UL Network Timing Synchronization

Network timing synchronization becomes an important issue when pico/femtocells are deployed together with overlaying macro/microcells, especially in a co-channel devel-opment scenario. Network timing offset will destroy the orthogonality of OFDM symbol and lead to severe interference cross whole operating frequency band such that any re-source planning fails. Network timing synchronization is essential and has to be kept so that radio signals from a pico/femto base station and an overlaying macro/micro base station over the air do not interfere with each other.

TDD OFDM comprises DL subframes, UL subframes and transmission time gaps.

Macro station transmits data during DL subframes and receives data during UL sub-frames. Each DL subframe is followed by an UL subframe after a predefined transmit transition gap (TTG) time, and each UL subframe is followed by a DL subframe after

a predefined receive transition gap (RTG) time. TTG is always larger than or equal to RTG.

Pico/femto base station is synchronized with overlaying macro, and has approxi-mately the same DL and UL transmission timing due to its physical proximity with BS.

For mobile station that is served by macro, it receives data during DL subframes and transmits data during UL subframes. After DL and UL synchronization with its serving BS, each DL subframe of MS synchronizes with each DL subframe of BS with a DL prop-agation delay, while each UL subframe of MS synchronizes with each UL subframe of BS with an UL timing advance. However, the Pico/femto BS can not directly synchronize with the local time of macro station. In Fig. 4.2, the femto/pico has to postpone its DL transmission due to DL propagation delay. Similarly, the UL subframes of femtocells are advanced. If not doing so, the femtocells will cause cross-tier interference to nearby macro users in DL subframes and UL subframes of femtocells will be interfered by macro users.

Figure 4.2: DL and UL subframes and transmission timing in cellular OFDM commu-nication system

The problem is not fully solved. As illustrated in Fig. 4.2, the pdefined TTG re-served to avoid the collision of downlink and uplink transmission may need to be adjusted

base on the deployment location of the pico/femto base station. However, for mobile sta-tions which handover to a pico/femto cell or camping on in a pico/femtocell, there is no mechanism for them to know whether TTG adjustment is applied in the pico/femtocell.

Without knowing this adjustment value, there will be different understandings about the uplink transmission timing between the mobile stations and the pico/femto BS. As illustrated in Fig. 4.3,the TTG adjustment causes the timing misalignment. The issue longer timing offset can be solved by extending CP. This solution, however, is associated with a few disadvantages. First, it requires a non-synchronous ranging channel to have a different CP length from a data channel in the same communication system. Second, the different CP lengths between non-synchronous ranging channel and data channel may result in interference with each other. Fatally, a non-synchronous ranging channel may have a different physical structure and code sequences than those of a synchronous rang-ing channel. Thus, without utilizrang-ing a unified synchronous rangrang-ing channel, hardware complexity and cost of a pico/femto BS may not be reduced. Although the distance between a fMs and a fBS is limited, but the information of TTG adjustment is not available at fMSs such that the time alignment at fBS will fail.

DL subframes UL subframes

DL subframes UL subframes DL subframes Femto BS

Figure 4.3: Timing misalignment is caused by TTG adjustment.

Specially, in IEEE 802.16m-D4 [14], the Padded Zadoff-Chu codes with cyclic shifts are used for the ranging preamble codes. The details of padded Zadoff-Chu sequences are described as follows. The p^th ranging preamble code xp{n, k} for the nth OFDMA symbol within a basic unit is defined by

x_p{n, k} = exp{−jπ(rp(n ∗ 71 + k)(n ∗ 71 + k + 1)

211 + 2 ∗ k ∗ s^p∗ N^{T CS}

NF F T )}, (4.1)

k = 0, 1, ..., NRP − 1; n = 0, 1, 2

where

• p is the index for p^th ranging preamble code within a basis unit which is made as the s^th_p cyclic shifted sequence from the root index rp of Zadoff-Chu sequence.

• r^p = mod((1 − 2mod(floor(P/M), 2)) ∗ (floor(p/(2M^s)) + rs0) + 211, 211)

• p = 0, 1, ..., N^cont− 1, ..., N^cont+ Ndedi− 1

• s^p = mode(p, Ms), p = 0, 1, ..., Ncont+ Ndedi− 1

• The start root index rs0 is broadcasted.

• M^s is the number if cyclic shift per ZC root index and defined by Ms = 1/G.

• N^{T CS} is the unit of time domain cyclic shift per OFDMA symbol according to the CP length and defined by NT CS = NF F T

• G and N^{F F T} are the are the CP ratio and FFT size respectively.

• N^RP is the length of ranging preamble code per OFDMA symbol, i.e., NRP = 72.

Originally, the padded Zaddoff-Chu code are not suitable for asynchronous MSs due to the asynchronous MSs have wide-range timing offsets. The wide-range timing offset caused by TTG adjustment is not expected and leads to severe false detection probability of padded Zaddoff-Chu sequence. To solve the issue, an amendment is proposed. In our

approach, the TTG adjustment information is quantized and broadcasted via broadcast channel. With the assisted information, the padded Zaddoff-Chu sequences can be used for femto MSs without causing any further side effects.