tailieunhanh - Recent Advances in Wireless Communications and Networks Part 3

Tham khảo tài liệu 'recent advances in wireless communications and networks part 3', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 50 Recent Advances in Wireless Communications and Networks The average ICI power across different sequences can be calculated as N-1 2 r 1 V I - k - m _ . I ơĩa E a cim X ÁX 25 k m As seen in 25 the average ICI power depends on the number of the subcarriers and P f at frequencies A k m k 0 1 . N 1 The system ICI power level can be evaluated by using the CIR Carrier-to-Interference power Ratio . While deriving the theoretical CIR expression the additive noise is omitted. By using 23 and 25 the CIR can be derived as Tan Beaulieu 2004 Mourad 2006 Kumbasar Kucur 2007 - 26 k m 5 Therefore the CIR of the OFDM systems only depends approximately on the normalized frequency offset. A commonly used pulse shaping function is the raised cosine function that is defined by 1 . 1 . nt Ế iC0Ý B ổ í 1 27 for 0 t aTs for aTs t Ts 11 2 cos for Ts t 1 Up- P 2 2 where a denotes the rolloff factor and the symbol interval Ts is shorter than the total symbol duration 1 a Ts because adjacent symbols are allowed to partially overlap in the rolloff region. Simulation shows that the benefit of the raised cosine function with respect to the ICI reduction is fairly low. A number of pulse shaping functions such as Rectangular pulse REC Raised Cosine pulse RC Better Than Raised Cosine pulse BTRC Sinc Power pulse SP and Improved Sinc Power pulse ISP have been introduced for ICI power reduction. Their Fourier transforms are given respectively as Kumbasar Kucur 2007 PrecÍD sinctfTf 28 rinrifT Ì cos t afTs P - Sinc jTs i 2afTsy 29 n ỉ-c .rr rr 1 -V PbtrcU - smc fls i WTsy 30 Psptn sincn fTf 31 Plsp f exp a fTs 2 sincn fTs 32 where a 0 a 1 is the rolloff factor p na in 2 a is a design parameter to adjust the amplitude and n is the degree of the sinc function. ICI Reduction Methods in OFDM Systems 51 Fig. 5. Comparison of REC RC BTRC SP and ISP spectrums Fig. 6. CIR performance for different pulse shapes The purpose of pulse shaping is to increase the width of the main .