Kitplus Magazine

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TECHNOLOGIES “Using different modes of modulation simultaneously allows for larger symbols.” Fig.3, A correctly confi gured guard interval in comparison to too short of an interval. This is where COFDM uses a very clever trick indeed. It uses a whole load of carriers within it’s permitted bandwidth (usually thousands) - and spaces them 90 degrees apart (see fi g.2 for a reminder of what this would look like) to ensure they don’t interfere with one another (hence ‘orthogonal’). The data is then divided between all of the carriers, dramatically reducing the data rate per carrier - and thusly slashes the baud rate (i.e. the amount of symbols that each carrier needs to send in a second). Now the ‘symbol period’ can be a lot longer - and thus ‘delayed’ information no longer poses as much as a threat as more time can be permitted for each symbol to be completely received - both directly and by refl ection - before silence is needed to accommodate for the next symbol. Ensuring that there is enough ‘silence’ between each symbol can be adjusted by adapting the ‘Guard Interval’. Put simply, adapting the guard interval is a way of avoiding inter symbol interference (often referred to as ISI) (see Fig. 3.) As aforementioned, multi path propagation means that some of the signal will take a longer route via refl ecting off of various surfaces (such as an offi ce building) to the receiver- whereas other parts of the exact same signal (a enquire symbol)- will take a direct route and thus arrive faster. In fi gure three (top half), we can see how the guard interval permits for this delay. The guard interval is the ‘silent’ time between the two symbols that allow all of the delayed signals to be received, or dissipate to a point where they will no longer cause interference before allowing the next packets to be transmitted. In fi gure three (bottom half), however, we note that the guard interval is no longer long enough. Whilst having a shorter guard interval does mean that more ‘symbols’, can pass through in less time, and thus stands to increase the total data rate - in this example we can now see that 72 | KITPLUS - THE TV-BAY MAGAZINE: ISSUE 96 DECEMBER 2014 the symbols that have been refl ected and thus delayed are now clashing with the next symbol that has been transmitted and has taken the shortest path. This will cause inter-symbol interference, which can effect the error correction capabilities of the system, potentially introducing artefacts. Adversely, however, care should be taken to not allow too long of a guard interval as this means that the ‘silence’ periods will become dominate in the overall transmission, effectively crippling the overall data rate, and thusly potentially hampering the level of quality the link can provide. This again brings us back to what has become a recurring theme throughout - quality verses stability. Of course, allowing a very long guard interval would effectively remove all risks of suffering multi path interference - in the same way that using PSK in comparison to QAM would mean that the chances of a mis-read, and thus the induction of interference, is considerably lower (although, not even PSK, FSK and ASK should be considered fool-proof as they all have a ‘turning point’ value where the ‘0’ becomes the ‘1’ and vice versa - such as a drastically degraded signal (i.e. one that has travelled to far and lost a lot of it’s amplitude) in an ASK system may cause an accidental switching - ergo again inducing errors into the system) - but this would all be at the cost of the total data rate of the link, which translates into the overall quality of the images that may be passing through it - which, ultimately, is where talented engineers come in. Although the technical systems that have been developed over the years have made achieving a wireless link thanks to these new technologies such as those discussed, for years to come yet - it will remain the jobs of the talented individual to pick the right ariel for the job, be it omni-direction, or super-focused - as well as to assess the link both subjectively, and objectively (by utilising such tools as an eye display, ensuring that effects such as jitter have not been permitted to reach a perceivable level) - and make that fi nal decision on where the fair balance between link security and quality lies.