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Uplink interference in WiMAX

Posted by Farrukh Feroze Ali on September 27, 2009 at 1:52pmView Blog

One of the prime concerns for WiMAX network designers is to eliminate the unwanted signals fromnetwork. Unlike traditional Single carrier/TDMA based systems, WiMAX 16e requires greaterunderstanding of frequency allocations because of OFDMA based scheduling. Most licenses issued by theregulators around the World do not allow leverage of planning with frequency re-use patterns of 3x3x3 anddesigners have to persist with re-use patterns of 1x3x3.

One of the factors that influence the inter-cell interference averaging and amount of frequency diversity isthe Permutation scheme being used. PUSC (Partial Usage of sub-channels) the default permutationscheme, provides some level of frequency diversity, since sub-channels consist of non-adjacent groups ofadjacent sub-carriers. FUSC (Full Usage of sub-carrier) provide maximum frequency diversity and is moreresilient to interference, since sub-channels consist of non-adjacent sub-carriers, but is only available inDL. With re-use pattern 1x3x3 same frequency channel is re-used among Co-Channel sectors. CCI(Co-Channel Interference) for DL transmission can be optimized, but it is difficult to optimize for the ULtransmission, hence there's a possibility that neighboring Co-Channel sectors UL signals will interfere witheach other.

In TDD systems complete channel (physical band) is occupied for UL transmission within dedicatedsymbols based on TDD ratio. Within a standard 5ms frame (assuming TDD Ratio 31:15), excluding DLsub-frame and TTG/RTG gaps UL transmission from all SS will last for about 1.5ms. During this time there

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is no DL (BS-CPE) transmission and all SS within one BS will occupy different slots in UL sub-frame.Since slot is itself a two-dimensional figure, either Time (Symbol) OR Frequency (sub-channel)differentiate same SBS (Serving BS) SS UL slots from interfering with each other.

Slot structure in UL sub-frame is such that its multiples expand in both (Time/Frequency) dimensionsforming a Data Region based on user QoS requirement within the frame. The expanding Data Regionconsists of multiples of 48 sb-carrier sub-channels which are prearranged and communicated in theUL-MAP (part of the DL sub-frame). The problematic part comes when two Co-Channel sectors assignData Regions that comprises of slots having common symbols/sub-channels. Since these Time overlappedsymbols are not Time synchronized, if the signal strength of respective SS reach out to each others SBS(Serving BS), these upstream transmission interfere. The amount of UL interference is driven by theoverlapping of Data Regions of the two subscribers, worst case interference occur if exactly same slots areassigned to SSs in Co-channel sectors.

The odds of interference lessens if system is on average having higher UL MCS (Modulation and CodingSequence), as less slots are occupied per subscriber to meet QoS requirement, but this raises the needs forhigh UL CINR levels to meet MCS requirements. On the other hand, sacrificing higher MCS levels andrunning a system with low MCS will allow SS to transmit less (Automatic UL Power Control), just to meetsensitivity thresholds (Adaptive Modulation and Coding) and reduce the probability of over shoots.Consequently its the traffic behavior in UL that determines this trade-off between UL interference andUL capacity.

Its difficult to visualize how UL signals reach beyond serving BS. It requires precise data of clutter of theterrain to determine how the radiation will be diffracted, reflected, absorbed and attenuated. High ULTX-Power combined with gain and diversity results in signals that reach beyond serving sectors. Oneviable solution is to use low gain antennas and allow attenuation of unwanted signals using lower SSheights, but there will always remain randomness with mobility users mixed with dynamic allocation ofresources in UL that changes the picture completely each instance. OFDMA resource schedulermechanism is efficient but is complex to simulate and anticipated interference is more probabilistic ratherthan pre-determined; combined with limited reuse factor interference avoidance remains one of the biggestchallenges for network designers.

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Tags: Uplink, WiMAX, interference

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Comment by Robert Syputa on October 2, 2009 at 10:06pm"The odds of interference lessens if system is on average having higher UL MCS (Modulation andCoding Sequence), as less slots are occupied per subscriber to meet QoS requirement, but this raisesthe needs for high UL CINR levels to meet MCS requirements."

Thanks for excellent discussion.

But your comment drives back to greater use of distributed network architectures: granular,aggregated layered network topologies. Reduce range, sub-channel, self-configure.

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Uplink interference in WiMAX - 4G360 http://4g360.com/profiles/blogs/uplink-interference-in-wimax

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