IEEE C802.16m-08/015r3

Project	IEEE 802.16 Broadband Wireless Access Working Group <http://ieee802.org/16>	
Title	Symbol duration Extended Interleaved FDMA as Uplink Multiple Access Technique for 802.16m	
Date Sub.	2008-01-16	
Source(s)	Arpita Thakre,
Arun Ayyar,
K. Giridhar,
Indian Institute of Technology Madras, India

J. Klutto Milleth
Center of Excellence in Wireless Tech, India	arpita@tenet.res.in
arun_ayyar@tenet.res.in
giri@tenet.res.in

klutto@cewit.org.in
	
Re:	IEEE C802.16m-07/047 - Call for Contributions on Project 802.16m System Description Document (SDD)	
Abstract	Symbol duration Extended I-FDMA (SE-IFDMA) using symbol repetition can provide low PAPR uplinks, when compared to OFDMA and DFT-spread OFDMA. In large cell sites that are coverage limited, and particularly in FDD deployments, the proposed SE-IFDMA can provide high quality UL channel estimation in a spectrally efficient and power efficient manner, and also enable flexible bit-rate allocations.	
 Purpose	To be discussed by TGm for incorporating the proposal into IEEE 802.16m standard	
Notice	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.	
Release	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.	
Patent Policy	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>.	

Symbol duration Extended Interleaved FDMA as Uplink Multiple Access Technique for 802.16m

Arpita Thakre		Arun Ayyar		K. Giridhar		J. Klutto Milleth* 

Dept. of Electrical Engg., Indian Institute of Technology Madras, Chennai 600036, India
* Center of Excellence in Wireless Technology, Chennai 600036, India

1. Motivation

OFDMA with tile based resource allocation is used in the uplink of IEEE 802.16e [1]. DFT-spread OFDMA (D-OFDMA) has CM and PAPR less than OFDMA [2]. Interleaved FDMA (I-FDMA) can be viewed as a special case of D-OFDMA, where it is possible to achieve a PAPR of unity for QPSK. However, unlike OFDMA or D-OFDMA where pilot sub-carriers can be embedded along with the data sub-carriers, I-FDMA typically requires time-multiplexed pilots in order to track fast fading channels. The other seeming limitation is that since (symbol) sequence repetition is used to create the equi-spaced sub-carriers in the frequency domain (which we will refer hereafter as a frequency  “comb”), the sub-carriers extend up to the edges of the frequency band, making it difficult to provide for guard-bands. Also, such comb-like structure could potentially reduce the flexibility of bit-rate allocation and interference averaging. The above considerations have perhaps prevented emerging access standards such as the LTE to consider symbol-repetition based I-FDMA for the uplink.

Yet, in many countries with large open and rural areas where macro-cells (~25km radius) are required, the unity PAPR of I-FDMA can significantly help in providing higher uplink margins, especially for fixed and pedestrian users. Moreover, such channels have low delay spread, and MIMO uplink may not be effective (other than perhaps virtual antenna or beam-forming modes using 2 or more UL transmit antennas). Also, with multiple operators, FDD may be the favoured duplexing method. Since channel reciprocity does not hold in FDD systems, high quality UL channel sounding in a power efficient manner is necessary for good link performance. 

For these reasons, we consider I-FDMA as an alternative to OFDMA in these coverage limited, rural macro scenarios. The functional diagram of the I-FDMA transceiver is shown in Fig.1.

FFT

FDE

IFFT

Phase Correct & Detect

Remove CP

ADC

Symbol Repetition

CHANNEL

Add CP

DAC

Phase Shift

Fig. 1: Block Diagram of Symbol duration Extended I-FDMA Uplink Transceiver

2. Proposed SE-IFDMA Technique

We propose a Symbol duration Extended Interleaved FDMA (SE-IFDMA) as the uplink multiple access technique where:

(1)	Unity PAPR is retained (for M-ary PSK)
(2)	The symbol duration is extended in order to ensure that the used band-width on the UL  is exactly equal to that on the OFDMA downlink (or D-OFDMA UL). An example for 10MHz deployment is given in Fig.2 below. In comparison to WiMaX, which has a useful OFDMA symbol duration of 91.43msec, the proposed SE-IFDMA will have a duration of 111.32msec (not counting the CP duration in both cases).
(3)	By using high-quality decision-directed channel tracking (DDCT), the need for temporally multiplexing pilot combs is avoided
(4)	The techniques in (2) and (3) ensure that the spectral efficiency of the SE-IFDMA scheme is equal to (or more than) that of OFDMA or D-OFDMA schemes
(5)	Flexible allocation of combs to various users is possible
(6)	Interference averaging is possible by hopping the comb allocations over time
(7)	As in diversity spaced OFDMA allocations and in the equi-spaced D-OFMDA scheme, this SE-IFDMA can also accrue the full frequency diversity by using the outer FEC code
(8)	If all the users on the uplink are power-controlled, then the impact of inter-carrier interference will be comparable to than in the localized D-OFDMA or OFDMA schemes

1024

1

933

92

WiMaX

Df2

Df1

1024 tones over 9.1984MHz

841 tones over 9.1984MHz

= 10.9375 KHz 

= 8.9828 KHz 

SE-IFDMA

Fig. 2: Comparision of sub-carrier widths in WiMaX (10MHz) and in the proposed SE-IFDMA. For the scalable-OFDMA cases, the (block) symbol duration increases by approximately 21%. However, when compared to WiMaX, the number of QAM symbols sent per second of channel use will be higher in the case of  SE-IFDMA since more data sub-carriers are available.

3. Illustrative Example of SE-IFDMA Uplink 
FFT size N=1024;  P1=256, K1=4;  P2=128, K2=8;  and  P3=128, K3=8

 User 3

 User 2

 User 1

P1 symbols

Repeat K1 times N=P1K1

Add CP

DAC

P2 symbols

Repeat K2 times N=P2K2

Add CP

DAC

P3 symbols

Repeat K3 times N=P3K3

Add CP

DAC

 n = [0 1 2 3 ….(N-1)]

                                        

Fig.3a: Transmission from 3 users on the uplink using SE-IFDMA; the value of the phase shift (which is assigned by BS) determines the location of the frequency comb for that particular user

 0   1   2   3  4   5   6  7   8   9  10 11      13                 17 18 19                                            1021  1023

User 1
User 2
User 3

 f

	
Fig.3b: SE-IFDMA Example with N=1024 : frequency combs for the three uplink users where the comb for user1 has 256 tones,  while user2 and user3 have 128 tones each

4. UL Zone for SE-IFDMA 
The uplink zone structure for SE-IFDMA is described, assuming 1024-pt FFT. This can be a time-multiplexed zone in the UL sub-frame, where the other part of the UL frame could use legacy OFDMA or any other generalized multi-carrier modulation.
(1)	The first three symbols (slot #0 in Fig.4 below) in the UL frame can be reserved for carrying a mix of long pilot (LP) combs and short pilot (SP) combs. Alternatively, the LP and SP combs can be distributed over the entire UL frame.
(2)	The duration of the SE-IFDMA sub-frame (or zone) should be at least 3 symbols long.
(3)	Long pilot combs will have either 256 or 128 sub-carriers. Short pilot combs are also defined with either 64 or 32 sub-carriers. 
(4)	LP Comb can also be used for estimating long-term statistics. These properties vary only over 50-100 frames, and these can be exploited to greatly reduce channel estimation overhead. On the other hand, SP Combs will aid instantaneous (short term) channel estimation and tracking.(5)	
(6)	Both reuse-1/1 and reuse-1/3 LP and SP combs can be considered, which are drawn from modified Frank-Zadoff sequences with uni-modular properties in both time domain and frequency domain. The sequences are specified in Table x (TBD).
(7)	UL ranging code will be assigned a comb with 128 sub-carriers from slot #0, and the ranging code shall be 2 symbols long. It will have the phase continuity property as in the 802.16e ranging mechanism, and will be drawn from the same set of UL ranging codes. Periodic ranging codes with comb of 64 tones can occur anywhere within this zone.
(8)	Every scheduled user in this zone, is assigned either a long pilot comb or a short pilot comb, followed by multiple data combs (if in SE-IFDMA zone) and/or tiles (if OFDMA, or D-OFDMA) over the remaining portion of the UL sub-frame.

20

14

20

OR

7

2

0

0

Slot 6

Slot 0

UL ranging code over 2T (comb with 128 sub-carriers)

Mix of long pilots (LP) with short pilots (SP); comb with DC unused
LP comb : 256 or 128 sub-carriers; SP comb: 64 or 32 sub-carriers;

Mix of data combs with short pilot combs; comb with DC is typically 
unused by any UL user

Fig.4: SE-IFDMA Framing (with N=1024). UL ranging code is always in slot #0, while the LP and SP combs can be either in slot #0, or distributed over the frame. 

5. Flexible Resource Allocation in SE-IFDMA
Flexible resource allocation on the uplink is enabled by IFDMA (and hence also in SE-IFDMA). A comb-tree as shown in Fig.5 below, which is similar to the OVSF code-tree used in W-CDMA, is used to provide user combs, and the assigned comb can be signaled on the downlink MAP with very low signaling overhead. (TBD – the MAP signaling overhead computation)

512

512

1024

256

256

Unused comb with dc sub-carrier

128

128

64

64

32

32

16

16

128

128

32  32

32

16

16

16

32

64

64

32

32  32

32

16

16

16

16

Fig. 5: Subcarrier (comb) assignment to UL users for FFT size N=1024

•	A node corresponds to a particular comb (equi-spaced sub-carrier allocation)
•	An user can occupy only one node, in order to retain the low PAPR property
•	The number associated with each node indicates the number of sub-carriers per symbol allocated to the user occupying the node
•	When a node is occupied by a user, none of the child-nodes below that node can be occupied by any other user
•	A user has a fixed number of comb/sub-carrier allocation choices for a given number of sub-carriers per symbol and FFT size
•	In the example above in Fig.5, one comb of 16 equi-spaced sub-carriers (which contains the DC sub-carrier) is not allocated to any user for UL transmission.

6. Coded BER Performance 
Since interference free, high-quality channel estimation is possible, coded BER performance is not only better than that of the localized D-OFDMA, but is also better than the BER performance of equi-spaced D-OFDMA and tile based OFDMA. We know that DFT-spread OFDMA and OFDMA perform very similar in terms of block error rate with gaps less than 0.5 dB [2]. Again Cubic Metric and PAPR of DFT-spread OFDMA is less than OFDMA [2]. We therefore compare our proposed uplink MA technique with DFT-spread OFDMA with localized sub-carrier allocation and DFT-spread OFDMA with equally spaced sub-carrier allocation.

6.1 Parameters used in Simulation [3], [4]

Symbol duration Extended I-FDMA	DFT-spread OFDMA	
Channel Model: WINNER Rural Macro-cell
Vehicle speed: 30 km/h
Modulation: QPSK
Coder: ½ rate Turbo coder
MIMO: 1X2
Pilot : Data = 1:7			
BW = 10 MHz
FFT size = 1024, 10MHz
Resource block: 64X3	Channel Model: WINNER Rural Macro-cell
Vehicle speed: 30 km/h
Modulation: QPSK
Coder: ½ rate Turbo coder
MIMO: 1X2
Pilot : Data = 1:7			
BW = 10 MHz
FFT size = 1024, 110MHz
Resource block: 48X7 (64X3) 	

6.2 Coded BER Plots
(a) Winner rural macro-cell model (only 2 taps at 10MHz resolution)

(b) Exponential PDF model (4 taps assumed at 10MHz resolution)

7. Note on Backward Compatibility
The proposed SE-IFDMA will seamlessly support sub-channel sizes that are integral multiples of 48 resource units. The other requirements for backward compatibility will be met by:
(a)	Making SE-IFDMA an UL zone
(b)	Ensure (by adjusting the length of CP) that the total length of this new UL zone still meets the legacy frame boundary requirements. In other words, ensure that when both OFDMA and SE-IFDMA symbols are used on the UL we have (for integers L1 and L2)
L1 Tofdma+cp1 = L2  Tse-ofdma+cp2

8. Summary
Power efficient UL is required in large cells, and SE-IFDMA is a good candidate to achieve this in a spectrally efficient and simple manner. In such rural macro cells, MIMO on the UL may not be useful since the delay spread may not be significant. Better UL link budget could come from transmit beam-forming as well as the ability to narrow-band (and reduce peak power) which is easy to do in IFDMA. SE-IFDMA can be used within an UL zone to provide in a power efficient manner high quality channel estimation for both SE-IFDMA as well as OFDMA users.

Comparison between Proposed SE-IFDMA and WiMAX (for 10MHz, 1024)
Symbol duration Extended I-FDMA	WiMAX	
Pilot : Data = 1:7			
BW = 10 MHz
FFT size = 1024
Subcarrier spacing = 8.9828 KHz
Used BW = 9.1984 MHz
Symbol duration (without CP) =111.32 msec
Oversampling – NIL
Used subcarriers per symbol = 1008
Data bits per msec = 7763
(assuming BPSK modulation)	Pilot : Data = 1:3			
BW = 10 MHz
FFT size = 1024
Subcarrier spacing = 10.9375 KHz
Used BW = 9.1984 MHz (841 tones)
Symbol duration (without CP) = 91.43msec
Oversampling factor = 28/25
Used subcarriers per symbol = 840
Data bits per msec = 6860 
(assuming BPSK modulation)	

8. References
[1]  IEEE 802.16e-2005
[2]  IEEE C802.16m-07/239r1, “Proposal for Incorporating Single-carrier FDMA into 802.16m”
[3]  IEEE 802.16m-07/002r4, “802.16m System Requirements”
[4]  IEEE 802.16m-07/037r2, “Draft 802.16m Evaluation Methodology”

9. Proposed Text for 16m SDD (IEEE C802.16m-07/320r1)
Insert the following text in Chapter 11 (Physical Layer)

11.x Uplink Modulation
In the uplink, the modulation can be OFDMA, or DFT-spread OFDMA, or Symbol duration Extended Interleaved FDMA (SE-IFDMA). 

The functional block diagram of SE-IFDMA is given below

Fig. x (insert Fig.1 here)

Extending the  symbol duration enables SE-IFDMA to have the same used bandwidth as that of legacy OFDMA schemes. An example for 10MHz, 1024pt FFT is provided below 

Fig. y (insert Fig.2 here)

11.y Uplink SE-IFDMA Zone Structure

(insert Section 4 titled “UL Zone for SE-IFDMA” here)

11.z UL Resource Allocation and MAP Overhead

(insert Section 5 titled “Flexible Resource Allocation in SE-IFDMA” here); 
computation of exact MAP overhead -- TBD