GPS World, December 2009

WIRELESS Receiver Design noise received PRN code noise IF Massively Parallel Correlator locally generated copy of PRN code RF Front End Å FIGURE 2 Massive parallel correllation concepts of fine time coarse time and massive parallel correlation Any assistance data helps reduce the frequency search The frequency search is just as you might scan the dial on a car radio looking for a radio station but the different GPS frequencies are affected by the satellite motion their Doppler effect If you know in advance whether the satellite is rising or setting then you can narrow the frequency search window The code delay is more subtle The entire C A code repeats every millisecond So narrowing the code delay search space requires knowledge of GPS time to better than one millisecond before you have acquired the signal We call this fine time Only two phone systems had this time accuracy CDMA and iDEN both synchronized to GPS time The largest networks GSM and now 3G are not synchronized to GPS time They are within 2 seconds of GPS time we call this coarsetime Initially only the two fine time systems adopted A GPS Then came massive parallel correlation technology number two and high sensitivity technology number three 2 3 MPC High Sensitivity A simplifi ed block diagram of a GPS receiver appears in FIGURE 2 Traditional GPS prior to 1999 had just two or three correlators per channel They would search the codedelay space until they found the signal and then track the signal by keeping one correlator slightly ahead early and one slightly behind late the correlation peak These are the so called early late correlators Massive parallel correlation is defined as enough correlators to search all C A code delays simultaneously on multiple channels In hardware this means tens of thousands of correlators The effect of massive parallel correlation is that all code delays are searched in parallel so the receiver can spend longer integrating the signal whether or not fine time is available So now we can be faster longer higher regardless of the phone system on which we implement A GPS Major milestones of massive parallel correlation MPC In 1999 MPC was done in software the most prominent example being by Snaptrack who did this with a fast Fourier transform FFT running on a digital signal processor DSP The first chip with MPC in hardware was the GL16000 produced by Global Locate then a small startup now owned by Broadcom In 2005 the first smartphone implementation of MPC the HP iPaq used the GL20000 GPS chip Today MPC is standard on GPS chips found in mobile phones 4 Coarse Time Navigation We have seen that A GPS assistance relieves the receiver from decoding orbit data making it faster and MPC means it can operate with coarse time longer higher But the time of week TOW still needed to be decoded for the position computation and navigation for unambiguous pseudoranges and to know the time of transmission Coarse time navigation is a technique for solving for TOW instead of decoding it A key part of the technique involves adding an extra state to the standard navigation equation and a corresponding extra column to the well known line of sight matrix The technical consequence of this technique is that you can get a position faster than it is possible to decode TOW for example in one two or three seconds or you can get a position when the signals are too weak to decode TOW And a practical consequence is longer battery life since you can get fast timeto first fix TTFF always without frequently waking and running the receiver to maintain it in a hot start state 5 Low Time of Week A parallel effort to coarse time navigation is low TOW decode that is lowering the threshold at which it is possible to decode the TOW data In 1999 it was widely accepted that 142 dBm was the lower limit of signal strength at which you could decode TOW This is because 142 dBm is where the energy in a single data bit is just observable if all you do is integrate for 20 ms However there have evolved better and better ways of decoding the TOW message so that now it can be done down to 152 dBm Today different manufacturers will quote you different levels for achievable TOW decode anywhere from 142 to 152 dBm depending on who you talk to But they will all tell you that they are at the theoretical minimum 6 7 Host Based GPS RF CMOS Host based GPS and RF CMOS are technologies six and seven if youre still counting with me We can understand the host based architecture best by starting with traditional system on chip SOC architecture An SOC GPS may come in a single package but inside that package you would fi nd three separate die three separate silicon chips packaged together A baseband die including the central processing unit CPU a separate radio frequency tuner and fl ash memory The only cost effective way of avoiding the fl ash memory is to have read only memory ROM which could be part of the baseband die but that means you cannot update the receiver software and keep up with the technological developments weve been talking about Hence state of the art GPS World December 2009 www gpsworld com 38