IEEE C802.16m-08/321r1
Project
IEEE 802.16 Broadband Wireless Access Working Group <http://ieee802.org/16>
Title
Design Considerations for UL Ranging Channel in 802.16m
Date
Submitted
2008-05-08
Source(s)
Yih-Shen Chen, Kelvin Chou, I-Kang Fu and
Paul Cheng
yihshen.chen@mediatek.com
Kelvin.Chou@mediatek.com
IK.Fu@mediatek.com
MediaTek Inc.
No.1, Dusing Rd. 1, HsinChu Science-Based
Industrial Park, HsinChu, Taiwan
Re:
IEEE 802.16m-08/016, “Call for Contributions on Project 802.16m System Description
Document (SDD)”. In response to the following topics:
• Uplink Control Structure
Abstract
This contribution discusses the configuration of UL ranging channel and its management.
Purpose
To be discussed and adopted by TGm for the P802.16m SDD.
Notice
Release
Patent
Policy
This document does not represent the agreed views of the IEEE 802.16 Working Group or any of its subgroups. It
represents only the views of the participants listed in the “Source(s)” field above. It is offered as a basis for
discussion. It is not binding on the contributor(s), who reserve(s) the right to add, amend or withdraw material
contained herein.
The contributor grants a free, irrevocable license to the IEEE to incorporate material contained in this contribution,
and any modifications thereof, in the creation of an IEEE Standards publication; to copyright in the IEEE’s name
any IEEE Standards publication even though it may include portions of this contribution; and at the IEEE’s sole
discretion to permit others to reproduce in whole or in part the resulting IEEE Standards publication. The
contributor also acknowledges and accepts that this contribution may be made public by IEEE 802.16.
The contributor is familiar with the IEEE-SA Patent Policy and Procedures:
<http://standards.ieee.org/guides/bylaws/sect6-7.html#6> and
<http://standards.ieee.org/guides/opman/sect6.html#6.3>.
Further information is located at <http://standards.ieee.org/board/pat/pat-material.html> and
<http://standards.ieee.org/board/pat>.
Design Considerations for UL Ranging Channel in 802.16m
Yih-Shen Chen, Kelvin Chou, I-Kang Fu, and Paul Cheng
MediaTek Inc.
1
IEEE C802.16m-08/321r1
1. Introduction
Uplink (UL) ranging channel is a region where mobile stations (MS) maintain link quality or perform accessing
to network. The ranging channel is split into many ranging opportunities (RO) and each RO is chosen randomly
with equal probability. To perform ranging, MS chooses a RO and sends a ranging code onto it and then waits
for the response from BS. The ranging code currently used is PRBS code which is a pseudo random sequence. If
more than one ranging code is received in a single RO, the codes may be still detectable but the detection rate
decreases as the number increases. Obviously, the number must be lower than a threshold; otherwise, code
collision occurs. Another code collision scenario comes from that at least two user selects identical ranging code
and RO.
In this contribution, we discuss the following issues for contention-based UL ranging channels:

Flexible UL ranging channel configuration

Fast response mechanism for UL ranging channels

Efficient BR ranging procedure

Parameters for ranging request
2. Partition of ranging channels and codes
In 802.16e, four kinds of ranging purposes are defined: initial ranging, HO ranging, periodic ranging and BR
ranging. They are further categorized into two ranging channels: initial ranging/HO ranging and periodic
ranging/BR ranging. However, the PRBS ranging codes are partitioned into four code sets for the four ranging
purposes. The number mismatch between ranging code sets and ranging channels seems to allow overlapping
configuration; that is, the initial ranging/HO ranging and periodic ranging/BR ranging may partially or overly
share the same physical OFDM time-subchannel region.
Advantage
Disadvantage
Overlapping partition
- High bandwidth efficiency
Non-overlapping partition
- No collision between high and low
priority rangings
- Larger code pools for each ranging
- Low bandwidth efficiency
- High-priority ranging may
collide with low-priority ranging
- Smaller code pools for each
ranging
Table 1: comparisons of ranging channel partition methods
Table 1 shows the comparisons between the two partition methods. When the number of MS is small, the
overlapping partition method is better due to the bandwidth sharing. As the number of MS increases, nonoverlapping partition method is better because the collision between different priority ranging requests can be
avoided. Obviously, the ranging channels should be partitioned according to the network loading. Therefore, in
the contribution, we propose a flexible UL ranging channel configuration. The UL ranging channels are
categorized into three kinds:

General-purpose ranging channel (G-RCH): G-RCH is applied for all ranging purposes.

Specific-purpose ranging channel (S-RCH): S-RCH is applied for a specific set of ranging purpose.

Dedicated ranging channel (D-RCH): D-RCH is applied upon the assignment from MAC management
message for quick ranging.
2
IEEE C802.16m-08/321r1
The ranging codes for the G-RCH and S-RCH are partitioned accordingly. For example, if the network wants to
reduce the forced termination rate for handover MSs, the HO ranging requests can be applied in S-RCH and the
other kinds of ranging requests are applied in G-RCH. And, the corresponding ranging code partition is
illustrated in Fig. 1. As to D-RCH, the ranging code is assigned by the MAC management messages, such as:
MOB_PAG-ADV and etc.
Initial ranging codes
Total number of
Ranging codes
BR ranging codes
Total number of
Ranging codes
HO ranging codes
Periodic ranging
codes
G-RCH
S-RCH
Figure 1: an example of ranging code partition
3. Fast response mechanism for UL ranging channel
Since BS may decode multiple ranging codes in ranging opportunities/ranging channels, it takes some time to
handle the corresponding procedures. It is not time-efficient for MS to simply wait for a contention resolution
time interval, without knowing contention results in advance. A ranging collision may happen when one of the
following scenarios happens:
1.
The low SNR caused by noise spreading or frequency-selective fading in the multi-carrier environments
leads to undecodable ranging codes, which usually happens when too many ranging codes are conveyed by
a single ranging slot, as illustrated in Fig. 2.
MS
BS
Ranging code
(collided)
T3
Guess
there is a
collision
Backoff
Ranging code
CDMA Allocation IE
Figure 2: Ranging failure caused by ranging code collision
As shown in Fig. 2, when an 802.16e MS sends a ranging code for initial ranging, it starts a T3 timer to
wait the response from BS. If the timer expires, the MS performs backoff algorithm to calculate the next
ranging instance. The overall delay causes by ranging collision can be calculated by
Delay = (T3+backoff_time)*R,
(1)
where T3 is the timer value ranging from 60ms to 200ms and R is the number of retry. As to the BR
ranging request, eq. (1) is still applied and the T3 timer is replaced by “Contention-Based Reservation
Timeout”.
3
2.
IEEE C802.16m-08/321r1
The BS successfully decoded the received ranging codes but has insufficient bandwidth to satisfy all
requests, as illustrated in Fig. 3.
MS
BS
Ranging code
T3
No idea
what’s wrong
Successfully
decoded, but
insufficient
bandwidth
Backoff
Ranging code
CDMA Allocation IE
Figure 3: Ranging failure due to insufficient bandwidth
3.
At least two MSs have selected identical ranging code and ranging slot, as illustrated in Fig. 4.
MS 1
MS 2
BS
f(x)
g(x)=f(x)
f(x)
decoded
RNG-RSP (success)
CDMA Allocation IE
RNG-REQ
RNG-REQ
T3
T3
Failed to
decode
RNG-REQ
Restart contention-based ranging
Figure 4: Two SSs selected identical ranging code and ranging slot
In all scenarios the MS has no way to detect the collision but waits for its timer expiration, which leads to long
delay in the network entry process.
The ranging collision in scenario 1 can be detected by the BS when it received a ranging slot which contains undecodable ranging code. In the case that either a ranging collision is detected (scenario 1) or the BS decides to
ignore the ranging request due to insufficient bandwidth (scenario 2), the BS should broadcast its decoding
status of each ranging slot in the subsequent DL sub-frame, as illustrated in Figure 5.
4
IEEE C802.16m-08/321r1
MS
BS
Some
ranging slots
cannot be
Report of decoding
decoded
status
Ranging code
T3
Backoff
ReTx Ranging code
CDMA Allocation IE
Figure 5: Report of the ranging code decoding status
The reporting of the decoding status can be in the form of a MAC management message or just a bitmap for
minimum overhead, as illustrated in Fig. 6. In this example, the ranging status indicator is a bitmap in which
each bit represents the decoding status of its corresponding ranging slot in the previous UL sub-frame. The
fourth 0 bit indicates a ranging collision, while the seventh bit indicates the drop of the ranging request by the
BS due to insufficient bandwidth. The MS, upon received the ranging status indicator, cannot know whether its
ranging request is collided or dropped by the BS. However, the MS is able to know and react to the failure of its
ranging immediately.
DL i
UL i
1110
DL-MAP
Preamble
Ranging slots
UL-MAP
FCH
DL-MAP
Preamble
FCH
UL-MAP
Collided slot
1101
Ranging status
indicator
DL i+1
UL i+1
Figure 6: Concept of ranging status indicator
As to the ranging collision in scenario 3, there is no way for either BS or MS to detect such collision until the
RNG-REQ messages are collided. However, by using mechanisms similar to the aforementioned one, the BS
can still announce the collision of RNG-REQ messages instantaneously to avoid MSs’ waiting for the expiration
of their ranging timers, as illustrated in Fig 7.
5
IEEE C802.16m-08/321r1
MS 1
MS 2
BS
f(x)
f(x)
decoded
g(x)=f(x)
RNG-RSP (success)
CDMA Allocation IE
Failed to
decode
RNG-REQ
RNG-REQ
RNG-REQ
T3
T3
Fast ranging status report
Restart contention-based ranging
Figure 7: Fast report of the RNG-REQ decoding status
4. BR ranging procedure
4.1. Negotiation procedure
Fig. 8 shows the typical BR ranging procedure which consists of contention resolution phase and bandwidth
negotiation phase. BS identifies the MS who makes a ranging request in the contention resolution phase and
allocate UL bandwidth in the bandwidth negotiation phase. From sending BR ranging code to the UL data
transmission, it takes 5-step message transaction and long transmission latency. For real-time services or timecritical MAC management messages, the latency may not be allowable. For some non-real-time services (ex:
short message service, instant message), the latency is not a problem but the transmission efficiency is low.
MS
BS
BR_RNG_CODE (1)
Contention resolution
phase
CDMA_ALLOC_IE (2)
BR header (3)
Resource grant (4)
Bandwidth negotiation
phase
UL data (5)
Fig 8: typical BR ranging procedure
In this contribution, we suggest that the BR ranging codes may be further classified into two categories:
1. Variable-size BR ranging codes
2. Fixed-size BR ranging codes
For the fixed-size BR ranging codes, a simplified BR ranging procedure is proposed. As shown in Fig. 9, the
simplified BR ranging procedure takes only 3-step message transaction. Since bandwidth negotiation phase is
skipped, the transmission latency is reduced. And, the UL bandwidth allocation may be pre-defined and
broadcasted by BS via broadcasting messages or negotiated during connection establishment. As for the
variables-size BR ranging codes, the typical procedure is still applied.
6
IEEE C802.16m-08/321r1
MS
BS
BR_RNG_CODE (1)
Contention resolution
phase
CDMA_ALLOC_IE (2)
UL data (5)
Fig 8: simplified BR ranging procedure
4.2. Parameters in the BR ranging procedure
During the bandwidth negotiation procedure, MS reports its bandwidth request in a BR header and BS
determines the UL bandwidth allocation and the corresponding transmission format, UIUC. In some cases, BS
may has no knowledge of absolute transmission power in the MS side because BS only changes the UL
transmission with adjustment by periodic ranging or other power control messages. In the error-prone wireless
channel, it’s hard for BS to exactly track the absolute power value after several adjustments. Without the power
information, it’s hard for BS to determine an efficient transmission format.
Therefore, in the contribution, we suggest the UL channel condition parameter may be also included into the BR
header. Here, the UL channel condition parameters may be UL Tx power, received CINR or recommended
UIUC. The detail format of BR header carrying UL channel condition parameters is for future study.
5. Proposed Text for SDD
[Insert the following text into TGm SDD ]
------------------------------- Text Start ---------------------------------------------------------------------------------11.x Uplink Control Structures
11.x.y UL ranging channel
11.x.y.1 Configuration of UL ranging channel
The UL ranging channel is flexibly configured into three kinds:
5.1. General-purpose ranging channel (G-RCH): G-RCH is applied for all ranging purposes.
5.2. Specific-purpose ranging channel (S-RCH): S-RCH is applied for a specific set of ranging purpose.
5.3. Dedicated ranging channel (D-RCH): D-RCH is applied upon the assignment from MAC management
message for quick ranging.
The ranging codes for the G-RCH and S-RCH are partitioned accordingly.
11. x.y.2 Fast response message for UL ranging channel
Since BS may decode multiple ranging codes in a ranging channel, it takes some time to handle the
corresponding procedures. It is not time-efficient for MS to simply wait for a contention resolution time interval,
without knowing contention results in advance. To reduce the unnecessary delay, a fast response message is
required so that the MS can determine its next operation properly. The fast response message may be a status
indicator to represent the contention results of a RO; or, it may be an overloading indicator to indicate that too
many MSs sent out ranging requests. The detail design of the fast ranging message is TBD.
11.x.y.3 BR ranging procedure
11.x.y.3.1 negotiation procedure
7
IEEE C802.16m-08/321r1
The typical BR ranging procedure consists of contention resolution phase and bandwidth negotiation phase. BS
identifies the MS who makes a ranging request in the contention resolution phase and allocate UL bandwidth in
the bandwidth negotiation phase. From sending BR ranging code to the UL data transmission, it takes 5-step
message transaction and long transmission latency. For real-time services or time-critical MAC management
messages, the latency may not be allowable. For some non-real-time services (ex: short message service, instant
message), the latency is not a problem but the transmission efficiency is low. Therefore, it is suggested that the
BR ranging codes may be further classified into two categories:
1. Variable-size BR ranging codes
2. Fixed-size BR ranging codes
For the fixed-size BR ranging codes, a simplified BR ranging procedure may skip the bandwidth negotiation
phase to reduce the transmission latency. As for the variables-size BR ranging codes, the typical procedure is
still applied.
11.x.y.3.2 Parameters in the BR ranging procedure
The BR header may consist of bandwidth requests as well as UL channel condition parameter. Here, the UL
channel condition parameters may be UL Tx power, received CINR or recommended UIUC. The detail format
of BR header carrying UL channel condition parameters is for future study.
------------------------------- Text End ------------------------------------------------------------------------------------
Appendix: Collision Probability of Ranging Codes
The collision probability, Pcollision, is defined as the probability that at least two users selected the identical code
and slot. It can be derived by
 NcNs  i 
 1
Nu 1
Pcollision
i 0
(2)
NcNsNu
where Nu is the number of contention users, Nc is the number of available ranging codes and Ns is the number
of available ranging slots. Figure 9 shows the collision probability versus the number of contention users. In the
figure, it can be seen that the collision probability increases as the number of the contention user increases. And
also, the collision probability decreases when the number of ranging slot increases. Obviously, there exists a
tradeoff between collision probability and bandwidth efficiency.
Fig. 9: the collision probability versus the number of contention user
8