IEEE C802.16j-07/410
Project
IEEE 802.16 Broadband Wireless Access Working Group <http://ieee802.org/16>
Title
DL and UL RS Assisted HARQ Scheme for Multiple RSs
Date
Submitted
2007-07-05
Source(s)
Israfil Bahceci, Senarath Gamini, Peiying
Zhu, Wen Tong, Hang Zhang, Mo Han
Fong, Derek Yu, David Steer, Mark
Naden
Voice: +613-763-1315
E-mail: wentong@nortel.com
Voice: +613-765-8983
E-mail: pyzhu@nortel.com
Nortel
3500 Carling Avenue
Ottawa, Ontario K2H 8E9
Re:
IEEE 802.16j-07/019: “Call for Technical Comments Regarding IEEE Project 802.16j”
Abstract
A hybrid analog-forward/decode-forward Type-I HARQ method is proposed for multiple RSs
assisted HARQ
Purpose
To incorporate the proposed text into the P802.16j Baseline Document (IEEE 802.16j-06/026r4)
Notice
Release
Patent
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DL and UL RS Assisted HARQ Scheme for Multiple RSs
Israfil Bahceci, Senarath Gamini, Peiying Zhu, Wen Tong, Hang Zhang, Mo Han Fong, Derek Yu,
David Steer, Mark Naden
Nortel Networks
Introduction
In the current baseline document, procedures for DL/UL HARQ are described for a centralized scheduler. The
centralized scheduler allocates resources for an initial transmission of HARQ packets on all links between the
MRBS and MS as well as the required ACK/NACK channels. If a decoding failure occurs at a RS, the RS that
received the HARQ packet may transmit to the next node in the path the pilot tones and null data. While
transmission of the pilots may be useful, the null data transfer may be wasteful since the received HARQ bursts
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IEEE C802.16j-07/410
still have soft information that may be useful for the subordinate nodes (e.g., in a typical HARQ, the received
HARQ packets may be combined with retransmitted packets to reduce the error rate by soft bit combining).
If there are multiple RSs along the path between MRBS and MS, they can improve the HARQ performance. The
reception by multiple RSs results in a multiple look to the HARQ packets (spatial diversity), thus, the
probability that all RSs decode the HARQ packet in error is very small. If at least one of them decodes the
HARQ packet successfully, it can forward it to the next node. On the other hand, if all nodes fail to decode
successfully, than again resources may be being wasted by transmitting null data by multiple RSs to the
destination node.
Especially in the presence of multiple RSs along a HARQ enabled path, the spatial diversity will be useful in
utilizing the HARQ packets: even if the reception quality at individual RSs may not be enough, soft
combination of them at a central node will reduce the error probability.
In this contribution, we propose a hybrid use of decode-and-forward strategy & analog forwarding strategy to
relay the HARQ bursts. This is applicable in Chase Combining HARQ when the MRBS-RS and RS-MS links
use the same burst profiles. The proposed strategy has the following advantages:
1. Delay problem: In case of error at all RSs, the RSs request retransmission from MRBS. Only after
successful reception by at least one of the relays can the HARQ burst be transmitted to the subordinate
node. The proposed scheme utilizes the spatial diversity for an HARQ burst that is received in error by
individual RSs but may be corrected by combing them at a destination node. The number of
retransmissions can thus be reduced.
2. Efficient use of resources: All signal received by the RSs can always be utilized whether decoding is
successful or not.
An other problem with the HARQ mechanism with multiple RSs described in the current baseline document is
that even if the MR-BS to RS link is good enough to allow for a successful decoding, the downstream for that
RS may be bad. So, even if we decode and forward at some RS, the link from that RS to the subordinate node
may be so weak that it can not be decoded thorough that path. Spatial diversity may improve the performance
of HARQ if some other multiple RSs involved in the same HARQ traffic have good quality links to MS even if
their decoding is not successful. Figure 1 depicts such a scenario.
NACK
ACK
NACK
Bad
Link
Good
Link
Good
Link
Figure 1 Multiple RSs in HARQ
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IEEE C802.16j-07/410
Analysis of the HARQ System
Assume non-MIMO mode for all links in the MR network in Figure 1, e.g., single antennas at all nodes. We
consider the analysis for DL HARQ system. Analysis for UL HARQ follows similarly. Let hk [n] be the channel
gain from MRBS to RSk, and g k [n] be the channel gain from RSk to its subordinate node at time n, such that
E{ hk }   k2 and E{ g k }   k2 . Assume the HARQ packet (after encoding and modulation) is given by a signal
2
2
sequence s[n], n=0,…,L-1, such that E{ s[n] }  1 . Received signal at RSk is
2
rk [n]  Phk [n]s[n]  ek [n] .
Let sˆk [n] denote the signal that RSk transmits to the subordinate node. Received signal at the subordinate node
is then given by
K
y[n]   Bk g k [n]sˆk [n]  z[n] .
k 1
The estimate sˆk [n] is equal to s[n] if RSk decodes the HARQ packet successfully. Otherwise,
sˆk [n] 
1
rk [n] ,
P   k2
2
k
that is, it is a scaled version of the signal received by RSk (so that the signal sˆk [n] is normalized to a unit power,
and the transmitted signal power at RSk is Bk.
Let RS1,…,RSK1 can decode the HARQ burst successfully, while the rest can not. According to baseline
document, one or more RS(s) that decoded successfully may be scheduled for forwarding the HARQ burst
(Scheme 0):
K1
y[n]   Bk g k [n]s[n]I k  z[n]
k 1
where Ik is the indicator function (e.g., it is equal to 1 if RSk is forwarding the HARQ burst, otherwise it is 01).
In this contribution, we propose that the RS(s) that have decoded erroneously also forwards the HARQ subburst,
thus, the received signal is given by (Scheme 1)
K1
K
Bk
x[n]   Bk g k [n]s[n]I k  
g [n]rk [n] z[n]
2
2 k
k 1
k  K1 1 Pk  k   k
K
K
 K1

Bk P
Bk

 s[n]  Bk g k [n]I k  
h
[
n
]
g
[
n
]

g [n]ek [n]  z[n]

k
k
2
2
2
2 k

k  K1 1 P k   k
 k 1
 k  K1 1 P k   k
Note that if K1=0, e.g., all RS(s) fail to decode, this scheme reduces to a pure amplify-and-forward relay
strategy.
The signal-to-noise ratio (SNR) in Scheme 0 is given by
1
We can also view BkIk as a power allocation to RSk. If transmitting the successfully decoded burst contributes to the overall performance of HARQ
burst decoding at the destination node, the power allocation to RS k is non-zero.
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IEEE C802.16j-07/410
K1
SNR0 
B 
k 1
k
2
k k
I
 z2
while the SNR in Scheme 0 is
K1
SNR1 
k 1
K
Bk
 2 2
2 k k
k  K1 1 P   k
.
K
Bk
2
2 2
z  
k k
2
2
k  K1 1 P k   k
 Bk  k2 I k  P

2
k
By an appropriate power allocation in Scheme 1, one can obtain an improved SNR value at the destination node
without wasting the resources.
Numerical Example
The proposed scheme can provide significant gains for a situation as described in Figure 1 where the RS(s) with
successful decoding may have lower quality link(s) to its subordinate node while the RS(s) with decoding
failure have better links. We provide a simple example here. Let signal-to-noise ratio (SNR) for MRBS-RSk link
P k2
(in dB),  k  2 ~ N (0,10) , and assume an error occurs if  k  0 dB. Assume that SNR for RSk-MS link is
k
by  k   k  (1  2 I k ) k , where I k  1 if RSk decodes HARQ subburst in success and I k  0 otherwise. Here,
 k (in dB) is the difference between the relay link SNR and access link SNR for RSk. Table 1 provides average
SNR gain with the proposed scheme against the decode-and-forward only scheme. It is assumed that the power
of the RSs is ~ 9 dB less than that of the MRBS (e.g., MRBS power ~ 43 dBm, RS power ~ 34 dBm). The
hybrid scheme promise further gains with optimized power allocation among the RS(s).
Table 1 SNR gain (in DB) with hybrid forwarding scheme for HARQ with 2, 3 and 4 RSs.
K
2
3
4

5
10
5
10
5
10
SNR Gain (dB)
1.92
6.12
2.46
7.46
2.5
7.63
Conclusions
The hybrid amplify-forward/decode-forward strategy can improve the HARQ performance in the presence of
multiple RSs assisting the HARQ. Enabling this scheme helps reduce the number of retransmissions in an
HARQ enabled traffic, and hence can increase the throughout of the system.
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IEEE C802.16j-07/410
Proposed Text Changes
[Using the HARQ DL and UL MAP subburst IEs, one shall be capable of enabling the hybrid HARQ subburst
forwarding at a relay station. The following text changes provide the necessary modifications.]
+++++++++++++ Start Text++++++++++++++++
[Change Row 9 of Table 286m (starting from the table header) as indicated. Chase HARQ subburst IE]
Reserved
AF IND
1 bit
Shall be set to zero.
1: This HARQ sub-burst is supported by analog forwarding
[Change Row 9 of Table 286s (starting from the table header) as indicated. MIMO STC Chase HARQ subburst
IE]
Reserved
AF IND
3 bits
0b111: This HARQ sub-burst is supported by analog forwarding
[MIMO DL Chase HARQ TBD]
[Change Row 14 of Table 302k (starting from the table header) as indicated. UL Chase HARQ subburst IE]
Reserved
AF IND
1 bit
1: This HARQ sub-burst is supported by analog forwarding
[Change Row 6 of Table 302q (starting from the table header) as indicated. UL MIMO STC HARQ subburst IE.
The padding bits for this IE requires correction in 16e.]
Reserved
AF IND
-
Shall be set to zero
If set to 1, this HARQ sub-burst is supported by analog forwarding
+++++++++++++++++End Text +++++++++++++++++++++
[MIMO UL Chase HARQ TBD]
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