Browse Prior Art Database

SWITCHED PILOT AVERAGING FOR A SUBCHANNEL DEMODULATOR

IP.com Disclosure Number: IPCOM000008036D
Original Publication Date: 1997-Mar-01
Included in the Prior Art Database: 2002-May-14
Document File: 5 page(s) / 237K

Publishing Venue

Motorola

Related People

Troy Beukema: AUTHOR

Abstract

An algorithm is described which improves mits reference symbols (called "pilots") at known the sensitivity of a pilot-based multiple sub-channel times in a fixed frame format. The receiver demod- modulation system while simultaneously main- ulates these symbols and uses them for gain/phase taining full carrier tracking bandwidth of the references in the process of decoding the transmit- demodulator. ted data symbols. Given a transmitted pilot P, a transmitted symbol S, a received (interpolated) BODY pilot Prx, and a received data symbol Srx the decoder computes normalized data symbol S' A pilot-based linear modulation system trans- according to: S' = PIprx * Srx = estimate of transmitted data symbol S (1) A problem with a system like this is direct cou- The problem described above is shown pictori- pling of noise on the pilot symbols to the normal- ally below in Figure 1. The problem with the coher- ized data symbols. This reduces the optimality of ent demodulator is that noise vector Np is coupled the decode process and can reduce the sensitivity of onto transmitted symbol S and increases the per- the system by 2dB or more. ceived noise on the received data symbol Srx beyond the channel noise Ns.

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Page 1 of 5

@ MOTOROLA Technical Developments

SWITCHED PILOT AVERAGING FOR

A SUBCHANNEL DEMODULATOR

by Troy Beukema

ABSTRACT

  An algorithm is described which improves mits reference symbols (called "pilots") at known the sensitivity of a pilot-based multiple sub-channel times in a fixed frame format. The receiver demod- modulation system while simultaneously main- ulates these symbols and uses them for gain/phase taining full carrier tracking bandwidth of the references in the process of decoding the transmit- demodulator. ted data symbols. Given a transmitted pilot P, a transmitted symbol S, a received (interpolated) BODY pilot Prx, and a received data symbol Srx the decoder computes normalized data symbol S' A pilot-based linear modulation system trans- according to:

S' = PIprx * Srx = estimate of transmitted data symbol S (1)

  A problem with a system like this is direct cou- The problem described above is shown pictori- pling of noise on the pilot symbols to the normal- ally below in Figure 1. The problem with the coher- ized data symbols. This reduces the optimality of ent demodulator is that noise vector Np is coupled the decode process and can reduce the sensitivity of onto transmitted symbol S and increases the per- the system by 2dB or more. ceived noise on the received data symbol Srx beyond the channel noise Ns.

P=TRANSYITTED PILOT, Prx=RECEIVED INTERPOLATED PILOT Np=PILOT NOISE

S=TRANSYITTED DATA SYMBOL, Srx=RECEIVED SIGNAL Ns=SYMBOL NOISE

Fig. 1 Vector Noise Model

0 Mommla, Inc. 1997 107 March 1997

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0 M

MOTOROLA Technical Developments

The mathematical relationship describing the sensitivity degradation of a pilot-based modulation system is given by the following approximation:

Nf(/$ k, '2. Np. Ns) z((( $+I) .f, (i!ki))+l)( ~~:N~).k<O.l (*)

where:

Nf = decoder noise figure R = power(Np) / power(Ns) = pilot noise power to symbol noise power ratio k = power(Np) /power(P) = pilot N/S ratio

a = power (P) / power(S) = Pilot power to average Symbol power ratio

Np = Pilots per unit time Ns = Symbols per unit time

  The function described in formula (2) is plotted in Figure 2 using parameters for the iDEN system. The current iDEN demodulator runs at a pilot noise to symbol noise ratio of about -0.5dB. Thus, decreasing the noise on the pilots by 4dB can give about 1dB of sensitivity improvement as shown in the figure.

  There are two simple ways to reduce noise on the pilots. The first way is to increase time domain averaging of time local pilots. This amounts to plac- ing a lowpass filter on the pilot sequence and limit- ing the interpolation bandwidth of the interpolation

process, Typically, a 100 Hz bandwidth interpola- tion filter will give 4 dB S/N improvement on the pilots and achieve the 1dB sensitivity gain. This approach has the disadvantage of constraining the interpolation bandwidth below rates which can often be experienced in a mobile fading environ- ment. Addition...