same sector do not share the same frequency at the same time. Reason for using power control in an OFDM system is to minimize co-channel interference.
A Multicarrier CDMA Architecture Figure 10: MC-CDMA simulation used for forward traffic
ANALYSIS OF MODEL PARAMETERS AND RESULTS Figure 13: The simulation in operation This is the start of the simulation for MC-CDMA (with BER values shown) in Figure 13.
Comparison of MC-CDMA and CDMA Spread Model
CDMA and MC-CDMA Comparison Summary MC-CDMA CDMA Spectral efficiency More efficient Less efficient Multipath Handles larger number of paths Some diversity benefits; performance sensitivity to number of path coherently combined Multiple Modulation support Downlink and Uplink Downlink only Resistance to Narrowband Interference Suited for multirate signal transmission due to the use of an offset stacked spreading modulation scheme Spread spectrum provides that Network Planning It simplifies the rate-matching algorithm relevant to multimedia services and asymmetric traffic in up and downlink transmission for IPbased applications Difficult for cell overlays; PN offset planning cell-breathing complications Power Control Required function Required function Peak-to-Average Ratio Varies Varies; can be as high as 11dB Standards Adoption WLAN, 3G, 4G WLAN, 3G Cost Less expensive Varies, may depend modem.
The Bit Error Rate (BER) Test Results for MC-CDMA TX Error Rate Calculation 1 2 3 4 5 6 7 BER 0 0.00072 0.00833 0.00227 0.00077 0.00091 0.00119 Error Count 0 0 1 2 3 4 6 Sample Count 0 80 120 880 3880 4360 5040 Total Sample Time T=0.085 secs
The Bit Error Rate (BER) Test Results for CMDA Number of TX Error Rate Calculation 1 2 3 4 5 6 7 BER 0 0 0.0174 0.0290 0.03516 0.0195 0.01896 Error Count 0 0 30 85 127 131 137 Sample Count 0 1032 1720 2924 3612 6078 7224 Total Sample time T=0.092 secs
Limitations There exit some technical limitations for the proposed cc-codes based MC- CDMA which ought to be addressed. If a long cc-code is employed in the MC-CDMA system, the number of different levels generated from the baseband could be a problem. For example, cc-codes with L = 4 will generate five possible levels from the offset stacked spreading; 0, ±2,and ±4. For cc-code with L=16 becomes 0, ±2, ±4 ±16, comprising 17 different levels. In general, it will yield L+1 different levels. Another concern is with the cc-code based CDMA is that, a relatively small number can be supported by a family of cc-codes. For cc-codes family with L = 64, only eight flock of codes, each of which can be assigned to one channel. If more users should be supported, long cc-codes have to be used. The cost for introducing a multilevel digital modem may be expensive varies depending on a user s requirements.
Advantages and Remarks The MC-CDMA can achieve spreading efficiency very close to one. It offers multiple access interference (MAI free operation in both up and down link transmission in MAI-AWGN channel. It can significantly reduce reduce the co-channel interference responsible for capacity decline in CDMA systems. Able to offer high bandwidth efficiency due to its use of unique spreading modulation scheme. Particularly suited for multi-rate signal transmission due to the use of an offset stacked modulation scheme. One possible solution to the MC-CDMA problem is to introduce a of multilevel digital modem capable of transmitting L+1 different levels in a symbol duration. An L+1 quadrature modulation (QAM) digital modem can be a suitable choice for its robustness in detection efficiency. Another possible solution is to introduce frequency division on top of the code division in each frequency band to create more transmission channels.
Conclusion The paper addresses several advantages of the MC-CDMA over conventional CDMA such as higher bandwidth efficiency, MAI-free operation in both synchronous and asynchronous MAI_AWGN channels. Also reduces co-channel interference and capacity increase in mobile cellular system. The paper addresses technical limitations of the new MC-CDMA architecture, such as relatively small family codes and the need for complex multilevel digital modems. Both CDMA and the MC-CDMA both have their respective advantages. Nevertheless, the proposed MC-CDMA architecture based on complete complimentary codes offers a new option to implement future wideband mobile communications beyond 3G. Future research work on this paper will be needed to be continued as I could not find all the answers to limitation problems.
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