1. Scanning Operation by Multi-Carrier AMS
Document Number: IEEE C802.16m-09/0967
Date Submitted:
Source:
I-Kang Fu, Kelvin Chou, YihShen Chen, Paul Cheng
MediaTek Inc.
Venue:
IEEE m-09/0020, “Call for Contributions on Project m Amendment Working Document (AWD) Content”
– AWD New Contribution for Multi-Carrier
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This is base contribution.
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Propose to be discussed and adopted by TGm for the use in Project m AWD.
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2. Motivation
Much longer time will be required to complete the scanning over each carrier
The scanning process in single carrier system already takes a long time
Number of cells in 16m system is expected to be higher than 16e system
Number of carriers in 16m system will also be more than 16e system
These result in the difficulty for AMS to arrange its scanning operation to complete scanning in time

3. Problem Analysis
When the carrier number is increased, the time to complete scanning is also increased.
It takes longer time to collect enough time domain statistics if the scanning opportunities doesn’t increase

4. Problem Analysis
For the multi-carrier AMS implemented by single RF (i.e. with single large FFT), the same problem on multi-carrier scanning also exists.

5. Problem Analysis
Therefore, the increase on the number of the carrier will result in higher difficulty for MS to complete scanning over each carriers for each BS.
If the scanning opportunities is the same, the scanning result will be unreliable by higher variance.
This is because of the less averaging duration when scanning each BS over each carrier
This will degrade the algorithm performances which rely on scanning results as their input
This will be more significant for the MS with higher speed or for the network with more cells (especially pico-/femto-cells) deployed into the network
If the scanning opportunities is increased in response to the number of carriers, the achievable user throughput will be degraded due to less transmission opportunities.
This will also result in the difficulty for BS resource scheduling, especially when the number of users is increased.
Autonomous scanning cannot resolve this problem since the MS can only perform background scanning over the same carrier as the one used for data transmission

6. Investigation
The received preamble power by MSj mainly depends on the preamble transmission power by BSi over each carrier
The received preamble signal power from BSi by MSj can be represented by the following equation:
PR,i,j,k = PT,i,k + Gi,j(θi,j) – PL(di,j,k) – χi,j dBm, (1)
where
PR,i,j,k : Received power of the preamble signal from BSi by MSj over carrier k.
PT,i,k : Transmission power on preamble signal by BSi over carrier k.
Gi,j : antenna gain of the signal transmitted from BSi to MSj .
θi,j : the direction of MSj the with respective to the steering direction of BSi.
PL(di,j,k) : path loss experienced by the signals transmitted from BSi to MSj.
χi,j : shadow fading experienced by the signals transmitted from BSi to MSj.
The multi-path fading effect is not shown in the equation (1) because the scanning operation will take average over the preamble measurement results.

7. Investigation
Because the impact by choosing different carrier is the difference on center frequency, Gi,j(θi,j) and χi,j in equation (1) are not function of the carrier index k.
For the path loss over the signals transmitted from BSi to MSj, the typical path loss function can be represented by the following equation:
PL(di,j,k) = Alog10(di,j) + B + Clog10(fc(k)/5) dB
The parameters {A, B, C} will be different for different environments. For example, the following path loss model may be used for Rural macro cell environment with LOS condition:
PL(di,j,k) = 40log10(di,j) log10(hBS)-18.5log10(hMS)+1.5log10(fc(k)/5) dB
,where
hBS : (m) BS antenna height, hMS : (m) MS antenna height, fc(k) : (GHz) center frequency of carrier k
In general, the bandwidth of each carrier is from 5MHz~20MHz. Therefore, the separation from the center frequencies of the adjacent carrier will be within 20MHz for most cases.
If the carriers supported by BS and MS are adjacent in frequency domain, the pathloss difference due to different carrier will be very small and can be neglected.
If the carriers supported by BS and MS are separated in frequency domain (e.g. one carrier in 2.5GHz and another at 3.5GHz), the path loss difference will be larger (e.g. around 3dB). But it can be pre-calculated once the MS can know parameter “C” in advance.

8. Investigation
Therefore, the main factor to impact the PR,i,j,k under different k is the PT,i,k.
The propagation loss in different carrier will not change significantly. The main difference comes from the BS transmit power over each carrier.
The received preamble signal power in carrier k’ can be easily derived by the scanning result obtained from carrier k by adding the following offsets:
PR,i,,j,k’≈ PR,i,j,k + PT,offset(k, k’) + PLoffset(k, k’)
PT,offset(k, k’) = PT,i,j,k’ - PT,i,j,k
PLoffset(k, k’) = C log10(fc(k)/fc(k’)) : the path loss difference between carrier k and k+1, which is function of the path loss exponent “C” as depicted in previous slide.
MS can always know fc(k) after synchronize with the serving BS over carrier k, and it can usually know fc(k’) from multi-carrier configuration information sent by BS. Then MS can derive the PLoffset(k, k’) once it knows the path loss exponent “C”.
The received preamble signal power is usually used for reporting to serving BS as the RSSI (Received Signal Strength Index) result
If carrier k and carrier k’ are very close in frequency domain (e.g within 100MHz), this path loss offset value will be very small and can be ignored.
BS may only need to inform MS the “C” to support multiple band classes scenarios

9. Investigation
The received CINR of carrier k’ can be further derived based on the derived preamble signal power of carrier k’
CINRi,j,k’ = PR,i,j,,k’ / (IR,i,j,k’ + N)
= PR,i,j,,k’ / (PR,total,j,k’ - PR,i,j,,k’)
,where
IR,j,k’ : received interference power when decoding the signal from BSi to MSj over carrier k’
PR,total,j,k’ : total received power by MSj over carrier k’, including the power of the signals transmitted from each BS and the thermal noise power.
Once the MSj can obtain the total power received from carrier k’, it can also derive the received CINR received from BSi over the carrier k’ before scanning the preamble signal transmitted by BSi over the carrier k’.
MS only need to measure the total received power from each carrier and perform the aforementioned derivation, then it can obtain the received CINR from each BS before scanning their preamble over each carrier.

10. Conclusion
In order to prevent unnecessary scanning over each carrier for each BS, the following steps can help AMS easily derive the RSSI and CINR over each carrier.
MS perform scanning over the neighboring BSs in primary carrier
MS can perform autonomous scanning to prevent service disruption
MS obtains the RSSI of neighboring BSs on primary carrier
Serving BS inform MS (1) the preamble transmit power offsets between primary carrier and secondary carriers be applied at serving/neighboring BSs (2) and/or the path loss exponent of the surrounding propagation environment
MS can derive the RSSI of the secondary carriers
MS measure the total received power over secondary carriers
MS further derive the received CINR of the neighboring BSs based on the RSSI derived from step 2 and the measurement result by step 3
Base on the derivation results, AMS can determine which carriers and BSs should be further scanned. But it’s up to AMS implementation algorithm.

11. Operation Flow Example
Based on the derivation results, MS can further perform scanning operation over the “selected” carriers and BSs.
e.g. MS can only select the BSs be scanned over specific carrier if the RSSI or CINR derivation result is good (i.e. possible handover target)

12. Table xxx MOB_MC-ADV message format
AWD Text Proposal
Start of the Text
[Add the following information into the multi-carrier advertisement message broadcasted by ABS]
End of the Text
Table xxx MOB_MC-ADV message format
Syntax
Size (bit)
Notes
MOB_MC-ADV_Message_format() { -
Management Message Type = xx 8
Propagation Loss Exponent 4
2~4; in unit of 0.125
N_NEIGHBORS
N_CARRIERS 3
For (i=0; i< N_NEIGHBORS; i++) {
i=0 refers to serving ABS
For (j=0; i< N_CARRIERS; j++) {
Preamble Transmit Power Offset
-16dB ~ +16dB; in unit of 0.125dB }